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iShares U.S. Long Government Bond Index Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Fund Overview Key Facts About iShares U.S. Long Government Bond Index Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
The investment objective of iShares U.S. Long Government Bond Index Fund (the “Fund”), a series of BlackRock FundsSM (the “Trust”), is to seek to provide investment results that correspond to the total return performance of fixed-income securities, in aggregate, as represented by the Bloomberg U.S. Long Government Bond Index (or the “Underlying Index”).
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.
 
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 June 30, 2033
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. There has been no portfolio turnover because the Fund has not commenced operations as of the date of this prospectus.
Other Expenses, New Fund, Based on Estimates [Text] rr_OtherExpensesNewFundBasedOnEstimates Other Expenses are based on estimated amounts for the Fund’s current fiscal year.
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, at least 90% of the value of the Fund’s assets, plus the amount of any borrowings for investment purposes, is invested in securities comprising the Bloomberg U.S. Long Government Bond Index, which, for the Fund, are considered bonds issued or guaranteed by the U.S. Government and its agencies or instrumentalities.
  
The Fund attempts to achieve, in both rising and falling markets, a correlation of at least 95% between the total return of its net assets before fees and expenses and the total return of the Fund’s benchmark index, the Bloomberg U.S. Long Government Bond Index. Notwithstanding the factors described below, perfect (100%) correlation would be achieved if the total return of the Fund’s net assets, before fees and expenses, increased or decreased exactly as the total return of the Fund’s benchmark index increased or decreased. The Fund’s ability to match its investment performance to the investment performance of its benchmark index may be affected by, among other things, the Fund’s expenses, the amount of cash and cash equivalents held by the Fund, the manner in which the total return of the Fund’s benchmark index is calculated, the size of the Fund’s investment portfolio, and the timing, frequency and size of shareholder purchases and redemptions. 
The Bloomberg U.S. Long Government Bond Index is a capitalization weighted index that measures US dollar-denominated, fixed-rate, nominal US Treasuries and US agency debentures (securities issued by US government owned or government sponsored entities, and debt explicitly guaranteed by the US government) with a maturity greater than 10 years. The index is rebalanced monthly. There were approximately 100 fixed-income securities included in the Bloomberg U.S. Long Government Bond Index as of December 31, 2021. All securities in the Bloomberg U.S. Long Government Bond Index are rated investment-grade. 
The Fund shall be invested and reinvested primarily in U.S. debt securities with maturities greater than ten years, with the objective of approximating as closely as practicable the total rate of return of the Bloomberg U.S. Long Government Bond Index. The dollar-weighted average maturity of the Fund’s portfolio, under normal circumstances, is expected to be more than ten years. 
The Fund utilizes sampling techniques that are designed to allow the Fund to duplicate substantially the investment performance of the Bloomberg U.S. Long Government Bond Index . However, the Fund is not expected to track the Bloomberg U.S. Long Government Bond Index with the same degree of accuracy that complete replication of the Bloomberg U.S. Long Government Bond Index would provide. No attempt is made to manage the Fund using economic, financial or market analysis. In addition, at times, the portfolio composition of the Fund may be altered (or “rebalanced”) to reflect changes in the characteristics of the index that the Fund tracks. 
The Fund also may engage in futures, forwards and options transactions and other derivative securities transactions and lend its portfolio securities, each of which involves risk. The Fund may use futures contracts, forwards, options and other derivative transactions to manage its short-term liquidity and/or as substitutes for comparable market positions in the securities in its benchmark index. The Fund may also invest in high-quality money market instruments, including shares of money market funds advised by BFA or its affiliates. 
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 the principal risks of investing in the Fund. The relative significance of each risk factor below may change over time and you should review each risk factor carefully.
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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%. (Duration is a measure of the price sensitivity of a debt security or portfolio of debt securities to relative changes in interest rates.) 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. 
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Index-Related Risk — There is no guarantee that the Fund’s investment results will have a high degree of correlation to those of the Underlying Index or that the Fund will achieve its investment objective. Market disruptions or high volatility, other unusual market circumstances and regulatory restrictions could have an adverse effect on the Fund’s ability to adjust its exposure to the required levels in order to track the Underlying Index. Errors in index data, index computations or the construction of the Underlying Index in accordance with its methodology may occur from time to time and may not be identified and corrected by the index provider for a period of time or at all, which may have an adverse impact on the Fund and its shareholders. Unusual market conditions may cause the index provider to postpone a scheduled rebalance, which could cause the Underlying Index to vary from its normal or expected composition. 
      An index fund has operating and other expenses while an index does not. As a result, while the Fund will attempt to track the Underlying Index as closely as possible, it will tend to underperform the Underlying Index to some degree over time. If an index fund is properly correlated to its stated index, the fund will perform poorly when the index performs poorly. 
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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. 
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Passive Investment Risk — Because BFA does not select individual companies in the index that the Fund tracks, the Fund may hold securities of companies that present risks that an investment adviser researching individual securities might seek to avoid. 
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Concentration Risk — The Fund reserves the right to concentrate its investments (i.e., invest 25% or more of its total assets in securities of issuers in a particular industry) to approximately the same extent that the Underlying Index concentrates in a particular industry. To the extent the Fund concentrates in a particular industry, it may be more susceptible to economic conditions and risks affecting that industry. 
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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 Illiquidity 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 (the “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, with respect to uncleared swaps, swap dealers are required to collect variation margin from the Fund and may be required by applicable regulations to collect initial margin from the Fund. Both initial and variation margin may be comprised of cash and/or securities, subject to applicable regulatory haircuts. Shares of investment companies (other than certain money market funds) may not be posted as collateral under applicable regulations. In addition, regulations adopted by global prudential regulators that are now in effect 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. 
      On October 28, 2020, the Securities and Exchange Commission adopted new regulations governing the use of derivatives by registered investment companies (“Rule 18f‑4”). The Fund will be required to implement and comply with Rule 18f‑4 by August 19, 2022. Once implemented, Rule 18f‑4 will impose limits on the amount of derivatives a fund can enter into, eliminate the asset segregation framework currently used by funds to comply with Section 18 of the Investment Company Act of 1940, as amended (the “Investment Company Act”), treat derivatives as senior securities and require funds whose use of derivatives is more than a limited specified exposure amount to establish and maintain a comprehensive derivatives risk management program and appoint a derivatives risk manager. 
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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. The value of a security or other asset may decline due to changes in general market conditions, economic trends or events that are not specifically related to the issuer of the security or other asset, or factors that affect a particular issuer or issuers, exchange, country, group of countries, region, market, industry, group of industries, sector or asset class. Local, regional or global events such as war, acts of terrorism, the spread of infectious illness or other public health issues like pandemics or epidemics, recessions, or other events could have a significant impact on the Fund and its investments. 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. 
      A recent outbreak of an infectious coronavirus has developed into a global pandemic that has resulted in numerous disruptions in the market and has had significant economic impact leaving general concern and uncertainty. The impact of this coronavirus, and other epidemics and pandemics that may arise in the future, could affect the economies of many nations, individual companies and the market in general ways that cannot necessarily be foreseen at the present time. 
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Money Market Securities Risk — If market conditions improve while the Fund has invested some or all of its assets in high quality money market securities, this strategy could result in reducing the potential gain from the market upswing, thus reducing the Fund’s opportunity to achieve its investment objective. 
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Representative Sampling Risk — Representative sampling is a method of indexing that involves investing in a representative sample of securities that collectively have a similar investment profile to the Underlying Index and resemble the Underlying Index in terms of risk factors and other key characteristics. The Fund may or may not hold every security in the Underlying Index. When the Fund deviates from a full replication indexing strategy to utilize a representative sampling strategy, the Fund is subject to an increased risk of tracking error, in that the securities selected in the aggregate for the Fund may not have an investment profile similar to those of the Underlying Index. 
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Securities Lending Risk — Securities lending involves the risk that the borrower may fail to return the securities in a timely manner or at all. As a result, the Fund may lose money and there may be a delay in recovering the loaned securities. The Fund could also lose money if it does not recover the securities and/or the value of the collateral falls, including the value of investments made with cash collateral. These events could trigger adverse tax consequences for the Fund. 
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Tracking Error Risk — Imperfect correlation between a passively managed fund’s portfolio securities and those in its index, rounding of prices, the timing of cash flows, the fund’s size, changes to the index and regulatory requirements may cause tracking error, which is the divergence of a fund’s performance from that of its underlying index. This risk may be heightened during times of increased market volatility or other unusual market conditions. Tracking error also may result because a fund incurs fees and expenses while its underlying index does not. 
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
Because the Fund has not commenced operations as of the date of this prospectus, it does not have performance information an investor would find useful in evaluating the risks of investing in the Fund. The Fund’s benchmark is the Bloomberg U.S. Long Government Bond Index.
Performance One Year or Less [Text] rr_PerformanceOneYearOrLess Because the Fund has not commenced operations as of the date of this prospectus, it does not have performance information an investor would find useful in evaluating the risks of investing in the Fund.
iShares U.S. Long Government Bond Index Fund | iShares U.S. Long Government Bond Index Fund  
Risk/Return: rr_RiskReturnAbstract  
Management Fee rr_ManagementFeesOverAssets 0.05% [1]
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Administration Fee rr_Component1OtherExpensesOverAssets 0.05% [1]
Independent Expenses rr_Component2OtherExpensesOverAssets [2]
Other Expenses rr_OtherExpensesOverAssets 0.05% [3]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.10%
Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.10%) [1],[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets none [1],[2]
1 Year rr_ExpenseExampleYear01 none
3 Years rr_ExpenseExampleYear03 none
[1]
As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 46, BlackRock Fund Advisors (“BFA”) and BlackRock Advisors, LLC (“BAL”), the Fund’s administrator, have contractually agreed to waive their management fee and administration fee, as applicable, through June 30, 2033. On July 1 of each year, the waiver agreement will renew automatically for an additional one year so that the agreement will have a perpetual ten-year term. 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.
[2]
Independent Expenses consist of the Fund’s allocable portion of the fees and expenses of the independent trustees of the Trust, counsel to such independent trustees and the independent registered public accounting firm that provides audit services to the Fund. BAL and BFA have contractually agreed to reimburse, or provide offsetting credits to, the Fund for Independent Expenses through June 30, 2033. On July 1 of each year, the waiver agreement will renew automatically for an additional one year so that the agreement will have a perpetual ten-year term. After giving effect to such contractual arrangements, Independent Expenses will be 0.00%. Such contractual arrangements 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.
[3]
Other Expenses are based on estimated amounts for the Fund’s current fiscal year.