485BPOS

As filed with the U.S. Securities and Exchange Commission on May 16, 2018

Securities Act File No. 333-142592

Investment Company Act File No. 811-22061

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933	☒
Pre-Effective Amendment No.	☐
Post-Effective Amendment No. 312	☒
and/or
REGISTRATION STATEMENT
UNDER
THE INVESTMENT COMPANY ACT OF 1940	☒
Amendment No. 314
(Check appropriate box or boxes)

BLACKROCK FUNDS II

(Exact Name of Registrant as Specified in Charter)

100 Bellevue Parkway

Wilmington, Delaware 19809

(Address of Principal Executive Office)

Registrant’s Telephone Number, including Area Code (800) 441-7762

John M. Perlowski

BLACKROCK FUNDS II

55 East 52^nd Street, New York, New York 10055

(Name and Address of Agent for Service)

Copies to:

Counsel for the Fund: Margery K. Neale, Esq. Willkie Farr & Gallagher LLP 787 Seventh Avenue New York, New York 10019-6099 Benjamin Archibald, Esq. BlackRock Advisors, LLC 55 East 52^nd Street New York, New York 10055

Continuous

(Approximate Date of Proposed Offering)

It is proposed that this filing will become effective:

☒ immediately upon filing pursuant to paragraph (b)

☐ on (date) pursuant to paragraph (b)

☐ 60 days after filing pursuant to paragraph (a)(1)

☐ on (date) pursuant to paragraph (a)(1)

☐ 75 days after filing pursuant to paragraph (a)(2)

☐ on (date) pursuant to paragraph (a)(2) of Rule 485.

If appropriate, check the following box:

☐ This post-effective amendment designates a new effective date for a previously filed post-effective amendment.

Title of Securities Being Registered: Shares of Beneficial Interest, par value, $0.001 per share.

This filing relates solely to: BlackRock Inflation Protected Bond Portfolio.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all the requirements for the effectiveness of this Post-Effective Amendment to its Registration Statement pursuant to Rule 485(b) under the Securities Act of 1933 and has duly caused this Post-Effective Amendment to its Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York and the State of New York, on May 16, 2018.

	BLACKROCK FUNDS II
	(Registrant)
	on behalf of BlackRock Inflation Protected Bond Portfolio
By:	/s/ JOHN M. PERLOWSKI
(John M. Perlowski,
President and Chief Executive Officer) (Principal Executive Officer)

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment to its Registration Statement has been signed below by the following persons in the capacities and on the dates indicated:

Signature	Title	Date
/s/ JOHN M. PERLOWSKI	Trustee, President and Chief Executive Officer	May 16, 2018
John M. Perlowski	(Principal Executive Officer)
/s/ NEAL J. ANDREWS	Chief Financial Officer (Principal	May 16, 2018
Neal J. Andrews	Financial and Accounting Officer)
JAMES H. BODURTHA*	Trustee
(James H. Bodurtha)
BRUCE R. BOND*	Trustee
(Bruce R. Bond)
STUART E. EIZENSTAT*	Trustee
(Stuart E. Eizenstat)
HENRY GABBAY*	Trustee
(Henry Gabbay)
LENA G. GOLDBERG*	Trustee
(Lena G. Goldberg)
ROBERT M. HERNANDEZ*	Trustee
(Robert M. Hernandez)
HENRY R. KEIZER*	Trustee
(Henry R. Keizer)

- 3 -

JOHN F. O’BRIEN*	Trustee
(John F. O’Brien)
DONALD C. OPATRNY*	Trustee
(Donald C. Opatrny)
ROBERT FAIRBAIRN*	Trustee
(Robert Fairbairn)

*By:	/s/ BENJAMIN ARCHIBALD	May 16, 2018
	Benjamin Archibald (Attorney-in-Fact)

- 4 -

SIGNATURES

Cayman Inflation Protected Bond Portfolio, Ltd. has duly caused this Post-Effective Amendment to the Registration Statement of BlackRock Funds II, with respect only to information that specifically relates to Cayman Inflation Protected Bond Portfolio, Ltd., to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York and the State of New York, on May 16, 2018.

	CAYMAN INFLATION PROTECTED BOND PORTFOLIO, LTD.
	(Registrant)
By:	/s/ JOHN M. PERLOWSKI
(John M. Perlowski,
Director)

This Post-Effective Amendment to the Registration Statement of BlackRock Funds II, with respect only to information that specifically relates to Cayman Inflation Protected Bond Portfolio, Ltd., has been signed below by the following persons in the capacities on the dates indicated:

Signature	Title	Date
/s/ JOHN M. PERLOWSKI	Director, Cayman Inflation Protected Bond Portfolio, Ltd.	May 16, 2018
John M. Perlowski
/s/ NEAL J. ANDREWS	Director, Cayman Inflation Protected Bond Portfolio, Ltd.	May 16, 2018
Neal J. Andrews

EXHIBIT INDEX

Index No.	Description of Exhibit
EX-101.INS	XBRL Instance Document
EX-101.SCH	XBRL Taxonomy Extension Schema Document
EX-101.CAL	XBRL Taxonomy Extension Calculation Linkbase
EX-101.DEF	XBRL Taxonomy Extension Definition Linkbase
EX-101.LAB	XBRL Taxonomy Extension Labels Linkbase
EX-101.PRE	XBRL Taxonomy Extension Presentation Linkbase

- 6 -

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Dec. 31, 2017
Registrant Name	dei_EntityRegistrantName	BlackRock Funds II
Central Index Key	dei_EntityCentralIndexKey	0001398078
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Apr. 30, 2018
Document Effective Date	dei_DocumentEffectiveDate	Apr. 30, 2018
Prospectus Date	rr_ProspectusDate	Apr. 30, 2018

XML

Investor A, C and Institutional | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO

Fund Overview

Key Facts About BlackRock Inflation Protected Bond Portfolio

Investment Objective

The investment objective of the BlackRock Inflation Protected Bond Portfolio (the “Inflation Protected Bond Portfolio” or the “Fund”) is to seek to maximize real return, consistent with preservation of real capital and prudent investment management.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $25,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates. More information about these and other discounts is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, The PNC Financial Services Group, Inc. (“PNC”) and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” and the “Intermediary-Defined Sales Charge Waiver Policies” sections on pages 25 and A-1, respectively, of the Fund’s prospectus and in the “Purchase of Shares” section on page II-74 of Part II of the Fund’s Statement of Additional Information.

Shareholder Fees
(fees paid directly from your investment)

Shareholder Fees - Investor A, C and Institutional - BLACKROCK INFLATION PROTECTED BOND PORTFOLIO	Investor A Shares		Investor C Shares		Institutional Shares
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	4.00%		none		none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	none	[1]	1.00%	[2]	none

Annual Fund Operating Expenses
(expenses that you pay each year as a
percentage of the value of your investment)

Annual Fund Operating Expenses - Investor A, C and Institutional - BLACKROCK INFLATION PROTECTED BOND PORTFOLIO		Investor A Shares	Investor C Shares	Institutional Shares
Management Fee	[1],[2]	0.24%	0.24%	0.24%
Distribution and/or Service (12b-1) Fees		0.25%	1.00%	none
Other Expenses		0.43%	0.31%	0.29%
Interest Expense		0.05%	0.05%	0.05%
Miscellaneous Other Expenses		0.38%	0.26%	0.24%
Other Expenses of the Subsidiary	[3]
Total Annual Fund Operating Expenses	[4]	0.92%	1.55%	0.53%
Fee Waivers and/or Expense Reimbursements	[1],[5]	(0.28%)	(0.16%)	(0.14%)
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	[1],[5]	0.64%	1.39%	0.39%

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example - Investor A, C and Institutional - BLACKROCK INFLATION PROTECTED BOND PORTFOLIO - USD ($)	1 Year	3 Years	5 Years	10 Years
Investor A Shares	463	626	835	1,435
Investor C Shares	242	457	814	1,818
Institutional Shares	40	141	268	637

You would pay the following expenses if you did not redeem your shares:

Expense Example, No Redemption	1 Year	3 Years	5 Years	10 Years
Investor A, C and Institutional | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Investor C Shares | USD ($)	142	457	814	1,818

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 76% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the Fund invests at least 80% of its assets in inflation-indexed bonds of varying maturities issued by the U.S. and non-U.S. governments, their agencies or instrumentalities, and U.S. and non-U.S. corporations.

The Fund maintains an average portfolio duration that is within ±20% of the duration of the Barclays U.S. Treasury Inflation Protected Securities Index (the benchmark).

The Fund may invest up to 20% of its assets in non-investment grade bonds (high yield or junk bonds) or securities of emerging market issuers. The Fund may also invest up to 20% of its assets in non-dollar denominated securities of non-U.S. issuers, and may invest without limit in U.S. dollar denominated securities of non-U.S. issuers.

The Fund may seek to provide exposure to the investment returns of real assets that trade in the commodity markets through investment in commodity-linked derivative instruments and investment vehicles such as exchange traded funds that exclusively invest in commodities and are designed to provide this exposure without direct investment in physical commodities. The Fund may also gain exposure to commodity markets by investing up to 25% of its total assets in the Subsidiary, a wholly owned subsidiary of the Fund formed in the Cayman Islands, which invests primarily in commodity-related instruments.

The Fund also makes investments in residential and commercial mortgage-backed securities and other asset-backed securities.

Non-investment grade bonds acquired by the Fund will generally be in the lower rating categories of the major rating agencies (BB or lower by S&P Global Ratings (“S&P”) or Ba or lower by Moody’s Investors Service, Inc. (“Moody’s”)) or will be determined by the management team to be of similar quality. Split rated bonds will be considered to have the higher credit rating. Split rated bonds are bonds that receive different ratings from two or more rating agencies.

The Fund may buy or sell options or futures, or enter into credit default swaps and interest rate or foreign currency transactions, including swaps (collectively, commonly known as derivatives). The Fund may seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques (such as reverse repurchase agreements or dollar rolls).

The Fund may engage in active and frequent trading of portfolio securities to achieve its primary investment strategies.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Commodities Related Investments Risks — Exposure to the commodities markets may subject the Fund to greater volatility than investments in traditional securities. The value of commodity-linked derivative investments may be affected by changes in overall market movements, commodity index volatility, changes in interest rates, or factors affecting a particular industry or commodity, such as drought, floods, weather, embargoes, tariffs and international economic, political and regulatory developments.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.
  
  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Deflation Risk — Deflation risk is the possibility that prices throughout the economy decline over time — the opposite of inflation. If inflation is negative, the principal and income of an inflation-protected bond will decline and could result in losses for the Fund.

• Derivatives Risk — The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including:
  
  Volatility Risk — Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets.
  
  Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation.
  
  Market and Liquidity Risk — The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately.
  
  Valuation Risk — Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them.
  
  Hedging Risk — Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences.
  
  Tax Risk — Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments, are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority. Such treatment may be less favorable than that given to a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments.
  
  Regulatory Risk — Derivative contracts, including, without limitation, swaps, currency forwards and non-deliverable forwards, are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (“Dodd Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, certain derivatives are subject to margin requirements and swap dealers are required to collect margin from the Fund with respect to such derivatives. Specifically, regulations are now in effect that require swap dealers to post and collect variation margin (comprised of specified liquid instruments and subject to a required haircut) in connection with trading of over-the-counter (“OTC”) swaps with the Fund. Shares of investment companies (other than certain money market funds) may not be posted as collateral under these regulations. Requirements for posting of initial margin in connection with OTC swaps will be phased-in through 2020. In addition, regulations adopted by prudential regulators that will begin to take effect in 2019 will require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund.

• Dollar Rolls Risk — Dollar rolls involve the risk that the market value of the securities that the Fund is committed to buy may decline below the price of the securities the Fund has sold. These transactions may involve leverage.

• Emerging Markets Risk — Emerging markets are riskier than more developed markets because they tend to develop unevenly and may never fully develop. Investments in emerging markets may be considered speculative. Emerging markets are more likely to experience hyperinflation and currency devaluations, which adversely affect returns to U.S. investors. In addition, many emerging securities markets have far lower trading volumes and less liquidity than developed markets.

• Foreign Securities Risk — Foreign investments often involve special risks not present in U.S. investments that can increase the chances that the Fund will lose money. These risks include:

• The Fund generally holds its foreign securities and cash in foreign banks and securities depositories, which may be recently organized or new to the foreign custody business and may be subject to only limited or no regulatory oversight.

• Changes in foreign currency exchange rates can affect the value of the Fund’s portfolio.

• The economies of certain foreign markets may not compare favorably with the economy of the United States with respect to such issues as growth of gross national product, reinvestment of capital, resources and balance of payments position.

• The governments of certain countries may prohibit or impose substantial restrictions on foreign investments in their capital markets or in certain industries.

• Many foreign governments do not supervise and regulate stock exchanges, brokers and the sale of securities to the same extent as does the United States and may not have laws to protect investors that are comparable to U.S. securities laws.

• Settlement and clearance procedures in certain foreign markets may result in delays in payment for or delivery of securities not typically associated with settlement and clearance of U.S. investments.

• High Portfolio Turnover Risk — The Fund may engage in active and frequent trading of its portfolio securities. High portfolio turnover (more than 100%) may result in increased transaction costs to the Fund, including brokerage commissions, dealer mark-ups and other transaction costs on the sale of the securities and on reinvestment in other securities. The sale of Fund portfolio securities may result in the realization and/or distribution to shareholders of higher capital gains or losses as compared to a fund with less active trading policies. These effects of higher than normal portfolio turnover may adversely affect Fund performance.

• Inflation-Indexed Bonds Risk — The principal value of an investment is not protected or otherwise guaranteed by virtue of the Fund’s investments in inflation-indexed bonds.
  
  Inflation-indexed bonds are fixed-income securities whose principal value is periodically adjusted according to the rate of inflation. If the index measuring inflation falls, the principal value of inflation-indexed bonds will be adjusted downward, and consequently the interest payable on these securities (calculated with respect to a smaller principal amount) will be reduced.
  
  Repayment of the original bond principal upon maturity (as adjusted for inflation) is guaranteed in the case of U.S. Treasury inflation-indexed bonds. For bonds that do not provide a similar guarantee, the adjusted principal value of the bond repaid at maturity may be less than the original principal value.
  
  The value of inflation-indexed bonds is expected to change in response to changes in real interest rates. Real interest rates are tied to the relationship between nominal interest rates and the rate of inflation. If nominal interest rates increase at a faster rate than inflation, real interest rates may rise, leading to a decrease in value of inflation-indexed bonds. Short-term increases in inflation may lead to a decline in value. Any increase in the principal amount of an inflation-indexed bond will be considered taxable ordinary income, even though investors do not receive their principal until maturity.
  
  Periodic adjustments for inflation to the principal amount of an inflation-indexed bond may give rise to original issue discount, which will be includable in the Fund’s gross income. Due to original issue discount, the Fund may be required to make annual distributions to shareholders that exceed the cash received, which may cause the Fund to liquidate certain investments when it is not advantageous to do so. Also, if the principal value of an inflation-indexed bond is adjusted downward due to deflation, amounts previously distributed in the taxable year may be characterized in some circumstances as a return of capital.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund’s portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund’s investments in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund’s principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquidity risk may be the result of, among other things, the reduced number and capacity of traditional market participants to make a market in fixed-income securities or the lack of an active market. Liquid investments may become illiquid or less liquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid and relatively less liquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. This may be magnified in a rising interest rate environment or other circumstances where investor redemptions from fixed-income mutual funds may be higher than normal. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Mortgage- and Asset-Backed Securities Risks — Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities.

• Non-Investment Grade Securities Risk — Although non-investment grade securities generally pay higher rates of interest than investment grade bonds, non-investment grade securities are high risk investments that are considered speculative and may cause income and principal losses for the Fund.

• Repurchase Agreements and Purchase and Sale Contracts Risk — If the other party to a repurchase agreement or purchase and sale contract defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security in either situation and the market value of the security declines, the Fund may lose money.

• Reverse Repurchase Agreements Risk — Reverse repurchase agreements involve the sale of securities held by the Fund with an agreement to repurchase the securities at an agreed-upon price, date and interest payment. Reverse repurchase agreements involve the risk that the other party may fail to return the securities in a timely manner or at all. The Fund could lose money if it is unable to recover the securities and the value of the collateral held by the Fund, including the value of the investments made with cash collateral, is less than the value of the securities. These events could also trigger adverse tax consequences to the Fund.

• Subsidiary Risk — By investing in the Subsidiary, the Fund is indirectly exposed to the risks associated with the Subsidiary’s investments. The commodity-related instruments held by the Subsidiary are generally similar to those that are permitted to be held by the Fund and are subject to the same risks that apply to similar investments if held directly by the Fund (see “Commodities Related Investments Risks” above). There can be no assurance that the investment objective of the Subsidiary will be achieved. The Subsidiary is not registered under the Investment Company Act of 1940, as amended (the “Investment Company Act”), and, unless otherwise noted in this prospectus, is not subject to all the investor protections of the Investment Company Act. However, the Fund wholly owns and controls the Subsidiary, and the Fund and the Subsidiary are both managed by BlackRock, making it unlikely that the Subsidiary will take action contrary to the interests of the Fund and its shareholders. The Board has oversight responsibility for the investment activities of the Fund, including its investment in the Subsidiary, and the Fund’s role as sole shareholder of the Subsidiary. The Subsidiary is subject to the same investment restrictions and limitations, and follows the same compliance policies and procedures, as the Fund. Changes in the laws of the United States and/or the Cayman Islands could result in the inability of the Fund and/or the Subsidiary to operate as described in this prospectus and the Statement of Additional Information and could adversely affect the Fund.
  
  In order to qualify as a regulated investment company, at least 90% of the Fund’s gross income for the taxable year must be “qualifying income.” The Fund anticipates treating the income and gain generated from investments in controlled foreign subsidiaries that invest in physical commodities and/or commodity-linked derivative instruments as “qualifying income” for regulated investment company qualification purposes. However, there can be no assurance that the Internal Revenue Service (“IRS”) will agree with treating such income and gain as “qualifying income.” If the IRS makes an adverse determination relating to the treatment of such income and gain, the Fund would likely need to change its investment strategies, which could adversely affect the Fund. The IRS has proposed regulations that, if finalized in current form, would specify that a subpart F income inclusion for U.S. federal income tax purposes will be treated as qualifying income only to the extent that the Subsidiary makes distributions out of its earnings and profits in the same taxable year.

• U.S. Government Issuer Risk — Treasury obligations may differ in their interest rates, maturities, times of issuance and other characteristics. Obligations of U.S. Government agencies and authorities are supported by varying degrees of credit but generally are not backed by the full faith and credit of the U.S. Government. No assurance can be given that the U.S. Government will provide financial support to its agencies and authorities if it is not obligated by law to do so.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com or can be obtained by phone at 800-882-0052.

Investor A Shares
ANNUAL TOTAL RETURNS
Inflation Protected Bond Portfolio
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 5.80% (quarter ended March 31, 2008) and the lowest return for a quarter was -6.63% (quarter ended June 30, 2013).

As of 12/31/17
Average Annual Total Returns

Average Annual Total Returns - Investor A, C and Institutional - BLACKROCK INFLATION PROTECTED BOND PORTFOLIO	1 Year	5 Years	10 Years
Investor A Shares	(1.36%)	(1.29%)	2.67%
Investor A Shares | Return After Taxes on Distributions	(2.23%)	(2.05%)	1.62%
Investor A Shares | Return After Taxes on Distributions and Sale of Fund Shares	(0.78%)	(1.29%)	1.69%
Investor C Shares	0.97%	(1.20%)	2.34%
Institutional Shares	3.09%	(0.18%)	3.40%
Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index (Reflects no deduction for fees, expenses or taxes)	3.01%	0.13%	3.53%

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns are shown for Investor A Shares only, and the after-tax returns for Investor C and Institutional Shares will vary.

[1] A contingent deferred sales charge (“CDSC”) of 0.15% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

[2] There is no CDSC on Investor C Shares after one year.

[1] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 40, BlackRock has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through April 30, 2020. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of BlackRock Funds II (the “Trust”) or by a vote of a majority of the outstanding voting securities of the Fund.

[2] The Management Fee has been restated to reflect current fees.

[3] Other Expenses of Cayman Inflation Protected Bond Portfolio, Ltd. (the “Subsidiary”) were less than 0.01% for the most recent fiscal year.

[4] The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current expense.

[5] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 40, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.59% (for Investor A Shares), 1.34% (for Investor C Shares) and 0.34% (for Institutional Shares) of average daily net assets through April 30, 2020. The Fund may have to repay some of these waivers and/or reimbursements to BlackRock in the two years following such waivers and/or reimbursements. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BlackRock Funds II
Prospectus Date	rr_ProspectusDate	Apr. 30, 2018
Investor A, C and Institutional | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock Inflation Protected Bond Portfolio
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of the BlackRock Inflation Protected Bond Portfolio (the “Inflation Protected Bond Portfolio” or the “Fund”) is to seek to maximize real return, consistent with preservation of real capital and prudent investment management.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $25,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates. More information about these and other discounts is available from your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock, The PNC Financial Services Group, Inc. (“PNC”) and their respective affiliates) (each a “Financial Intermediary”) and in the “Details About the Share Classes” and the “Intermediary-Defined Sales Charge Waiver Policies” sections on pages 25 and A-1, respectively, of the Fund’s prospectus and in the “Purchase of Shares” section on page II-74 of Part II of the Fund’s Statement of Additional Information.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	April 30, 2020
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 76% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	76.00%
Expenses Deferred Charges [Text Block]	rr_ExpensesDeferredChargesTextBlock	A contingent deferred sales charge (“CDSC”) of 0.15% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.
Expense Breakpoint Discounts [Text]	rr_ExpenseBreakpointDiscounts	You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $25,000 in the fund complex advised by BlackRock Advisors, LLC (“BlackRock”) or its affiliates.
Expense Breakpoint, Minimum Investment Required [Amount]	rr_ExpenseBreakpointMinimumInvestmentRequiredAmount	$ 25,000
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	The Management Fee has been restated to reflect current fees.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example, No Redemption, By Year, Caption [Text]	rr_ExpenseExampleNoRedemptionByYearCaption	You would pay the following expenses if you did not redeem your shares:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 80% of its assets in inflation-indexed bonds of varying maturities issued by the U.S. and non-U.S. governments, their agencies or instrumentalities, and U.S. and non-U.S. corporations. The Fund maintains an average portfolio duration that is within ±20% of the duration of the Barclays U.S. Treasury Inflation Protected Securities Index (the benchmark). The Fund may invest up to 20% of its assets in non-investment grade bonds (high yield or junk bonds) or securities of emerging market issuers. The Fund may also invest up to 20% of its assets in non-dollar denominated securities of non-U.S. issuers, and may invest without limit in U.S. dollar denominated securities of non-U.S. issuers. The Fund may seek to provide exposure to the investment returns of real assets that trade in the commodity markets through investment in commodity-linked derivative instruments and investment vehicles such as exchange traded funds that exclusively invest in commodities and are designed to provide this exposure without direct investment in physical commodities. The Fund may also gain exposure to commodity markets by investing up to 25% of its total assets in the Subsidiary, a wholly owned subsidiary of the Fund formed in the Cayman Islands, which invests primarily in commodity-related instruments. The Fund also makes investments in residential and commercial mortgage-backed securities and other asset-backed securities. Non-investment grade bonds acquired by the Fund will generally be in the lower rating categories of the major rating agencies (BB or lower by S&P Global Ratings (“S&P”) or Ba or lower by Moody’s Investors Service, Inc. (“Moody’s”)) or will be determined by the management team to be of similar quality. Split rated bonds will be considered to have the higher credit rating. Split rated bonds are bonds that receive different ratings from two or more rating agencies. The Fund may buy or sell options or futures, or enter into credit default swaps and interest rate or foreign currency transactions, including swaps (collectively, commonly known as derivatives). The Fund may seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques (such as reverse repurchase agreements or dollar rolls). The Fund may engage in active and frequent trading of portfolio securities to achieve its primary investment strategies.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Commodities Related Investments Risks — Exposure to the commodities markets may subject the Fund to greater volatility than investments in traditional securities. The value of commodity-linked derivative investments may be affected by changes in overall market movements, commodity index volatility, changes in interest rates, or factors affecting a particular industry or commodity, such as drought, floods, weather, embargoes, tariffs and international economic, political and regulatory developments. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Deflation Risk — Deflation risk is the possibility that prices throughout the economy decline over time — the opposite of inflation. If inflation is negative, the principal and income of an inflation-protected bond will decline and could result in losses for the Fund. Derivatives Risk — The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including: Volatility Risk — Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets. Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. Market and Liquidity Risk — The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation Risk — Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Hedging Risk — Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences. Tax Risk — Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments, are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority. Such treatment may be less favorable than that given to a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments. Regulatory Risk — Derivative contracts, including, without limitation, swaps, currency forwards and non-deliverable forwards, are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (“Dodd Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, certain derivatives are subject to margin requirements and swap dealers are required to collect margin from the Fund with respect to such derivatives. Specifically, regulations are now in effect that require swap dealers to post and collect variation margin (comprised of specified liquid instruments and subject to a required haircut) in connection with trading of over-the-counter (“OTC”) swaps with the Fund. Shares of investment companies (other than certain money market funds) may not be posted as collateral under these regulations. Requirements for posting of initial margin in connection with OTC swaps will be phased-in through 2020. In addition, regulations adopted by prudential regulators that will begin to take effect in 2019 will require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund. Dollar Rolls Risk — Dollar rolls involve the risk that the market value of the securities that the Fund is committed to buy may decline below the price of the securities the Fund has sold. These transactions may involve leverage. Emerging Markets Risk — Emerging markets are riskier than more developed markets because they tend to develop unevenly and may never fully develop. Investments in emerging markets may be considered speculative. Emerging markets are more likely to experience hyperinflation and currency devaluations, which adversely affect returns to U.S. investors. In addition, many emerging securities markets have far lower trading volumes and less liquidity than developed markets. Foreign Securities Risk — Foreign investments often involve special risks not present in U.S. investments that can increase the chances that the Fund will lose money. These risks include: The Fund generally holds its foreign securities and cash in foreign banks and securities depositories, which may be recently organized or new to the foreign custody business and may be subject to only limited or no regulatory oversight. Changes in foreign currency exchange rates can affect the value of the Fund’s portfolio. The economies of certain foreign markets may not compare favorably with the economy of the United States with respect to such issues as growth of gross national product, reinvestment of capital, resources and balance of payments position. The governments of certain countries may prohibit or impose substantial restrictions on foreign investments in their capital markets or in certain industries. Many foreign governments do not supervise and regulate stock exchanges, brokers and the sale of securities to the same extent as does the United States and may not have laws to protect investors that are comparable to U.S. securities laws. Settlement and clearance procedures in certain foreign markets may result in delays in payment for or delivery of securities not typically associated with settlement and clearance of U.S. investments. High Portfolio Turnover Risk — The Fund may engage in active and frequent trading of its portfolio securities. High portfolio turnover (more than 100%) may result in increased transaction costs to the Fund, including brokerage commissions, dealer mark-ups and other transaction costs on the sale of the securities and on reinvestment in other securities. The sale of Fund portfolio securities may result in the realization and/or distribution to shareholders of higher capital gains or losses as compared to a fund with less active trading policies. These effects of higher than normal portfolio turnover may adversely affect Fund performance. Inflation-Indexed Bonds Risk — The principal value of an investment is not protected or otherwise guaranteed by virtue of the Fund’s investments in inflation-indexed bonds. Inflation-indexed bonds are fixed-income securities whose principal value is periodically adjusted according to the rate of inflation. If the index measuring inflation falls, the principal value of inflation-indexed bonds will be adjusted downward, and consequently the interest payable on these securities (calculated with respect to a smaller principal amount) will be reduced. Repayment of the original bond principal upon maturity (as adjusted for inflation) is guaranteed in the case of U.S. Treasury inflation-indexed bonds. For bonds that do not provide a similar guarantee, the adjusted principal value of the bond repaid at maturity may be less than the original principal value. The value of inflation-indexed bonds is expected to change in response to changes in real interest rates. Real interest rates are tied to the relationship between nominal interest rates and the rate of inflation. If nominal interest rates increase at a faster rate than inflation, real interest rates may rise, leading to a decrease in value of inflation-indexed bonds. Short-term increases in inflation may lead to a decline in value. Any increase in the principal amount of an inflation-indexed bond will be considered taxable ordinary income, even though investors do not receive their principal until maturity. Periodic adjustments for inflation to the principal amount of an inflation-indexed bond may give rise to original issue discount, which will be includable in the Fund’s gross income. Due to original issue discount, the Fund may be required to make annual distributions to shareholders that exceed the cash received, which may cause the Fund to liquidate certain investments when it is not advantageous to do so. Also, if the principal value of an inflation-indexed bond is adjusted downward due to deflation, amounts previously distributed in the taxable year may be characterized in some circumstances as a return of capital. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund’s portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund’s investments in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund’s principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquidity risk may be the result of, among other things, the reduced number and capacity of traditional market participants to make a market in fixed-income securities or the lack of an active market. Liquid investments may become illiquid or less liquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid and relatively less liquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. This may be magnified in a rising interest rate environment or other circumstances where investor redemptions from fixed-income mutual funds may be higher than normal. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Mortgage- and Asset-Backed Securities Risks — Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities. Non-Investment Grade Securities Risk — Although non-investment grade securities generally pay higher rates of interest than investment grade bonds, non-investment grade securities are high risk investments that are considered speculative and may cause income and principal losses for the Fund. Repurchase Agreements and Purchase and Sale Contracts Risk — If the other party to a repurchase agreement or purchase and sale contract defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security in either situation and the market value of the security declines, the Fund may lose money. Reverse Repurchase Agreements Risk — Reverse repurchase agreements involve the sale of securities held by the Fund with an agreement to repurchase the securities at an agreed-upon price, date and interest payment. Reverse repurchase agreements involve the risk that the other party may fail to return the securities in a timely manner or at all. The Fund could lose money if it is unable to recover the securities and the value of the collateral held by the Fund, including the value of the investments made with cash collateral, is less than the value of the securities. These events could also trigger adverse tax consequences to the Fund. Subsidiary Risk — By investing in the Subsidiary, the Fund is indirectly exposed to the risks associated with the Subsidiary’s investments. The commodity-related instruments held by the Subsidiary are generally similar to those that are permitted to be held by the Fund and are subject to the same risks that apply to similar investments if held directly by the Fund (see “Commodities Related Investments Risks” above). There can be no assurance that the investment objective of the Subsidiary will be achieved. The Subsidiary is not registered under the Investment Company Act of 1940, as amended (the “Investment Company Act”), and, unless otherwise noted in this prospectus, is not subject to all the investor protections of the Investment Company Act. However, the Fund wholly owns and controls the Subsidiary, and the Fund and the Subsidiary are both managed by BlackRock, making it unlikely that the Subsidiary will take action contrary to the interests of the Fund and its shareholders. The Board has oversight responsibility for the investment activities of the Fund, including its investment in the Subsidiary, and the Fund’s role as sole shareholder of the Subsidiary. The Subsidiary is subject to the same investment restrictions and limitations, and follows the same compliance policies and procedures, as the Fund. Changes in the laws of the United States and/or the Cayman Islands could result in the inability of the Fund and/or the Subsidiary to operate as described in this prospectus and the Statement of Additional Information and could adversely affect the Fund. In order to qualify as a regulated investment company, at least 90% of the Fund’s gross income for the taxable year must be “qualifying income.” The Fund anticipates treating the income and gain generated from investments in controlled foreign subsidiaries that invest in physical commodities and/or commodity-linked derivative instruments as “qualifying income” for regulated investment company qualification purposes. However, there can be no assurance that the Internal Revenue Service (“IRS”) will agree with treating such income and gain as “qualifying income.” If the IRS makes an adverse determination relating to the treatment of such income and gain, the Fund would likely need to change its investment strategies, which could adversely affect the Fund. The IRS has proposed regulations that, if finalized in current form, would specify that a subpart F income inclusion for U.S. federal income tax purposes will be treated as qualifying income only to the extent that the Subsidiary makes distributions out of its earnings and profits in the same taxable year. U.S. Government Issuer Risk — Treasury obligations may differ in their interest rates, maturities, times of issuance and other characteristics. Obligations of U.S. Government agencies and authorities are supported by varying degrees of credit but generally are not backed by the full faith and credit of the U.S. Government. No assurance can be given that the U.S. Government will provide financial support to its agencies and authorities if it is not obligated by law to do so.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com or can be obtained by phone at 800-882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Investor A Shares ANNUAL TOTAL RETURNS Inflation Protected Bond Portfolio As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 5.80% (quarter ended March 31, 2008) and the lowest return for a quarter was -6.63% (quarter ended June 30, 2013).
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/17 Average Annual Total Returns
Performance Table Does Reflect Sales Loads	rr_PerformanceTableDoesReflectSalesLoads	However, the table includes all applicable fees and sales charges.
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table One Class of after Tax Shown [Text]	rr_PerformanceTableOneClassOfAfterTaxShown	After-tax returns are shown for Investor A Shares only, and the after-tax returns for Investor C and Institutional Shares will vary.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts. After-tax returns are shown for Investor A Shares only, and the after-tax returns for Investor C and Institutional Shares will vary.
Investor A, C and Institutional | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	4.00%
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none	[1]
Management Fee	rr_ManagementFeesOverAssets	0.24%	[2],[3]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.05%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.38%
Other Expenses of the Subsidiary	rr_Component3OtherExpensesOverAssets		[4]
Other Expenses	rr_OtherExpensesOverAssets	0.43%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.92%	[5]
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.28%)	[2],[6]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.64%	[2],[6]
1 Year	rr_ExpenseExampleYear01	$ 463
3 Years	rr_ExpenseExampleYear03	626
5 Years	rr_ExpenseExampleYear05	835
10 Years	rr_ExpenseExampleYear10	$ 1,435
2008	rr_AnnualReturn2008	0.23%
2009	rr_AnnualReturn2009	10.15%
2010	rr_AnnualReturn2010	5.82%
2011	rr_AnnualReturn2011	11.47%
2012	rr_AnnualReturn2012	6.62%
2013	rr_AnnualReturn2013	(8.34%)
2014	rr_AnnualReturn2014	2.27%
2015	rr_AnnualReturn2015	(2.43%)
2016	rr_AnnualReturn2016	3.89%
2017	rr_AnnualReturn2017	2.75%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	5.80%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2013
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(6.63%)
1 Year	rr_AverageAnnualReturnYear01	(1.36%)
5 Years	rr_AverageAnnualReturnYear05	(1.29%)
10 Years	rr_AverageAnnualReturnYear10	2.67%
Investor A, C and Institutional | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Investor C Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	1.00%	[7]
Management Fee	rr_ManagementFeesOverAssets	0.24%	[2],[3]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.05%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.26%
Other Expenses of the Subsidiary	rr_Component3OtherExpensesOverAssets		[4]
Other Expenses	rr_OtherExpensesOverAssets	0.31%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.55%	[5]
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.16%)	[2],[6]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	1.39%	[2],[6]
1 Year	rr_ExpenseExampleYear01	$ 242
3 Years	rr_ExpenseExampleYear03	457
5 Years	rr_ExpenseExampleYear05	814
10 Years	rr_ExpenseExampleYear10	1,818
1 Year	rr_ExpenseExampleNoRedemptionYear01	142
3 Years	rr_ExpenseExampleNoRedemptionYear03	457
5 Years	rr_ExpenseExampleNoRedemptionYear05	814
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,818
1 Year	rr_AverageAnnualReturnYear01	0.97%
5 Years	rr_AverageAnnualReturnYear05	(1.20%)
10 Years	rr_AverageAnnualReturnYear10	2.34%
Investor A, C and Institutional | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price)	rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice	none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower)	rr_MaximumDeferredSalesChargeOverOfferingPrice	none
Management Fee	rr_ManagementFeesOverAssets	0.24%	[2],[3]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Interest Expense	rr_Component1OtherExpensesOverAssets	0.05%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.24%
Other Expenses of the Subsidiary	rr_Component3OtherExpensesOverAssets		[4]
Other Expenses	rr_OtherExpensesOverAssets	0.29%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.53%	[5]
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.14%)	[2],[6]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.39%	[2],[6]
1 Year	rr_ExpenseExampleYear01	$ 40
3 Years	rr_ExpenseExampleYear03	141
5 Years	rr_ExpenseExampleYear05	268
10 Years	rr_ExpenseExampleYear10	$ 637
1 Year	rr_AverageAnnualReturnYear01	3.09%
5 Years	rr_AverageAnnualReturnYear05	(0.18%)
10 Years	rr_AverageAnnualReturnYear10	3.40%
Investor A, C and Institutional | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Return After Taxes on Distributions | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.23%)
5 Years	rr_AverageAnnualReturnYear05	(2.05%)
10 Years	rr_AverageAnnualReturnYear10	1.62%
Investor A, C and Institutional | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Return After Taxes on Distributions and Sale of Fund Shares | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(0.78%)
5 Years	rr_AverageAnnualReturnYear05	(1.29%)
10 Years	rr_AverageAnnualReturnYear10	1.69%
Investor A, C and Institutional | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	3.01%
5 Years	rr_AverageAnnualReturnYear05	0.13%
10 Years	rr_AverageAnnualReturnYear10	3.53%

[1] A contingent deferred sales charge (“CDSC”) of 0.15% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at the time of purchase as part of an investment of $1,000,000 or more.

[2] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 40, BlackRock has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through April 30, 2020. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of BlackRock Funds II (the “Trust”) or by a vote of a majority of the outstanding voting securities of the Fund.

[3] The Management Fee has been restated to reflect current fees.

[4] Other Expenses of Cayman Inflation Protected Bond Portfolio, Ltd. (the “Subsidiary”) were less than 0.01% for the most recent fiscal year.

[5] The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current expense.

[6] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 40, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.59% (for Investor A Shares), 1.34% (for Investor C Shares) and 0.34% (for Institutional Shares) of average daily net assets through April 30, 2020. The Fund may have to repay some of these waivers and/or reimbursements to BlackRock in the two years following such waivers and/or reimbursements. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

[7] There is no CDSC on Investor C Shares after one year.

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Class K | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO

Fund Overview

Key Facts About BlackRock Inflation Protected Bond Portfolio

Investment Objective

The investment objective of the BlackRock Inflation Protected Bond Portfolio (the “Inflation Protected Bond Portfolio” or the “Fund”) is to seek to maximize real return, consistent with preservation of real capital and prudent investment management.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold Class K Shares of the Fund.

Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses	Class K BLACKROCK INFLATION PROTECTED BOND PORTFOLIO Class K Shares
Management Fee	0.24%	[1],[2]
Distribution and/or Service (12b-1) Fees	none
Other Expenses	0.16%
Interest Expense	0.05%
Miscellaneous Other Expenses	0.11%
Other Expenses of the Subsidiary		[3]
Total Annual Fund Operating Expenses	0.40%	[4]
Fee Waivers and/or Expense Reimbursements	(0.06%)	[1],[5]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	0.34%	[1],[5]

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
Class K | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Class K Shares | USD ($)	35	116	212	493

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 76% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the Fund invests at least 80% of its assets in inflation-indexed bonds of varying maturities issued by the U.S. and non-U.S. governments, their agencies or instrumentalities, and U.S. and non-U.S. corporations.

The Fund maintains an average portfolio duration that is within ±20% of the duration of the Barclays U.S. Treasury Inflation Protected Securities Index (the benchmark).

The Fund may invest up to 20% of its assets in non-investment grade bonds (high yield or junk bonds) or securities of emerging market issuers. The Fund may also invest up to 20% of its assets in non-dollar denominated securities of non-U.S. issuers, and may invest without limit in U.S. dollar denominated securities of non-U.S. issuers.

The Fund may seek to provide exposure to the investment returns of real assets that trade in the commodity markets through investment in commodity-linked derivative instruments and investment vehicles such as exchange traded funds that exclusively invest in commodities and are designed to provide this exposure without direct investment in physical commodities. The Fund may also gain exposure to commodity markets by investing up to 25% of its total assets in the Subsidiary, a wholly owned subsidiary of the Fund formed in the Cayman Islands, which invests primarily in commodity-related instruments.

The Fund also makes investments in residential and commercial mortgage-backed securities and other asset-backed securities.

Non-investment grade bonds acquired by the Fund will generally be in the lower rating categories of the major rating agencies (BB or lower by S&P Global Ratings (“S&P”) or Ba or lower by Moody’s Investors Service, Inc. (“Moody’s”)) or will be determined by the management team to be of similar quality. Split rated bonds will be considered to have the higher credit rating. Split rated bonds are bonds that receive different ratings from two or more rating agencies.

The Fund may buy or sell options or futures, or enter into credit default swaps and interest rate or foreign currency transactions, including swaps (collectively, commonly known as derivatives). The Fund may seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques (such as reverse repurchase agreements or dollar rolls).

The Fund may engage in active and frequent trading of portfolio securities to achieve its primary investment strategies.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Commodities Related Investments Risks — Exposure to the commodities markets may subject the Fund to greater volatility than investments in traditional securities. The value of commodity-linked derivative investments may be affected by changes in overall market movements, commodity index volatility, changes in interest rates, or factors affecting a particular industry or commodity, such as drought, floods, weather, embargoes, tariffs and international economic, political and regulatory developments.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.
  
  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Deflation Risk — Deflation risk is the possibility that prices throughout the economy decline over time — the opposite of inflation. If inflation is negative, the principal and income of an inflation-protected bond will decline and could result in losses for the Fund.

• Derivatives Risk — The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including:
  
  Volatility Risk — Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets.
  
  Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation.
  
  Market and Liquidity Risk — The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately.
  
  Valuation Risk — Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them.
  
  Hedging Risk — Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences.
  
  Tax Risk — Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments, are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority. Such treatment may be less favorable than that given to a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments.
  
  Regulatory Risk — Derivative contracts, including, without limitation, swaps, currency forwards and non-deliverable forwards, are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (“Dodd Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, certain derivatives are subject to margin requirements and swap dealers are required to collect margin from the Fund with respect to such derivatives. Specifically, regulations are now in effect that require swap dealers to post and collect variation margin (comprised of specified liquid instruments and subject to a required haircut) in connection with trading of over-the-counter (“OTC”) swaps with the Fund. Shares of investment companies (other than certain money market funds) may not be posted as collateral under these regulations. Requirements for posting of initial margin in connection with OTC swaps will be phased-in through 2020. In addition, regulations adopted by prudential regulators that will begin to take effect in 2019 will require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund.

• Dollar Rolls Risk — Dollar rolls involve the risk that the market value of the securities that the Fund is committed to buy may decline below the price of the securities the Fund has sold. These transactions may involve leverage.

• Emerging Markets Risk — Emerging markets are riskier than more developed markets because they tend to develop unevenly and may never fully develop. Investments in emerging markets may be considered speculative. Emerging markets are more likely to experience hyperinflation and currency devaluations, which adversely affect returns to U.S. investors. In addition, many emerging securities markets have far lower trading volumes and less liquidity than developed markets.

• Foreign Securities Risk — Foreign investments often involve special risks not present in U.S. investments that can increase the chances that the Fund will lose money. These risks include:

• The Fund generally holds its foreign securities and cash in foreign banks and securities depositories, which may be recently organized or new to the foreign custody business and may be subject to only limited or no regulatory oversight.

• Changes in foreign currency exchange rates can affect the value of the Fund’s portfolio.

• The economies of certain foreign markets may not compare favorably with the economy of the United States with respect to such issues as growth of gross national product, reinvestment of capital, resources and balance of payments position.

• The governments of certain countries may prohibit or impose substantial restrictions on foreign investments in their capital markets or in certain industries.

• Many foreign governments do not supervise and regulate stock exchanges, brokers and the sale of securities to the same extent as does the United States and may not have laws to protect investors that are comparable to U.S. securities laws.

• Settlement and clearance procedures in certain foreign markets may result in delays in payment for or delivery of securities not typically associated with settlement and clearance of U.S. investments.

• High Portfolio Turnover Risk — The Fund may engage in active and frequent trading of its portfolio securities. High portfolio turnover (more than 100%) may result in increased transaction costs to the Fund, including brokerage commissions, dealer mark-ups and other transaction costs on the sale of the securities and on reinvestment in other securities. The sale of Fund portfolio securities may result in the realization and/or distribution to shareholders of higher capital gains or losses as compared to a fund with less active trading policies. These effects of higher than normal portfolio turnover may adversely affect Fund performance.

• Inflation-Indexed Bonds Risk — The principal value of an investment is not protected or otherwise guaranteed by virtue of the Fund’s investments in inflation-indexed bonds.
  
  Inflation-indexed bonds are fixed-income securities whose principal value is periodically adjusted according to the rate of inflation. If the index measuring inflation falls, the principal value of inflation-indexed bonds will be adjusted downward, and consequently the interest payable on these securities (calculated with respect to a smaller principal amount) will be reduced.
  
  Repayment of the original bond principal upon maturity (as adjusted for inflation) is guaranteed in the case of U.S. Treasury inflation-indexed bonds. For bonds that do not provide a similar guarantee, the adjusted principal value of the bond repaid at maturity may be less than the original principal value.
  
  The value of inflation-indexed bonds is expected to change in response to changes in real interest rates. Real interest rates are tied to the relationship between nominal interest rates and the rate of inflation. If nominal interest rates increase at a faster rate than inflation, real interest rates may rise, leading to a decrease in value of inflation-indexed bonds. Short-term increases in inflation may lead to a decline in value. Any increase in the principal amount of an inflation-indexed bond will be considered taxable ordinary income, even though investors do not receive their principal until maturity.
  
  Periodic adjustments for inflation to the principal amount of an inflation-indexed bond may give rise to original issue discount, which will be includable in the Fund’s gross income. Due to original issue discount, the Fund may be required to make annual distributions to shareholders that exceed the cash received, which may cause the Fund to liquidate certain investments when it is not advantageous to do so. Also, if the principal value of an inflation-indexed bond is adjusted downward due to deflation, amounts previously distributed in the taxable year may be characterized in some circumstances as a return of capital.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund’s portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund’s investments in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund’s principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquidity risk may be the result of, among other things, the reduced number and capacity of traditional market participants to make a market in fixed-income securities or the lack of an active market. Liquid investments may become illiquid or less liquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid and relatively less liquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. This may be magnified in a rising interest rate environment or other circumstances where investor redemptions from fixed-income mutual funds may be higher than normal. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Mortgage- and Asset-Backed Securities Risks — Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities.

• Non-Investment Grade Securities Risk — Although non-investment grade securities generally pay higher rates of interest than investment grade bonds, non-investment grade securities are high risk investments that are considered speculative and may cause income and principal losses for the Fund.

• Repurchase Agreements and Purchase and Sale Contracts Risk — If the other party to a repurchase agreement or purchase and sale contract defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security in either situation and the market value of the security declines, the Fund may lose money.

• Reverse Repurchase Agreements Risk — Reverse repurchase agreements involve the sale of securities held by the Fund with an agreement to repurchase the securities at an agreed-upon price, date and interest payment. Reverse repurchase agreements involve the risk that the other party may fail to return the securities in a timely manner or at all. The Fund could lose money if it is unable to recover the securities and the value of the collateral held by the Fund, including the value of the investments made with cash collateral, is less than the value of the securities. These events could also trigger adverse tax consequences to the Fund.

• Subsidiary Risk — By investing in the Subsidiary, the Fund is indirectly exposed to the risks associated with the Subsidiary’s investments. The commodity-related instruments held by the Subsidiary are generally similar to those that are permitted to be held by the Fund and are subject to the same risks that apply to similar investments if held directly by the Fund (see “Commodities Related Investments Risks” above). There can be no assurance that the investment objective of the Subsidiary will be achieved. The Subsidiary is not registered under the Investment Company Act of 1940, as amended (the “Investment Company Act”), and, unless otherwise noted in this prospectus, is not subject to all the investor protections of the Investment Company Act. However, the Fund wholly owns and controls the Subsidiary, and the Fund and the Subsidiary are both managed by BlackRock, making it unlikely that the Subsidiary will take action contrary to the interests of the Fund and its shareholders. The Board has oversight responsibility for the investment activities of the Fund, including its investment in the Subsidiary, and the Fund’s role as sole shareholder of the Subsidiary. The Subsidiary is subject to the same investment restrictions and limitations, and follows the same compliance policies and procedures, as the Fund. Changes in the laws of the United States and/or the Cayman Islands could result in the inability of the Fund and/or the Subsidiary to operate as described in this prospectus and the Statement of Additional Information and could adversely affect the Fund.
  
  In order to qualify as a regulated investment company, at least 90% of the Fund’s gross income for the taxable year must be “qualifying income.” The Fund anticipates treating the income and gain generated from investments in controlled foreign subsidiaries that invest in physical commodities and/or commodity-linked derivative instruments as “qualifying income” for regulated investment company qualification purposes. However, there can be no assurance that the Internal Revenue Service (“IRS”) will agree with treating such income and gain as “qualifying income.” If the IRS makes an adverse determination relating to the treatment of such income and gain, the Fund would likely need to change its investment strategies, which could adversely affect the Fund. The IRS has proposed regulations that, if finalized in current form, would specify that a subpart F income inclusion for U.S. federal income tax purposes will be treated as qualifying income only to the extent that the Subsidiary makes distributions out of its earnings and profits in the same taxable year.

• U.S. Government Issuer Risk — Treasury obligations may differ in their interest rates, maturities, times of issuance and other characteristics. Obligations of U.S. Government agencies and authorities are supported by varying degrees of credit but generally are not backed by the full faith and credit of the U.S. Government. No assurance can be given that the U.S. Government will provide financial support to its agencies and authorities if it is not obligated by law to do so.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index. The returns for Class K Shares prior to September 1, 2015, the commencement of operations of Class K Shares, are based upon performance of the Fund’s BlackRock Shares. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not an indication of future results. The table includes all applicable fees. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com or can be obtained by phone at 800-882-0052.

Class K Shares
ANNUAL TOTAL RETURNS
Inflation Protected Bond Portfolio
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 5.87% (quarter ended March 31, 2008) and the lowest return for a quarter was -6.54% (quarter ended June 30, 2013).

As of 12/31/17
Average Annual Total Returns

Average Annual Total Returns - Class K - BLACKROCK INFLATION PROTECTED BOND PORTFOLIO	1 Year	5 Years	10 Years
Class K Shares	3.14%	(0.07%)	3.52%
Class K Shares | Return After Taxes on Distributions	2.15%	(0.88%)	2.39%
Class K Shares | Return After Taxes on Distributions and Sale of Fund Shares	1.77%	(0.40%)	2.33%
Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index (Reflects no deduction for fees, expenses or taxes)	3.01%	0.13%	3.53%

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

[1] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 30, BlackRock Advisors, LLC (“BlackRock”) has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through April 30, 2020. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of BlackRock Funds II (the “Trust”) or by a vote of a majority of the outstanding voting securities of the Fund.

[2] The Management Fee has been restated to reflect current fees.

[3] Other Expenses of Cayman Inflation Protected Bond Portfolio, Ltd. (the “Subsidiary”) were less than 0.01% for the most recent fiscal year.

[4] The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current expenses.

[5] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 30, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.29% of average daily net assets through April 30, 2020. The Fund may have to repay some of these waivers and/or reimbursements to BlackRock in the two years following such waivers and/or reimbursements. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BlackRock Funds II
Prospectus Date	rr_ProspectusDate	Apr. 30, 2018
Class K | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock Inflation Protected Bond Portfolio
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of the BlackRock Inflation Protected Bond Portfolio (the “Inflation Protected Bond Portfolio” or the “Fund”) is to seek to maximize real return, consistent with preservation of real capital and prudent investment management.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Class K Shares of the Fund.
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	April 30, 2020
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 76% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	76.00%
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	The Management Fee has been restated to reflect current fees.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 80% of its assets in inflation-indexed bonds of varying maturities issued by the U.S. and non-U.S. governments, their agencies or instrumentalities, and U.S. and non-U.S. corporations. The Fund maintains an average portfolio duration that is within ±20% of the duration of the Barclays U.S. Treasury Inflation Protected Securities Index (the benchmark). The Fund may invest up to 20% of its assets in non-investment grade bonds (high yield or junk bonds) or securities of emerging market issuers. The Fund may also invest up to 20% of its assets in non-dollar denominated securities of non-U.S. issuers, and may invest without limit in U.S. dollar denominated securities of non-U.S. issuers. The Fund may seek to provide exposure to the investment returns of real assets that trade in the commodity markets through investment in commodity-linked derivative instruments and investment vehicles such as exchange traded funds that exclusively invest in commodities and are designed to provide this exposure without direct investment in physical commodities. The Fund may also gain exposure to commodity markets by investing up to 25% of its total assets in the Subsidiary, a wholly owned subsidiary of the Fund formed in the Cayman Islands, which invests primarily in commodity-related instruments. The Fund also makes investments in residential and commercial mortgage-backed securities and other asset-backed securities. Non-investment grade bonds acquired by the Fund will generally be in the lower rating categories of the major rating agencies (BB or lower by S&P Global Ratings (“S&P”) or Ba or lower by Moody’s Investors Service, Inc. (“Moody’s”)) or will be determined by the management team to be of similar quality. Split rated bonds will be considered to have the higher credit rating. Split rated bonds are bonds that receive different ratings from two or more rating agencies. The Fund may buy or sell options or futures, or enter into credit default swaps and interest rate or foreign currency transactions, including swaps (collectively, commonly known as derivatives). The Fund may seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques (such as reverse repurchase agreements or dollar rolls). The Fund may engage in active and frequent trading of portfolio securities to achieve its primary investment strategies.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Commodities Related Investments Risks — Exposure to the commodities markets may subject the Fund to greater volatility than investments in traditional securities. The value of commodity-linked derivative investments may be affected by changes in overall market movements, commodity index volatility, changes in interest rates, or factors affecting a particular industry or commodity, such as drought, floods, weather, embargoes, tariffs and international economic, political and regulatory developments. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Deflation Risk — Deflation risk is the possibility that prices throughout the economy decline over time — the opposite of inflation. If inflation is negative, the principal and income of an inflation-protected bond will decline and could result in losses for the Fund. Derivatives Risk — The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including: Volatility Risk — Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets. Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. Market and Liquidity Risk — The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation Risk — Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Hedging Risk — Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences. Tax Risk — Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments, are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority. Such treatment may be less favorable than that given to a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments. Regulatory Risk — Derivative contracts, including, without limitation, swaps, currency forwards and non-deliverable forwards, are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (“Dodd Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, certain derivatives are subject to margin requirements and swap dealers are required to collect margin from the Fund with respect to such derivatives. Specifically, regulations are now in effect that require swap dealers to post and collect variation margin (comprised of specified liquid instruments and subject to a required haircut) in connection with trading of over-the-counter (“OTC”) swaps with the Fund. Shares of investment companies (other than certain money market funds) may not be posted as collateral under these regulations. Requirements for posting of initial margin in connection with OTC swaps will be phased-in through 2020. In addition, regulations adopted by prudential regulators that will begin to take effect in 2019 will require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund. Dollar Rolls Risk — Dollar rolls involve the risk that the market value of the securities that the Fund is committed to buy may decline below the price of the securities the Fund has sold. These transactions may involve leverage. Emerging Markets Risk — Emerging markets are riskier than more developed markets because they tend to develop unevenly and may never fully develop. Investments in emerging markets may be considered speculative. Emerging markets are more likely to experience hyperinflation and currency devaluations, which adversely affect returns to U.S. investors. In addition, many emerging securities markets have far lower trading volumes and less liquidity than developed markets. Foreign Securities Risk — Foreign investments often involve special risks not present in U.S. investments that can increase the chances that the Fund will lose money. These risks include: The Fund generally holds its foreign securities and cash in foreign banks and securities depositories, which may be recently organized or new to the foreign custody business and may be subject to only limited or no regulatory oversight. Changes in foreign currency exchange rates can affect the value of the Fund’s portfolio. The economies of certain foreign markets may not compare favorably with the economy of the United States with respect to such issues as growth of gross national product, reinvestment of capital, resources and balance of payments position. The governments of certain countries may prohibit or impose substantial restrictions on foreign investments in their capital markets or in certain industries. Many foreign governments do not supervise and regulate stock exchanges, brokers and the sale of securities to the same extent as does the United States and may not have laws to protect investors that are comparable to U.S. securities laws. Settlement and clearance procedures in certain foreign markets may result in delays in payment for or delivery of securities not typically associated with settlement and clearance of U.S. investments. High Portfolio Turnover Risk — The Fund may engage in active and frequent trading of its portfolio securities. High portfolio turnover (more than 100%) may result in increased transaction costs to the Fund, including brokerage commissions, dealer mark-ups and other transaction costs on the sale of the securities and on reinvestment in other securities. The sale of Fund portfolio securities may result in the realization and/or distribution to shareholders of higher capital gains or losses as compared to a fund with less active trading policies. These effects of higher than normal portfolio turnover may adversely affect Fund performance. Inflation-Indexed Bonds Risk — The principal value of an investment is not protected or otherwise guaranteed by virtue of the Fund’s investments in inflation-indexed bonds. Inflation-indexed bonds are fixed-income securities whose principal value is periodically adjusted according to the rate of inflation. If the index measuring inflation falls, the principal value of inflation-indexed bonds will be adjusted downward, and consequently the interest payable on these securities (calculated with respect to a smaller principal amount) will be reduced. Repayment of the original bond principal upon maturity (as adjusted for inflation) is guaranteed in the case of U.S. Treasury inflation-indexed bonds. For bonds that do not provide a similar guarantee, the adjusted principal value of the bond repaid at maturity may be less than the original principal value. The value of inflation-indexed bonds is expected to change in response to changes in real interest rates. Real interest rates are tied to the relationship between nominal interest rates and the rate of inflation. If nominal interest rates increase at a faster rate than inflation, real interest rates may rise, leading to a decrease in value of inflation-indexed bonds. Short-term increases in inflation may lead to a decline in value. Any increase in the principal amount of an inflation-indexed bond will be considered taxable ordinary income, even though investors do not receive their principal until maturity. Periodic adjustments for inflation to the principal amount of an inflation-indexed bond may give rise to original issue discount, which will be includable in the Fund’s gross income. Due to original issue discount, the Fund may be required to make annual distributions to shareholders that exceed the cash received, which may cause the Fund to liquidate certain investments when it is not advantageous to do so. Also, if the principal value of an inflation-indexed bond is adjusted downward due to deflation, amounts previously distributed in the taxable year may be characterized in some circumstances as a return of capital. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund’s portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund’s investments in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund’s principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquidity risk may be the result of, among other things, the reduced number and capacity of traditional market participants to make a market in fixed-income securities or the lack of an active market. Liquid investments may become illiquid or less liquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid and relatively less liquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. This may be magnified in a rising interest rate environment or other circumstances where investor redemptions from fixed-income mutual funds may be higher than normal. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Mortgage- and Asset-Backed Securities Risks — Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities. Non-Investment Grade Securities Risk — Although non-investment grade securities generally pay higher rates of interest than investment grade bonds, non-investment grade securities are high risk investments that are considered speculative and may cause income and principal losses for the Fund. Repurchase Agreements and Purchase and Sale Contracts Risk — If the other party to a repurchase agreement or purchase and sale contract defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security in either situation and the market value of the security declines, the Fund may lose money. Reverse Repurchase Agreements Risk — Reverse repurchase agreements involve the sale of securities held by the Fund with an agreement to repurchase the securities at an agreed-upon price, date and interest payment. Reverse repurchase agreements involve the risk that the other party may fail to return the securities in a timely manner or at all. The Fund could lose money if it is unable to recover the securities and the value of the collateral held by the Fund, including the value of the investments made with cash collateral, is less than the value of the securities. These events could also trigger adverse tax consequences to the Fund. Subsidiary Risk — By investing in the Subsidiary, the Fund is indirectly exposed to the risks associated with the Subsidiary’s investments. The commodity-related instruments held by the Subsidiary are generally similar to those that are permitted to be held by the Fund and are subject to the same risks that apply to similar investments if held directly by the Fund (see “Commodities Related Investments Risks” above). There can be no assurance that the investment objective of the Subsidiary will be achieved. The Subsidiary is not registered under the Investment Company Act of 1940, as amended (the “Investment Company Act”), and, unless otherwise noted in this prospectus, is not subject to all the investor protections of the Investment Company Act. However, the Fund wholly owns and controls the Subsidiary, and the Fund and the Subsidiary are both managed by BlackRock, making it unlikely that the Subsidiary will take action contrary to the interests of the Fund and its shareholders. The Board has oversight responsibility for the investment activities of the Fund, including its investment in the Subsidiary, and the Fund’s role as sole shareholder of the Subsidiary. The Subsidiary is subject to the same investment restrictions and limitations, and follows the same compliance policies and procedures, as the Fund. Changes in the laws of the United States and/or the Cayman Islands could result in the inability of the Fund and/or the Subsidiary to operate as described in this prospectus and the Statement of Additional Information and could adversely affect the Fund. In order to qualify as a regulated investment company, at least 90% of the Fund’s gross income for the taxable year must be “qualifying income.” The Fund anticipates treating the income and gain generated from investments in controlled foreign subsidiaries that invest in physical commodities and/or commodity-linked derivative instruments as “qualifying income” for regulated investment company qualification purposes. However, there can be no assurance that the Internal Revenue Service (“IRS”) will agree with treating such income and gain as “qualifying income.” If the IRS makes an adverse determination relating to the treatment of such income and gain, the Fund would likely need to change its investment strategies, which could adversely affect the Fund. The IRS has proposed regulations that, if finalized in current form, would specify that a subpart F income inclusion for U.S. federal income tax purposes will be treated as qualifying income only to the extent that the Subsidiary makes distributions out of its earnings and profits in the same taxable year. U.S. Government Issuer Risk — Treasury obligations may differ in their interest rates, maturities, times of issuance and other characteristics. Obligations of U.S. Government agencies and authorities are supported by varying degrees of credit but generally are not backed by the full faith and credit of the U.S. Government. No assurance can be given that the U.S. Government will provide financial support to its agencies and authorities if it is not obligated by law to do so.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index. The returns for Class K Shares prior to September 1, 2015, the commencement of operations of Class K Shares, are based upon performance of the Fund’s BlackRock Shares. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not an indication of future results. The table includes all applicable fees. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com or can be obtained by phone at 800-882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Class K Shares ANNUAL TOTAL RETURNS Inflation Protected Bond Portfolio As of 12/31
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 5.87% (quarter ended March 31, 2008) and the lowest return for a quarter was -6.54% (quarter ended June 30, 2013).
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/17 Average Annual Total Returns
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor's tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Class K | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Class K Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.24%	[1],[2]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Interest Expense	rr_Component1OtherExpensesOverAssets	0.05%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.11%
Other Expenses of the Subsidiary	rr_Component3OtherExpensesOverAssets		[3]
Other Expenses	rr_OtherExpensesOverAssets	0.16%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.40%	[4]
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.06%)	[1],[5]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.34%	[1],[5]
1 Year	rr_ExpenseExampleYear01	$ 35
3 Years	rr_ExpenseExampleYear03	116
5 Years	rr_ExpenseExampleYear05	212
10 Years	rr_ExpenseExampleYear10	$ 493
2008	rr_AnnualReturn2008	0.58%
2009	rr_AnnualReturn2009	10.66%
2010	rr_AnnualReturn2010	6.33%
2011	rr_AnnualReturn2011	11.95%
2012	rr_AnnualReturn2012	7.08%
2013	rr_AnnualReturn2013	(7.98%)
2014	rr_AnnualReturn2014	2.68%
2015	rr_AnnualReturn2015	(1.95%)
2016	rr_AnnualReturn2016	4.29%
2017	rr_AnnualReturn2017	3.14%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	5.87%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2013
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(6.54%)
1 Year	rr_AverageAnnualReturnYear01	3.14%
5 Years	rr_AverageAnnualReturnYear05	(0.07%)
10 Years	rr_AverageAnnualReturnYear10	3.52%
Class K | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Return After Taxes on Distributions | Class K Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	2.15%
5 Years	rr_AverageAnnualReturnYear05	(0.88%)
10 Years	rr_AverageAnnualReturnYear10	2.39%
Class K | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Return After Taxes on Distributions and Sale of Fund Shares | Class K Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	1.77%
5 Years	rr_AverageAnnualReturnYear05	(0.40%)
10 Years	rr_AverageAnnualReturnYear10	2.33%
Class K | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	3.01%
5 Years	rr_AverageAnnualReturnYear05	0.13%
10 Years	rr_AverageAnnualReturnYear10	3.53%

[1] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 30, BlackRock Advisors, LLC (“BlackRock”) has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through April 30, 2020. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of BlackRock Funds II (the “Trust”) or by a vote of a majority of the outstanding voting securities of the Fund.

[2] The Management Fee has been restated to reflect current fees.

[3] Other Expenses of Cayman Inflation Protected Bond Portfolio, Ltd. (the “Subsidiary”) were less than 0.01% for the most recent fiscal year.

[4] The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current expenses.

[5] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 30, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.29% of average daily net assets through April 30, 2020. The Fund may have to repay some of these waivers and/or reimbursements to BlackRock in the two years following such waivers and/or reimbursements. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

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Service | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO

Fund Overview

Key Facts About BlackRock Inflation Protected Bond Portfolio

Investment Objective

The investment objective of the BlackRock Inflation Protected Bond Portfolio (the “Inflation Protected Bond Portfolio” or the “Fund”) is to seek to maximize real return, consistent with preservation of real capital and prudent investment management.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold Service Shares of the Fund.

Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses	Service BLACKROCK INFLATION PROTECTED BOND PORTFOLIO Service Shares
Management Fee	0.24%	[1],[2]
Distribution and/or Service (12b-1) Fees	0.25%
Other Expenses	0.30%
Interest Expense	0.05%
Miscellaneous Other Expenses	0.25%
Other Expenses of the Subsidiary		[3]
Total Annual Fund Operating Expenses	0.79%	[4]
Fee Waivers and/or Expense Reimbursements	(0.15%)	[1],[5]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	0.64%	[1],[5]

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
Service | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Service Shares | USD ($)	65	221	408	949

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year the Fund’s portfolio turnover rate was 76% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the Fund invests at least 80% of its assets in inflation-indexed bonds of varying maturities issued by the U.S. and non-U.S. governments, their agencies or instrumentalities, and U.S. and non-U.S. corporations.

The Fund maintains an average portfolio duration that is within ±20% of the duration of the Barclays U.S. Treasury Inflation Protected Securities Index (the benchmark).

The Fund may invest up to 20% of its assets in non-investment grade bonds (high yield or junk bonds) or securities of emerging market issuers. The Fund may also invest up to 20% of its assets in non-dollar denominated securities of non-U.S. issuers, and may invest without limit in U.S. dollar denominated securities of non-U.S. issuers.

The Fund may seek to provide exposure to the investment returns of real assets that trade in the commodity markets through investment in commodity-linked derivative instruments and investment vehicles such as exchange traded funds that exclusively invest in commodities and are designed to provide this exposure without direct investment in physical commodities. The Fund may also gain exposure to commodity markets by investing up to 25% of its total assets in the Subsidiary, a wholly owned subsidiary of the Fund formed in the Cayman Islands, which invests primarily in commodity-related instruments.

The Fund also makes investments in residential and commercial mortgage-backed securities and other asset-backed securities.

Non-investment grade bonds acquired by the Fund will generally be in the lower rating categories of the major rating agencies (BB or lower by S&P Global Ratings (“S&P”) or Ba or lower by Moody’s Investors Service, Inc. (“Moody’s”)) or will be determined by the management team to be of similar quality. Split rated bonds will be considered to have the higher credit rating. Split rated bonds are bonds that receive different ratings from two or more rating agencies.

The Fund may buy or sell options or futures, or enter into credit default swaps and interest rate or foreign currency transactions, including swaps (collectively, commonly known as derivatives). The Fund may seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques (such as reverse repurchase agreements or dollar rolls).

The Fund may engage in active and frequent trading of portfolio securities to achieve its primary investment strategies.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Commodities Related Investments Risks — Exposure to the commodities markets may subject the Fund to greater volatility than investments in traditional securities. The value of commodity-linked derivative investments may be affected by changes in overall market movements, commodity index volatility, changes in interest rates, or factors affecting a particular industry or commodity, such as drought, floods, weather, embargoes, tariffs and international economic, political and regulatory developments.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.
  
  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Deflation Risk — Deflation risk is the possibility that prices throughout the economy decline over time — the opposite of inflation. If inflation is negative, the principal and income of an inflation-protected bond will decline and could result in losses for the Fund.

• Derivatives Risk — The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including:
  
  Volatility Risk — Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets.
  
  Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation.
  
  Market and Liquidity Risk — The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately.
  
  Valuation Risk — Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them.
  
  Hedging Risk — Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences.
  
  Tax Risk — Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments, are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority. Such treatment may be less favorable than that given to a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments.
  
  Regulatory Risk — Derivative contracts, including, without limitation, swaps, currency forwards and non-deliverable forwards, are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (“Dodd Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, certain derivatives are subject to margin requirements and swap dealers are required to collect margin from the Fund with respect to such derivatives. Specifically, regulations are now in effect that require swap dealers to post and collect variation margin (comprised of specified liquid instruments and subject to a required haircut) in connection with trading of over-the-counter (“OTC”) swaps with the Fund. Shares of investment companies (other than certain money market funds) may not be posted as collateral under these regulations. Requirements for posting of initial margin in connection with OTC swaps will be phased-in through 2020. In addition, regulations adopted by prudential regulators that will begin to take effect in 2019 will require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund.

• Dollar Rolls Risk — Dollar rolls involve the risk that the market value of the securities that the Fund is committed to buy may decline below the price of the securities the Fund has sold. These transactions may involve leverage.

• Emerging Markets Risk — Emerging markets are riskier than more developed markets because they tend to develop unevenly and may never fully develop. Investments in emerging markets may be considered speculative. Emerging markets are more likely to experience hyperinflation and currency devaluations, which adversely affect returns to U.S. investors. In addition, many emerging securities markets have far lower trading volumes and less liquidity than developed markets.

• Foreign Securities Risk — Foreign investments often involve special risks not present in U.S. investments that can increase the chances that the Fund will lose money. These risks include:

• The Fund generally holds its foreign securities and cash in foreign banks and securities depositories, which may be recently organized or new to the foreign custody business and may be subject to only limited or no regulatory oversight.

• Changes in foreign currency exchange rates can affect the value of the Fund’s portfolio.

• The economies of certain foreign markets may not compare favorably with the economy of the United States with respect to such issues as growth of gross national product, reinvestment of capital, resources and balance of payments position.

• The governments of certain countries may prohibit or impose substantial restrictions on foreign investments in their capital markets or in certain industries.

• Many foreign governments do not supervise and regulate stock exchanges, brokers and the sale of securities to the same extent as does the United States and may not have laws to protect investors that are comparable to U.S. securities laws.

• Settlement and clearance procedures in certain foreign markets may result in delays in payment for or delivery of securities not typically associated with settlement and clearance of U.S. investments.

• High Portfolio Turnover Risk — The Fund may engage in active and frequent trading of its portfolio securities. High portfolio turnover (more than 100%) may result in increased transaction costs to the Fund, including brokerage commissions, dealer mark-ups and other transaction costs on the sale of the securities and on reinvestment in other securities. The sale of Fund portfolio securities may result in the realization and/or distribution to shareholders of higher capital gains or losses as compared to a fund with less active trading policies. These effects of higher than normal portfolio turnover may adversely affect Fund performance.

• Inflation-Indexed Bonds Risk — The principal value of an investment is not protected or otherwise guaranteed by virtue of the Fund’s investments in inflation-indexed bonds.
  
  Inflation-indexed bonds are fixed-income securities whose principal value is periodically adjusted according to the rate of inflation. If the index measuring inflation falls, the principal value of inflation-indexed bonds will be adjusted downward, and consequently the interest payable on these securities (calculated with respect to a smaller principal amount) will be reduced.
  
  Repayment of the original bond principal upon maturity (as adjusted for inflation) is guaranteed in the case of U.S. Treasury inflation-indexed bonds. For bonds that do not provide a similar guarantee, the adjusted principal value of the bond repaid at maturity may be less than the original principal value.
  
  The value of inflation-indexed bonds is expected to change in response to changes in real interest rates. Real interest rates are tied to the relationship between nominal interest rates and the rate of inflation. If nominal interest rates increase at a faster rate than inflation, real interest rates may rise, leading to a decrease in value of inflation-indexed bonds. Short-term increases in inflation may lead to a decline in value. Any increase in the principal amount of an inflation-indexed bond will be considered taxable ordinary income, even though investors do not receive their principal until maturity.
  
  Periodic adjustments for inflation to the principal amount of an inflation-indexed bond may give rise to original issue discount, which will be includable in the Fund’s gross income. Due to original issue discount, the Fund may be required to make annual distributions to shareholders that exceed the cash received, which may cause the Fund to liquidate certain investments when it is not advantageous to do so. Also, if the principal value of an inflation-indexed bond is adjusted downward due to deflation, amounts previously distributed in the taxable year may be characterized in some circumstances as a return of capital.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund’s portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund’s investments in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund’s principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquidity risk may be the result of, among other things, the reduced number and capacity of traditional market participants to make a market in fixed-income securities or the lack of an active market. Liquid investments may become illiquid or less liquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid and relatively less liquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. This may be magnified in a rising interest rate environment or other circumstances where investor redemptions from fixed-income mutual funds may be higher than normal. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Mortgage- and Asset-Backed Securities Risks — Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities.

• Non-Investment Grade Securities Risk — Although non-investment grade securities generally pay higher rates of interest than investment grade bonds, non-investment grade securities are high risk investments that are considered speculative and may cause income and principal losses for the Fund.

• Repurchase Agreements and Purchase and Sale Contracts Risk — If the other party to a repurchase agreement or purchase and sale contract defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security in either situation and the market value of the security declines, the Fund may lose money.

• Reverse Repurchase Agreements Risk — Reverse repurchase agreements involve the sale of securities held by the Fund with an agreement to repurchase the securities at an agreed-upon price, date and interest payment. Reverse repurchase agreements involve the risk that the other party may fail to return the securities in a timely manner or at all. The Fund could lose money if it is unable to recover the securities and the value of the collateral held by the Fund, including the value of the investments made with cash collateral, is less than the value of the securities. These events could also trigger adverse tax consequences to the Fund.

• Subsidiary Risk — By investing in the Subsidiary, the Fund is indirectly exposed to the risks associated with the Subsidiary’s investments. The commodity-related instruments held by the Subsidiary are generally similar to those that are permitted to be held by the Fund and are subject to the same risks that apply to similar investments if held directly by the Fund (see “Commodities Related Investments Risks” above). There can be no assurance that the investment objective of the Subsidiary will be achieved. The Subsidiary is not registered under the Investment Company Act of 1940, as amended (the “Investment Company Act”), and, unless otherwise noted in this prospectus, is not subject to all the investor protections of the Investment Company Act. However, the Fund wholly owns and controls the Subsidiary, and the Fund and the Subsidiary are both managed by BlackRock, making it unlikely that the Subsidiary will take action contrary to the interests of the Fund and its shareholders. The Board has oversight responsibility for the investment activities of the Fund, including its investment in the Subsidiary, and the Fund’s role as sole shareholder of the Subsidiary. The Subsidiary is subject to the same investment restrictions and limitations, and follows the same compliance policies and procedures, as the Fund. Changes in the laws of the United States and/or the Cayman Islands could result in the inability of the Fund and/or the Subsidiary to operate as described in this prospectus and the Statement of Additional Information and could adversely affect the Fund.
  
  In order to qualify as a regulated investment company, at least 90% of the Fund’s gross income for the taxable year must be “qualifying income.” The Fund anticipates treating the income and gain generated from investments in controlled foreign subsidiaries that invest in physical commodities and/or commodity-linked derivative instruments as “qualifying income” for regulated investment company qualification purposes. However, there can be no assurance that the Internal Revenue Service (“IRS”) will agree with treating such income and gain as “qualifying income.” If the IRS makes an adverse determination relating to the treatment of such income and gain, the Fund would likely need to change its investment strategies, which could adversely affect the Fund. The IRS has proposed regulations that, if finalized in current form, would specify that a subpart F income inclusion for U.S. federal income tax purposes will be treated as qualifying income only to the extent that the Subsidiary makes distributions out of its earnings and profits in the same taxable year.

• U.S. Government Issuer Risk — Treasury obligations may differ in their interest rates, maturities, times of issuance and other characteristics. Obligations of U.S. Government agencies and authorities are supported by varying degrees of credit but generally are not backed by the full faith and credit of the U.S. Government. No assurance can be given that the U.S. Government will provide financial support to its agencies and authorities if it is not obligated by law to do so.

Performance Information

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not an indication of future results. The table includes all applicable fees. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com or can be obtained by phone at 800-882-0052.

Service Shares
ANNUAL TOTAL RETURNS
Inflation Protected Bond Portfolio
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 5.86% (quarter ended March 31, 2008) and the lowest return for a quarter was -6.65% (quarter ended June 30, 2013).

As of 12/31/17
Average Annual Total Returns

Average Annual Total Returns - Service - BLACKROCK INFLATION PROTECTED BOND PORTFOLIO	1 Year	5 Years	10 Years
Service Shares	2.74%	(0.47%)	3.10%
Service Shares | Return After Taxes on Distributions	1.84%	(1.23%)	2.05%
Service Shares | Return After Taxes on Distributions and Sale of Fund Shares	1.54%	(0.67%)	2.03%
Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index (Reflects no deduction for fees, expenses or taxes)	3.01%	0.13%	3.53%

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

[1] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 29, BlackRock Advisors, LLC (“BlackRock”) has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through April 30, 2020. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of BlackRock Funds II (the “Trust”) or by a vote of a majority of the outstanding voting securities of the Fund.

[2] The Management Fee has been restated to reflect current fees.

[3] Other Expenses of Cayman Inflation Protected Bond Portfolio, Ltd. (the “Subsidiary”) were less than 0.01% for the most recent fiscal year.

[4] The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current expense.

[5] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 29, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.59% of average daily net assets through April 30, 2020. The Fund may have to repay some of these waivers and/or reimbursements to BlackRock in the two years following such waivers and/or reimbursements. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BlackRock Funds II
Prospectus Date	rr_ProspectusDate	Apr. 30, 2018
Service | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock Inflation Protected Bond Portfolio
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of the BlackRock Inflation Protected Bond Portfolio (the “Inflation Protected Bond Portfolio” or the “Fund”) is to seek to maximize real return, consistent with preservation of real capital and prudent investment management.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Service Shares of the Fund.
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	April 30, 2020
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year the Fund’s portfolio turnover rate was 76% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	76.00%
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	The Management Fee has been restated to reflect current fees.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 80% of its assets in inflation-indexed bonds of varying maturities issued by the U.S. and non-U.S. governments, their agencies or instrumentalities, and U.S. and non-U.S. corporations. The Fund maintains an average portfolio duration that is within ±20% of the duration of the Barclays U.S. Treasury Inflation Protected Securities Index (the benchmark). The Fund may invest up to 20% of its assets in non-investment grade bonds (high yield or junk bonds) or securities of emerging market issuers. The Fund may also invest up to 20% of its assets in non-dollar denominated securities of non-U.S. issuers, and may invest without limit in U.S. dollar denominated securities of non-U.S. issuers. The Fund may seek to provide exposure to the investment returns of real assets that trade in the commodity markets through investment in commodity-linked derivative instruments and investment vehicles such as exchange traded funds that exclusively invest in commodities and are designed to provide this exposure without direct investment in physical commodities. The Fund may also gain exposure to commodity markets by investing up to 25% of its total assets in the Subsidiary, a wholly owned subsidiary of the Fund formed in the Cayman Islands, which invests primarily in commodity-related instruments. The Fund also makes investments in residential and commercial mortgage-backed securities and other asset-backed securities. Non-investment grade bonds acquired by the Fund will generally be in the lower rating categories of the major rating agencies (BB or lower by S&P Global Ratings (“S&P”) or Ba or lower by Moody’s Investors Service, Inc. (“Moody’s”)) or will be determined by the management team to be of similar quality. Split rated bonds will be considered to have the higher credit rating. Split rated bonds are bonds that receive different ratings from two or more rating agencies. The Fund may buy or sell options or futures, or enter into credit default swaps and interest rate or foreign currency transactions, including swaps (collectively, commonly known as derivatives). The Fund may seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques (such as reverse repurchase agreements or dollar rolls). The Fund may engage in active and frequent trading of portfolio securities to achieve its primary investment strategies.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Commodities Related Investments Risks — Exposure to the commodities markets may subject the Fund to greater volatility than investments in traditional securities. The value of commodity-linked derivative investments may be affected by changes in overall market movements, commodity index volatility, changes in interest rates, or factors affecting a particular industry or commodity, such as drought, floods, weather, embargoes, tariffs and international economic, political and regulatory developments. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Deflation Risk — Deflation risk is the possibility that prices throughout the economy decline over time — the opposite of inflation. If inflation is negative, the principal and income of an inflation-protected bond will decline and could result in losses for the Fund. Derivatives Risk — The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including: Volatility Risk — Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets. Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. Market and Liquidity Risk — The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation Risk — Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Hedging Risk — Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences. Tax Risk — Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments, are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority. Such treatment may be less favorable than that given to a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments. Regulatory Risk — Derivative contracts, including, without limitation, swaps, currency forwards and non-deliverable forwards, are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (“Dodd Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, certain derivatives are subject to margin requirements and swap dealers are required to collect margin from the Fund with respect to such derivatives. Specifically, regulations are now in effect that require swap dealers to post and collect variation margin (comprised of specified liquid instruments and subject to a required haircut) in connection with trading of over-the-counter (“OTC”) swaps with the Fund. Shares of investment companies (other than certain money market funds) may not be posted as collateral under these regulations. Requirements for posting of initial margin in connection with OTC swaps will be phased-in through 2020. In addition, regulations adopted by prudential regulators that will begin to take effect in 2019 will require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund. Dollar Rolls Risk — Dollar rolls involve the risk that the market value of the securities that the Fund is committed to buy may decline below the price of the securities the Fund has sold. These transactions may involve leverage. Emerging Markets Risk — Emerging markets are riskier than more developed markets because they tend to develop unevenly and may never fully develop. Investments in emerging markets may be considered speculative. Emerging markets are more likely to experience hyperinflation and currency devaluations, which adversely affect returns to U.S. investors. In addition, many emerging securities markets have far lower trading volumes and less liquidity than developed markets. Foreign Securities Risk — Foreign investments often involve special risks not present in U.S. investments that can increase the chances that the Fund will lose money. These risks include: The Fund generally holds its foreign securities and cash in foreign banks and securities depositories, which may be recently organized or new to the foreign custody business and may be subject to only limited or no regulatory oversight. Changes in foreign currency exchange rates can affect the value of the Fund’s portfolio. The economies of certain foreign markets may not compare favorably with the economy of the United States with respect to such issues as growth of gross national product, reinvestment of capital, resources and balance of payments position. The governments of certain countries may prohibit or impose substantial restrictions on foreign investments in their capital markets or in certain industries. Many foreign governments do not supervise and regulate stock exchanges, brokers and the sale of securities to the same extent as does the United States and may not have laws to protect investors that are comparable to U.S. securities laws. Settlement and clearance procedures in certain foreign markets may result in delays in payment for or delivery of securities not typically associated with settlement and clearance of U.S. investments. High Portfolio Turnover Risk — The Fund may engage in active and frequent trading of its portfolio securities. High portfolio turnover (more than 100%) may result in increased transaction costs to the Fund, including brokerage commissions, dealer mark-ups and other transaction costs on the sale of the securities and on reinvestment in other securities. The sale of Fund portfolio securities may result in the realization and/or distribution to shareholders of higher capital gains or losses as compared to a fund with less active trading policies. These effects of higher than normal portfolio turnover may adversely affect Fund performance. Inflation-Indexed Bonds Risk — The principal value of an investment is not protected or otherwise guaranteed by virtue of the Fund’s investments in inflation-indexed bonds. Inflation-indexed bonds are fixed-income securities whose principal value is periodically adjusted according to the rate of inflation. If the index measuring inflation falls, the principal value of inflation-indexed bonds will be adjusted downward, and consequently the interest payable on these securities (calculated with respect to a smaller principal amount) will be reduced. Repayment of the original bond principal upon maturity (as adjusted for inflation) is guaranteed in the case of U.S. Treasury inflation-indexed bonds. For bonds that do not provide a similar guarantee, the adjusted principal value of the bond repaid at maturity may be less than the original principal value. The value of inflation-indexed bonds is expected to change in response to changes in real interest rates. Real interest rates are tied to the relationship between nominal interest rates and the rate of inflation. If nominal interest rates increase at a faster rate than inflation, real interest rates may rise, leading to a decrease in value of inflation-indexed bonds. Short-term increases in inflation may lead to a decline in value. Any increase in the principal amount of an inflation-indexed bond will be considered taxable ordinary income, even though investors do not receive their principal until maturity. Periodic adjustments for inflation to the principal amount of an inflation-indexed bond may give rise to original issue discount, which will be includable in the Fund’s gross income. Due to original issue discount, the Fund may be required to make annual distributions to shareholders that exceed the cash received, which may cause the Fund to liquidate certain investments when it is not advantageous to do so. Also, if the principal value of an inflation-indexed bond is adjusted downward due to deflation, amounts previously distributed in the taxable year may be characterized in some circumstances as a return of capital. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund’s portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund’s investments in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund’s principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquidity risk may be the result of, among other things, the reduced number and capacity of traditional market participants to make a market in fixed-income securities or the lack of an active market. Liquid investments may become illiquid or less liquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid and relatively less liquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. This may be magnified in a rising interest rate environment or other circumstances where investor redemptions from fixed-income mutual funds may be higher than normal. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Mortgage- and Asset-Backed Securities Risks — Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities. Non-Investment Grade Securities Risk — Although non-investment grade securities generally pay higher rates of interest than investment grade bonds, non-investment grade securities are high risk investments that are considered speculative and may cause income and principal losses for the Fund. Repurchase Agreements and Purchase and Sale Contracts Risk — If the other party to a repurchase agreement or purchase and sale contract defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security in either situation and the market value of the security declines, the Fund may lose money. Reverse Repurchase Agreements Risk — Reverse repurchase agreements involve the sale of securities held by the Fund with an agreement to repurchase the securities at an agreed-upon price, date and interest payment. Reverse repurchase agreements involve the risk that the other party may fail to return the securities in a timely manner or at all. The Fund could lose money if it is unable to recover the securities and the value of the collateral held by the Fund, including the value of the investments made with cash collateral, is less than the value of the securities. These events could also trigger adverse tax consequences to the Fund. Subsidiary Risk — By investing in the Subsidiary, the Fund is indirectly exposed to the risks associated with the Subsidiary’s investments. The commodity-related instruments held by the Subsidiary are generally similar to those that are permitted to be held by the Fund and are subject to the same risks that apply to similar investments if held directly by the Fund (see “Commodities Related Investments Risks” above). There can be no assurance that the investment objective of the Subsidiary will be achieved. The Subsidiary is not registered under the Investment Company Act of 1940, as amended (the “Investment Company Act”), and, unless otherwise noted in this prospectus, is not subject to all the investor protections of the Investment Company Act. However, the Fund wholly owns and controls the Subsidiary, and the Fund and the Subsidiary are both managed by BlackRock, making it unlikely that the Subsidiary will take action contrary to the interests of the Fund and its shareholders. The Board has oversight responsibility for the investment activities of the Fund, including its investment in the Subsidiary, and the Fund’s role as sole shareholder of the Subsidiary. The Subsidiary is subject to the same investment restrictions and limitations, and follows the same compliance policies and procedures, as the Fund. Changes in the laws of the United States and/or the Cayman Islands could result in the inability of the Fund and/or the Subsidiary to operate as described in this prospectus and the Statement of Additional Information and could adversely affect the Fund. In order to qualify as a regulated investment company, at least 90% of the Fund’s gross income for the taxable year must be “qualifying income.” The Fund anticipates treating the income and gain generated from investments in controlled foreign subsidiaries that invest in physical commodities and/or commodity-linked derivative instruments as “qualifying income” for regulated investment company qualification purposes. However, there can be no assurance that the Internal Revenue Service (“IRS”) will agree with treating such income and gain as “qualifying income.” If the IRS makes an adverse determination relating to the treatment of such income and gain, the Fund would likely need to change its investment strategies, which could adversely affect the Fund. The IRS has proposed regulations that, if finalized in current form, would specify that a subpart F income inclusion for U.S. federal income tax purposes will be treated as qualifying income only to the extent that the Subsidiary makes distributions out of its earnings and profits in the same taxable year. U.S. Government Issuer Risk — Treasury obligations may differ in their interest rates, maturities, times of issuance and other characteristics. Obligations of U.S. Government agencies and authorities are supported by varying degrees of credit but generally are not backed by the full faith and credit of the U.S. Government. No assurance can be given that the U.S. Government will provide financial support to its agencies and authorities if it is not obligated by law to do so.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not an indication of future results. The table includes all applicable fees. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com or can be obtained by phone at 800-882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Service Shares ANNUAL TOTAL RETURNS Inflation Protected Bond Portfolio As of 12/31
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 5.86% (quarter ended March 31, 2008) and the lowest return for a quarter was -6.65% (quarter ended June 30, 2013).
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/17 Average Annual Total Returns
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Service | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Service Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.24%	[1],[2]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.05%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.25%
Other Expenses of the Subsidiary	rr_Component3OtherExpensesOverAssets		[3]
Other Expenses	rr_OtherExpensesOverAssets	0.30%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.79%	[4]
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.15%)	[1],[5]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.64%	[1],[5]
1 Year	rr_ExpenseExampleYear01	$ 65
3 Years	rr_ExpenseExampleYear03	221
5 Years	rr_ExpenseExampleYear05	408
10 Years	rr_ExpenseExampleYear10	$ 949
2008	rr_AnnualReturn2008	0.14%
2009	rr_AnnualReturn2009	10.20%
2010	rr_AnnualReturn2010	5.90%
2011	rr_AnnualReturn2011	11.55%
2012	rr_AnnualReturn2012	6.58%
2013	rr_AnnualReturn2013	(8.34%)
2014	rr_AnnualReturn2014	2.24%
2015	rr_AnnualReturn2015	(2.31%)
2016	rr_AnnualReturn2016	3.87%
2017	rr_AnnualReturn2017	2.74%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	5.86%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2013
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(6.65%)
1 Year	rr_AverageAnnualReturnYear01	2.74%
5 Years	rr_AverageAnnualReturnYear05	(0.47%)
10 Years	rr_AverageAnnualReturnYear10	3.10%
Service | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Return After Taxes on Distributions | Service Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	1.84%
5 Years	rr_AverageAnnualReturnYear05	(1.23%)
10 Years	rr_AverageAnnualReturnYear10	2.05%
Service | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Return After Taxes on Distributions and Sale of Fund Shares | Service Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	1.54%
5 Years	rr_AverageAnnualReturnYear05	(0.67%)
10 Years	rr_AverageAnnualReturnYear10	2.03%
Service | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	3.01%
5 Years	rr_AverageAnnualReturnYear05	0.13%
10 Years	rr_AverageAnnualReturnYear10	3.53%

[1] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 29, BlackRock Advisors, LLC (“BlackRock”) has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through April 30, 2020. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of BlackRock Funds II (the “Trust”) or by a vote of a majority of the outstanding voting securities of the Fund.

[2] The Management Fee has been restated to reflect current fees.

[3] Other Expenses of Cayman Inflation Protected Bond Portfolio, Ltd. (the “Subsidiary”) were less than 0.01% for the most recent fiscal year.

[4] The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current expense.

[5] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 29, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.59% of average daily net assets through April 30, 2020. The Fund may have to repay some of these waivers and/or reimbursements to BlackRock in the two years following such waivers and/or reimbursements. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

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Class R | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO

Fund Overview

Key Facts About BlackRock Inflation Protected Bond Portfolio

Investment Objective

The investment objective of the BlackRock Inflation Protected Bond Portfolio (the “Inflation Protected Bond Portfolio” or the “Fund”) is to seek to maximize real return, consistent with preservation of real capital and prudent investment management.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy and hold Class R Shares of the Fund.

Annual Fund Operating Expenses
(expenses that you pay each year as a percentage of the value of your investment)

Annual Fund Operating Expenses	Class R BLACKROCK INFLATION PROTECTED BOND PORTFOLIO Class R Shares
Management Fee	0.24%	[1],[2]
Distribution and/or Service (12b-1) Fees	0.50%
Other Expenses	0.43%	[3]
Interest Expense	0.05%
Miscellaneous Other Expenses	0.38%	[3]
Other Expenses of the Subsidiary		[4]
Total Annual Fund Operating Expenses	1.17%	[5]
Fee Waivers and/or Expense Reimbursements	(0.28%)	[1],[5]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	0.89%	[1],[5]

Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

Expense Example	1 Year	3 Years	5 Years	10 Years
Class R | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Class R Shares | USD ($)	91	315	588	1,369

Portfolio Turnover:

The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 76% of the average value of its portfolio.

Principal Investment Strategies of the Fund

Under normal circumstances, the Fund invests at least 80% of its assets in inflation-indexed bonds of varying maturities issued by the U.S. and non-U.S. governments, their agencies or instrumentalities, and U.S. and non-U.S. corporations.

The Fund maintains an average portfolio duration that is within ±20% of the duration of the Barclays U.S. Treasury Inflation Protected Securities Index (the benchmark).

The Fund may invest up to 20% of its assets in non-investment grade bonds (high yield or junk bonds) or securities of emerging market issuers. The Fund may also invest up to 20% of its assets in non-dollar denominated securities of non-U.S. issuers, and may invest without limit in U.S. dollar denominated securities of non-U.S. issuers.

The Fund may seek to provide exposure to the investment returns of real assets that trade in the commodity markets through investment in commodity-linked derivative instruments and investment vehicles such as exchange traded funds that exclusively invest in commodities and are designed to provide this exposure without direct investment in physical commodities. The Fund may also gain exposure to commodity markets by investing up to 25% of its total assets in the Subsidiary, a wholly owned subsidiary of the Fund formed in the Cayman Islands, which invests primarily in commodity-related instruments.

The Fund also makes investments in residential and commercial mortgage-backed securities and other asset-backed securities.

Non-investment grade bonds acquired by the Fund will generally be in the lower rating categories of the major rating agencies (BB or lower by S&P Global Ratings (“S&P”) or Ba or lower by Moody’s Investors Service, Inc. (“Moody’s”)) or will be determined by the management team to be of similar quality. Split rated bonds will be considered to have the higher credit rating. Split rated bonds are bonds that receive different ratings from two or more rating agencies.

The Fund may buy or sell options or futures, or enter into credit default swaps and interest rate or foreign currency transactions, including swaps (collectively, commonly known as derivatives). The Fund may seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques (such as reverse repurchase agreements or dollar rolls).

The Fund may engage in active and frequent trading of portfolio securities to achieve its primary investment strategies.

Principal Risks of Investing in the Fund

Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.

• Commodities Related Investments Risks — Exposure to the commodities markets may subject the Fund to greater volatility than investments in traditional securities. The value of commodity-linked derivative investments may be affected by changes in overall market movements, commodity index volatility, changes in interest rates, or factors affecting a particular industry or commodity, such as drought, floods, weather, embargoes, tariffs and international economic, political and regulatory developments.

• Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.
  
  Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance.
  
  Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation.
  
  Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
  
  Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.

• Deflation Risk — Deflation risk is the possibility that prices throughout the economy decline over time — the opposite of inflation. If inflation is negative, the principal and income of an inflation-protected bond will decline and could result in losses for the Fund.

• Derivatives Risk — The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including:
  
  Volatility Risk — Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets.
  
  Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation.
  
  Market and Liquidity Risk — The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately.
  
  Valuation Risk — Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them.
  
  Hedging Risk — Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences.
  
  Tax Risk — Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments, are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority. Such treatment may be less favorable than that given to a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments.
  
  Regulatory Risk — Derivative contracts, including, without limitation, swaps, currency forwards and non-deliverable forwards, are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (“Dodd Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, certain derivatives are subject to margin requirements and swap dealers are required to collect margin from the Fund with respect to such derivatives. Specifically, regulations are now in effect that require swap dealers to post and collect variation margin (comprised of specified liquid instruments and subject to a required haircut) in connection with trading of over-the-counter (“OTC”) swaps with the Fund. Shares of investment companies (other than certain money market funds) may not be posted as collateral under these regulations. Requirements for posting of initial margin in connection with OTC swaps will be phased-in through 2020. In addition, regulations adopted by prudential regulators that will begin to take effect in 2019 will require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives, may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund.

• Dollar Rolls Risk — Dollar rolls involve the risk that the market value of the securities that the Fund is committed to buy may decline below the price of the securities the Fund has sold. These transactions may involve leverage.

• Emerging Markets Risk — Emerging markets are riskier than more developed markets because they tend to develop unevenly and may never fully develop. Investments in emerging markets may be considered speculative. Emerging markets are more likely to experience hyperinflation and currency devaluations, which adversely affect returns to U.S. investors. In addition, many emerging securities markets have far lower trading volumes and less liquidity than developed markets.

• Foreign Securities Risk — Foreign investments often involve special risks not present in U.S. investments that can increase the chances that the Fund will lose money. These risks include:

• The Fund generally holds its foreign securities and cash in foreign banks and securities depositories, which may be recently organized or new to the foreign custody business and may be subject to only limited or no regulatory oversight.

• Changes in foreign currency exchange rates can affect the value of the Fund’s portfolio.

• The economies of certain foreign markets may not compare favorably with the economy of the United States with respect to such issues as growth of gross national product, reinvestment of capital, resources and balance of payments position.

• The governments of certain countries may prohibit or impose substantial restrictions on foreign investments in their capital markets or in certain industries.

• Many foreign governments do not supervise and regulate stock exchanges, brokers and the sale of securities to the same extent as does the United States and may not have laws to protect investors that are comparable to U.S. securities laws.

• Settlement and clearance procedures in certain foreign markets may result in delays in payment for or delivery of securities not typically associated with settlement and clearance of U.S. investments.

• High Portfolio Turnover Risk — The Fund may engage in active and frequent trading of its portfolio securities. High portfolio turnover (more than 100%) may result in increased transaction costs to the Fund, including brokerage commissions, dealer mark-ups and other transaction costs on the sale of the securities and on reinvestment in other securities. The sale of Fund portfolio securities may result in the realization and/or distribution to shareholders of higher capital gains or losses as compared to a fund with less active trading policies. These effects of higher than normal portfolio turnover may adversely affect Fund performance.

• Inflation-Indexed Bonds Risk — The principal value of an investment is not protected or otherwise guaranteed by virtue of the Fund’s investments in inflation-indexed bonds.
  
  Inflation-indexed bonds are fixed-income securities whose principal value is periodically adjusted according to the rate of inflation. If the index measuring inflation falls, the principal value of inflation-indexed bonds will be adjusted downward, and consequently the interest payable on these securities (calculated with respect to a smaller principal amount) will be reduced.
  
  Repayment of the original bond principal upon maturity (as adjusted for inflation) is guaranteed in the case of U.S. Treasury inflation-indexed bonds. For bonds that do not provide a similar guarantee, the adjusted principal value of the bond repaid at maturity may be less than the original principal value.
  
  The value of inflation-indexed bonds is expected to change in response to changes in real interest rates. Real interest rates are tied to the relationship between nominal interest rates and the rate of inflation. If nominal interest rates increase at a faster rate than inflation, real interest rates may rise, leading to a decrease in value of inflation-indexed bonds. Short-term increases in inflation may lead to a decline in value. Any increase in the principal amount of an inflation-indexed bond will be considered taxable ordinary income, even though investors do not receive their principal until maturity.
  
  Periodic adjustments for inflation to the principal amount of an inflation-indexed bond may give rise to original issue discount, which will be includable in the Fund’s gross income. Due to original issue discount, the Fund may be required to make annual distributions to shareholders that exceed the cash received, which may cause the Fund to liquidate certain investments when it is not advantageous to do so. Also, if the principal value of an inflation-indexed bond is adjusted downward due to deflation, amounts previously distributed in the taxable year may be characterized in some circumstances as a return of capital.

• Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund’s portfolio will be magnified when the Fund uses leverage.

• Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund’s investments in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund’s principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquidity risk may be the result of, among other things, the reduced number and capacity of traditional market participants to make a market in fixed-income securities or the lack of an active market. Liquid investments may become illiquid or less liquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid and relatively less liquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. This may be magnified in a rising interest rate environment or other circumstances where investor redemptions from fixed-income mutual funds may be higher than normal. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.

• Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.

• Mortgage- and Asset-Backed Securities Risks — Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities.

• Non-Investment Grade Securities Risk — Although non-investment grade securities generally pay higher rates of interest than investment grade bonds, non-investment grade securities are high risk investments that are considered speculative and may cause income and principal losses for the Fund.

• Repurchase Agreements and Purchase and Sale Contracts Risk — If the other party to a repurchase agreement or purchase and sale contract defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security in either situation and the market value of the security declines, the Fund may lose money.

• Reverse Repurchase Agreements Risk — Reverse repurchase agreements involve the sale of securities held by the Fund with an agreement to repurchase the securities at an agreed-upon price, date and interest payment. Reverse repurchase agreements involve the risk that the other party may fail to return the securities in a timely manner or at all. The Fund could lose money if it is unable to recover the securities and the value of the collateral held by the Fund, including the value of the investments made with cash collateral, is less than the value of the securities. These events could also trigger adverse tax consequences to the Fund.

• Subsidiary Risk — By investing in the Subsidiary, the Fund is indirectly exposed to the risks associated with the Subsidiary’s investments. The commodity-related instruments held by the Subsidiary are generally similar to those that are permitted to be held by the Fund and are subject to the same risks that apply to similar investments if held directly by the Fund (see “Commodities Related Investments Risks” above). There can be no assurance that the investment objective of the Subsidiary will be achieved. The Subsidiary is not registered under the Investment Company Act of 1940, as amended (the “Investment Company Act”), and, unless otherwise noted in this prospectus, is not subject to all the investor protections of the Investment Company Act. However, the Fund wholly owns and controls the Subsidiary, and the Fund and the Subsidiary are both managed by BlackRock, making it unlikely that the Subsidiary will take action contrary to the interests of the Fund and its shareholders. The Board has oversight responsibility for the investment activities of the Fund, including its investment in the Subsidiary, and the Fund’s role as sole shareholder of the Subsidiary. The Subsidiary is subject to the same investment restrictions and limitations, and follows the same compliance policies and procedures, as the Fund. Changes in the laws of the United States and/or the Cayman Islands could result in the inability of the Fund and/or the Subsidiary to operate as described in this prospectus and the Statement of Additional Information and could adversely affect the Fund.
  
  In order to qualify as a regulated investment company, at least 90% of the Fund’s gross income for the taxable year must be “qualifying income.” The Fund anticipates treating the income and gain generated from investments in controlled foreign subsidiaries that invest in physical commodities and/or commodity-linked derivative instruments as “qualifying income” for regulated investment company qualification purposes. However, there can be no assurance that the Internal Revenue Service (“IRS”) will agree with treating such income and gain as “qualifying income.” If the IRS makes an adverse determination relating to the treatment of such income and gain, the Fund would likely need to change its investment strategies, which could adversely affect the Fund. The IRS has proposed regulations that, if finalized in current form, would specify that a subpart F income inclusion for U.S. federal income tax purposes will be treated as qualifying income only to the extent that the Subsidiary makes distributions out of its earnings and profits in the same taxable year.

• U.S. Government Issuer Risk — Treasury obligations may differ in their interest rates, maturities, times of issuance and other characteristics. Obligations of U.S. Government agencies and authorities are supported by varying degrees of credit but generally are not backed by the full faith and credit of the U.S. Government. No assurance can be given that the U.S. Government will provide financial support to its agencies and authorities if it is not obligated by law to do so.

Performance Information

Since Class R Shares of the Fund have no performance history, the chart and table below give you a picture of the Fund’s long-term performance for Investor A Shares. Although the chart and table show returns for the Investor A Shares which are not offered in this Prospectus, the Investor A Shares would have substantially similar annual returns as the Class R Shares offered in this Prospectus because the Investor A Shares and the Class R Shares are invested in the same portfolio of securities and the annual returns would differ only to the extent that the Investor A Shares and the Class R Shares do not have the same expenses. The actual returns of Class R Shares would have been lower than those of Investor A Shares because Class R Shares have higher expenses than Investor A Shares. Investor A Shares of the Fund are estimated to have expenses of 0.64% of average daily net assets (after waivers and reimbursements) for the current fiscal year and Class R Shares of the Fund are expected to have expenses of 0.89% of average daily net assets (after waivers and reimbursements) for the current fiscal year.

The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com or can be obtained by phone at 800-882-0052.

Investor A Shares
ANNUAL TOTAL RETURNS
Inflation Protected Bond Portfolio
As of 12/31

During the ten-year period shown in the bar chart, the highest return for a quarter was 5.80% (quarter ended March 31, 2008) and the lowest return for a quarter was -6.63% (quarter ended June 30, 2013).

As of 12/31/17
Average Annual Total Returns

Average Annual Total Returns - Class R - BLACKROCK INFLATION PROTECTED BOND PORTFOLIO	1 Year	5 Years	10 Years
Investor A Shares	(1.36%)	(1.29%)	2.67%
Investor A Shares | Return After Taxes on Distributions	(2.23%)	(2.05%)	1.62%
Investor A Shares | Return After Taxes on Distributions and Sale of Fund Shares	(0.78%)	(1.29%)	1.69%
Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index (Reflects no deduction for fees, expenses or taxes)	3.01%	0.13%	3.53%

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.

[1] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 29, BlackRock Advisors, LLC (“BlackRock”) has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through April 30, 2020. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of BlackRock Funds II (the “Trust”) or by a vote of a majority of the outstanding voting securities of the Fund.

[2] The Management Fee has been restated to reflect current fees.

[3] Other Expenses are based on estimated amounts for the current fiscal year.

[4] Other Expenses of Cayman Inflation Protected Bond Portfolio, Ltd. (the “Subsidiary”) were less than 0.01% for the most recent fiscal year.

[5] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 29, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.84% of average daily net assets through April 30, 2020. The Fund may have to repay some of these waivers and/or reimbursements to BlackRock in the two years following such waivers and/or reimbursements. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BlackRock Funds II
Prospectus Date	rr_ProspectusDate	Apr. 30, 2018
Class R | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock Inflation Protected Bond Portfolio
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of the BlackRock Inflation Protected Bond Portfolio (the “Inflation Protected Bond Portfolio” or the “Fund”) is to seek to maximize real return, consistent with preservation of real capital and prudent investment management.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	This table describes the fees and expenses that you may pay if you buy and hold Class R Shares of the Fund.
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	April 30, 2020
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the Example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 76% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	76.00%
Other Expenses, New Fund, Based on Estimates [Text]	rr_OtherExpensesNewFundBasedOnEstimates	Other Expenses are based on estimated amounts for the current fiscal year.
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	The Management Fee has been restated to reflect current fees.
Expense Example [Heading]	rr_ExpenseExampleHeading	Example:
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Under normal circumstances, the Fund invests at least 80% of its assets in inflation-indexed bonds of varying maturities issued by the U.S. and non-U.S. governments, their agencies or instrumentalities, and U.S. and non-U.S. corporations. The Fund maintains an average portfolio duration that is within ±20% of the duration of the Barclays U.S. Treasury Inflation Protected Securities Index (the benchmark). The Fund may invest up to 20% of its assets in non-investment grade bonds (high yield or junk bonds) or securities of emerging market issuers. The Fund may also invest up to 20% of its assets in non-dollar denominated securities of non-U.S. issuers, and may invest without limit in U.S. dollar denominated securities of non-U.S. issuers. The Fund may seek to provide exposure to the investment returns of real assets that trade in the commodity markets through investment in commodity-linked derivative instruments and investment vehicles such as exchange traded funds that exclusively invest in commodities and are designed to provide this exposure without direct investment in physical commodities. The Fund may also gain exposure to commodity markets by investing up to 25% of its total assets in the Subsidiary, a wholly owned subsidiary of the Fund formed in the Cayman Islands, which invests primarily in commodity-related instruments. The Fund also makes investments in residential and commercial mortgage-backed securities and other asset-backed securities. Non-investment grade bonds acquired by the Fund will generally be in the lower rating categories of the major rating agencies (BB or lower by S&P Global Ratings (“S&P”) or Ba or lower by Moody’s Investors Service, Inc. (“Moody’s”)) or will be determined by the management team to be of similar quality. Split rated bonds will be considered to have the higher credit rating. Split rated bonds are bonds that receive different ratings from two or more rating agencies. The Fund may buy or sell options or futures, or enter into credit default swaps and interest rate or foreign currency transactions, including swaps (collectively, commonly known as derivatives). The Fund may seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques (such as reverse repurchase agreements or dollar rolls). The Fund may engage in active and frequent trading of portfolio securities to achieve its primary investment strategies.
Risk [Heading]	rr_RiskHeading	Principal Risks of Investing in the Fund
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund. Commodities Related Investments Risks — Exposure to the commodities markets may subject the Fund to greater volatility than investments in traditional securities. The value of commodity-linked derivative investments may be affected by changes in overall market movements, commodity index volatility, changes in interest rates, or factors affecting a particular industry or commodity, such as drought, floods, weather, embargoes, tariffs and international economic, political and regulatory developments. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. Deflation Risk — Deflation risk is the possibility that prices throughout the economy decline over time — the opposite of inflation. If inflation is negative, the principal and income of an inflation-protected bond will decline and could result in losses for the Fund. Derivatives Risk — The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including: Volatility Risk — Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets. Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will not fulfill its contractual obligation. Market and Liquidity Risk — The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately. Valuation Risk — Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Hedging Risk — Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences. Tax Risk — Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments, are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority. Such treatment may be less favorable than that given to a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments. Regulatory Risk — Derivative contracts, including, without limitation, swaps, currency forwards and non-deliverable forwards, are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (“Dodd Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, certain derivatives are subject to margin requirements and swap dealers are required to collect margin from the Fund with respect to such derivatives. Specifically, regulations are now in effect that require swap dealers to post and collect variation margin (comprised of specified liquid instruments and subject to a required haircut) in connection with trading of over-the-counter (“OTC”) swaps with the Fund. Shares of investment companies (other than certain money market funds) may not be posted as collateral under these regulations. Requirements for posting of initial margin in connection with OTC swaps will be phased-in through 2020. In addition, regulations adopted by prudential regulators that will begin to take effect in 2019 will require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives, may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund. Dollar Rolls Risk — Dollar rolls involve the risk that the market value of the securities that the Fund is committed to buy may decline below the price of the securities the Fund has sold. These transactions may involve leverage. Emerging Markets Risk — Emerging markets are riskier than more developed markets because they tend to develop unevenly and may never fully develop. Investments in emerging markets may be considered speculative. Emerging markets are more likely to experience hyperinflation and currency devaluations, which adversely affect returns to U.S. investors. In addition, many emerging securities markets have far lower trading volumes and less liquidity than developed markets. Foreign Securities Risk — Foreign investments often involve special risks not present in U.S. investments that can increase the chances that the Fund will lose money. These risks include: The Fund generally holds its foreign securities and cash in foreign banks and securities depositories, which may be recently organized or new to the foreign custody business and may be subject to only limited or no regulatory oversight. Changes in foreign currency exchange rates can affect the value of the Fund’s portfolio. The economies of certain foreign markets may not compare favorably with the economy of the United States with respect to such issues as growth of gross national product, reinvestment of capital, resources and balance of payments position. The governments of certain countries may prohibit or impose substantial restrictions on foreign investments in their capital markets or in certain industries. Many foreign governments do not supervise and regulate stock exchanges, brokers and the sale of securities to the same extent as does the United States and may not have laws to protect investors that are comparable to U.S. securities laws. Settlement and clearance procedures in certain foreign markets may result in delays in payment for or delivery of securities not typically associated with settlement and clearance of U.S. investments. High Portfolio Turnover Risk — The Fund may engage in active and frequent trading of its portfolio securities. High portfolio turnover (more than 100%) may result in increased transaction costs to the Fund, including brokerage commissions, dealer mark-ups and other transaction costs on the sale of the securities and on reinvestment in other securities. The sale of Fund portfolio securities may result in the realization and/or distribution to shareholders of higher capital gains or losses as compared to a fund with less active trading policies. These effects of higher than normal portfolio turnover may adversely affect Fund performance. Inflation-Indexed Bonds Risk — The principal value of an investment is not protected or otherwise guaranteed by virtue of the Fund’s investments in inflation-indexed bonds. Inflation-indexed bonds are fixed-income securities whose principal value is periodically adjusted according to the rate of inflation. If the index measuring inflation falls, the principal value of inflation-indexed bonds will be adjusted downward, and consequently the interest payable on these securities (calculated with respect to a smaller principal amount) will be reduced. Repayment of the original bond principal upon maturity (as adjusted for inflation) is guaranteed in the case of U.S. Treasury inflation-indexed bonds. For bonds that do not provide a similar guarantee, the adjusted principal value of the bond repaid at maturity may be less than the original principal value. The value of inflation-indexed bonds is expected to change in response to changes in real interest rates. Real interest rates are tied to the relationship between nominal interest rates and the rate of inflation. If nominal interest rates increase at a faster rate than inflation, real interest rates may rise, leading to a decrease in value of inflation-indexed bonds. Short-term increases in inflation may lead to a decline in value. Any increase in the principal amount of an inflation-indexed bond will be considered taxable ordinary income, even though investors do not receive their principal until maturity. Periodic adjustments for inflation to the principal amount of an inflation-indexed bond may give rise to original issue discount, which will be includable in the Fund’s gross income. Due to original issue discount, the Fund may be required to make annual distributions to shareholders that exceed the cash received, which may cause the Fund to liquidate certain investments when it is not advantageous to do so. Also, if the principal value of an inflation-indexed bond is adjusted downward due to deflation, amounts previously distributed in the taxable year may be characterized in some circumstances as a return of capital. Leverage Risk — Some transactions may give rise to a form of economic leverage. These transactions may include, among others, derivatives, and may expose the Fund to greater risk and increase its costs. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet any required asset segregation requirements. Increases and decreases in the value of the Fund’s portfolio will be magnified when the Fund uses leverage. Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund’s investments in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund’s principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquidity risk may be the result of, among other things, the reduced number and capacity of traditional market participants to make a market in fixed-income securities or the lack of an active market. Liquid investments may become illiquid or less liquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid and relatively less liquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. This may be magnified in a rising interest rate environment or other circumstances where investor redemptions from fixed-income mutual funds may be higher than normal. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions. Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Mortgage- and Asset-Backed Securities Risks — Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities. Non-Investment Grade Securities Risk — Although non-investment grade securities generally pay higher rates of interest than investment grade bonds, non-investment grade securities are high risk investments that are considered speculative and may cause income and principal losses for the Fund. Repurchase Agreements and Purchase and Sale Contracts Risk — If the other party to a repurchase agreement or purchase and sale contract defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security in either situation and the market value of the security declines, the Fund may lose money. Reverse Repurchase Agreements Risk — Reverse repurchase agreements involve the sale of securities held by the Fund with an agreement to repurchase the securities at an agreed-upon price, date and interest payment. Reverse repurchase agreements involve the risk that the other party may fail to return the securities in a timely manner or at all. The Fund could lose money if it is unable to recover the securities and the value of the collateral held by the Fund, including the value of the investments made with cash collateral, is less than the value of the securities. These events could also trigger adverse tax consequences to the Fund. Subsidiary Risk — By investing in the Subsidiary, the Fund is indirectly exposed to the risks associated with the Subsidiary’s investments. The commodity-related instruments held by the Subsidiary are generally similar to those that are permitted to be held by the Fund and are subject to the same risks that apply to similar investments if held directly by the Fund (see “Commodities Related Investments Risks” above). There can be no assurance that the investment objective of the Subsidiary will be achieved. The Subsidiary is not registered under the Investment Company Act of 1940, as amended (the “Investment Company Act”), and, unless otherwise noted in this prospectus, is not subject to all the investor protections of the Investment Company Act. However, the Fund wholly owns and controls the Subsidiary, and the Fund and the Subsidiary are both managed by BlackRock, making it unlikely that the Subsidiary will take action contrary to the interests of the Fund and its shareholders. The Board has oversight responsibility for the investment activities of the Fund, including its investment in the Subsidiary, and the Fund’s role as sole shareholder of the Subsidiary. The Subsidiary is subject to the same investment restrictions and limitations, and follows the same compliance policies and procedures, as the Fund. Changes in the laws of the United States and/or the Cayman Islands could result in the inability of the Fund and/or the Subsidiary to operate as described in this prospectus and the Statement of Additional Information and could adversely affect the Fund. In order to qualify as a regulated investment company, at least 90% of the Fund’s gross income for the taxable year must be “qualifying income.” The Fund anticipates treating the income and gain generated from investments in controlled foreign subsidiaries that invest in physical commodities and/or commodity-linked derivative instruments as “qualifying income” for regulated investment company qualification purposes. However, there can be no assurance that the Internal Revenue Service (“IRS”) will agree with treating such income and gain as “qualifying income.” If the IRS makes an adverse determination relating to the treatment of such income and gain, the Fund would likely need to change its investment strategies, which could adversely affect the Fund. The IRS has proposed regulations that, if finalized in current form, would specify that a subpart F income inclusion for U.S. federal income tax purposes will be treated as qualifying income only to the extent that the Subsidiary makes distributions out of its earnings and profits in the same taxable year. U.S. Government Issuer Risk — Treasury obligations may differ in their interest rates, maturities, times of issuance and other characteristics. Obligations of U.S. Government agencies and authorities are supported by varying degrees of credit but generally are not backed by the full faith and credit of the U.S. Government. No assurance can be given that the U.S. Government will provide financial support to its agencies and authorities if it is not obligated by law to do so.
Risk Lose Money [Text]	rr_RiskLoseMoney	You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	Since Class R Shares of the Fund have no performance history, the chart and table below give you a picture of the Fund’s long-term performance for Investor A Shares. Although the chart and table show returns for the Investor A Shares which are not offered in this Prospectus, the Investor A Shares would have substantially similar annual returns as the Class R Shares offered in this Prospectus because the Investor A Shares and the Class R Shares are invested in the same portfolio of securities and the annual returns would differ only to the extent that the Investor A Shares and the Class R Shares do not have the same expenses. The actual returns of Class R Shares would have been lower than those of Investor A Shares because Class R Shares have higher expenses than Investor A Shares. Investor A Shares of the Fund are estimated to have expenses of 0.64% of average daily net assets (after waivers and reimbursements) for the current fiscal year and Class R Shares of the Fund are expected to have expenses of 0.89% of average daily net assets (after waivers and reimbursements) for the current fiscal year. The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to that of the Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance, including its current net asset value, can be obtained by visiting http://www.blackrock.com or can be obtained by phone at 800-882-0052.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance One Year or Less [Text]	rr_PerformanceOneYearOrLess	Since Class R Shares of the Fund have no performance history, the chart and table below give you a picture of the Fund’s long-term performance for Investor A Shares.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	800-882-0052
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	http://www.blackrock.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading]	rr_BarChartHeading	Investor A Shares ANNUAL TOTAL RETURNS Inflation Protected Bond Portfolio As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text]	rr_BarChartDoesNotReflectSalesLoads	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 5.80% (quarter ended March 31, 2008) and the lowest return for a quarter was -6.63% (quarter ended June 30, 2013).
Bar Chart, Returns for Class Not Offered in Prospectus [Text]	rr_BarChartReturnsForClassNotOfferedInProspectus	Since Class R Shares of the Fund have no performance history, the chart and table below give you a picture of the Fund’s long-term performance for Investor A Shares. Although the chart and table show returns for the Investor A Shares which are not offered in this Prospectus, the Investor A Shares would have substantially similar annual returns as the Class R Shares offered in this Prospectus because the Investor A Shares and the Class R Shares are invested in the same portfolio of securities and the annual returns would differ only to the extent that the Investor A Shares and the Class R Shares do not have the same expenses.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/17 Average Annual Total Returns
Performance Table Does Reflect Sales Loads	rr_PerformanceTableDoesReflectSalesLoads	However, the table includes all applicable fees and sales charges.
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Performance Table Narrative	rr_PerformanceTableNarrativeTextBlock	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor’s tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Class R | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Class R Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.24%	[1],[2]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.50%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.05%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.38%	[3]
Other Expenses of the Subsidiary	rr_Component3OtherExpensesOverAssets		[4]
Other Expenses	rr_OtherExpensesOverAssets	0.43%	[3]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.17%	[5]
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.28%)	[1],[5]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.89%	[1],[5]
1 Year	rr_ExpenseExampleYear01	$ 91
3 Years	rr_ExpenseExampleYear03	315
5 Years	rr_ExpenseExampleYear05	588
10 Years	rr_ExpenseExampleYear10	$ 1,369
Class R | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
2008	rr_AnnualReturn2008	0.23%
2009	rr_AnnualReturn2009	10.15%
2010	rr_AnnualReturn2010	5.82%
2011	rr_AnnualReturn2011	11.47%
2012	rr_AnnualReturn2012	6.62%
2013	rr_AnnualReturn2013	(8.34%)
2014	rr_AnnualReturn2014	2.27%
2015	rr_AnnualReturn2015	(2.43%)
2016	rr_AnnualReturn2016	3.89%
2017	rr_AnnualReturn2017	2.75%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2008
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	5.80%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2013
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(6.63%)
1 Year	rr_AverageAnnualReturnYear01	(1.36%)
5 Years	rr_AverageAnnualReturnYear05	(1.29%)
10 Years	rr_AverageAnnualReturnYear10	2.67%
Class R | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Return After Taxes on Distributions | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(2.23%)
5 Years	rr_AverageAnnualReturnYear05	(2.05%)
10 Years	rr_AverageAnnualReturnYear10	1.62%
Class R | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Return After Taxes on Distributions and Sale of Fund Shares | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	(0.78%)
5 Years	rr_AverageAnnualReturnYear05	(1.29%)
10 Years	rr_AverageAnnualReturnYear10	1.69%
Class R | BLACKROCK INFLATION PROTECTED BOND PORTFOLIO | Bloomberg Barclays U.S. Treasury Inflation Protected Securities Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	3.01%
5 Years	rr_AverageAnnualReturnYear05	0.13%
10 Years	rr_AverageAnnualReturnYear10	3.53%

[1] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 29, BlackRock Advisors, LLC (“BlackRock”) has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates that have a contractual management fee, through April 30, 2020. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of BlackRock Funds II (the “Trust”) or by a vote of a majority of the outstanding voting securities of the Fund.

[2] The Management Fee has been restated to reflect current fees.

[3] Other Expenses are based on estimated amounts for the current fiscal year.

[4] Other Expenses of Cayman Inflation Protected Bond Portfolio, Ltd. (the “Subsidiary”) were less than 0.01% for the most recent fiscal year.

[5] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 29, BlackRock has contractually agreed to waive and/or reimburse fees or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.84% of average daily net assets through April 30, 2020. The Fund may have to repay some of these waivers and/or reimbursements to BlackRock in the two years following such waivers and/or reimbursements. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or by a vote of a majority of the outstanding voting securities of the Fund.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BlackRock Funds II
Prospectus Date	rr_ProspectusDate	Apr. 30, 2018
Document Creation Date	dei_DocumentCreationDate	Apr. 30, 2018