Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BlackRock Funds III
Prospectus Date	rr_ProspectusDate	Apr. 30, 2013
Investor A, Institutional | BlackRock ACWI ex-US Index Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock ACWI ex-US Index Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of BlackRock ACWI ex-US Index Fund (“ACWI ex-US Index Fund” or the “Fund”), a series of BlackRock Funds III (the “Trust”), is to match the performance of the MSCI All Country World ex-US Index (the “MSCI ACWI ex-US Index”) in U.S. dollars with net dividends as closely as possible before the deduction of Fund expenses.
Expense [Heading]	rr_ExpenseHeading	Fees and Expenses of the Fund
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The table below describes the fees and expenses that you may pay if you buy and hold shares of the ACWI ex-US Index 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	May 1, 2014
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover:
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	The Master Portfolio pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover 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 Master Portfolio’s portfolio turnover rate was 42% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	42.00%
Expenses Represent Both Master and Feeder [Text]	rr_ExpensesRepresentBothMasterAndFeeder	The fees and expenses shown in the table and the example that follows include both the expenses of the Fund and its share of the allocated expenses of ACWI ex-US Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio (“MIP”). The management fees are paid by the Master Portfolio.
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Other Expenses and Miscellaneous Other Expenses have been restated to reflect current fees.
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]	rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees	The Total Annual Fund Operating Expenses do not correlate to the ratio of expenses to average net assets given in the Fund’s most recent annual report which does not include the Acquired Fund Fees and Expenses or the restatement of Other Expenses and Miscellaneous Other Expenses 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 shares of the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in shares of 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 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	The ACWI ex-US Index Fund employs a “passive” management approach, attempting to invest in a portfolio of assets whose performance is expected to match approximately the performance of the MSCI ACWI ex-US Index. The Fund will be substantially invested in equity securities in the MSCI ACWI ex-US Index, and will invest, under normal circumstances, at least 80% of its assets in securities or other financial instruments that are components of or have economic characteristics similar to the securities included in the MSCI ACWI ex-US Index. Equity securities consist primarily of common stock, preferred stock, securities convertible into common stock and securities or other instruments whose price is linked to the value of common stock. The Fund will invest in the common stocks represented in the MSCI ACWI ex-US Index in roughly the same proportions as their weightings in the MSCI ACWI ex-US Index. The MSCI ACWI ex-US Index is a free float-adjusted market capitalization index designed to measure the combined equity market performance of developed and emerging market countries, excluding the United States. The component stocks have a market capitalization between $190 million and $416 billion as of March 29, 2013. The Fund may also engage in futures transactions. At times, the Fund may not invest in all of the common stocks in the MSCI ACWI ex-US Index, or in the same weightings as in the MSCI ACWI ex-US Index. At those times, the Fund chooses investments so that the market capitalizations, industry weightings and other fundamental characteristics of the stocks chosen are similar to the MSCI ACWI ex-US Index as a whole. The Fund may lend securities with a value up to 33 1/3% of its total assets to financial institutions that provide cash or securities issued or guaranteed by the U.S. Government as collateral. The Fund will concentrate its investments (i.e., hold 25% or more of its total assets) in a particular industry or group of industries to approximately the same extent that the MSCI ACWI ex-US Index is concentrated. The Fund is a non-diversified fund, which means that it can invest more of its assets in fewer companies than a diversified fund. The Fund is a “feeder” fund that invests all of its assets in the Master Portfolio, which has the same investment objective and strategies as the Fund. All investments are made at the Master Portfolio level. This structure is sometimes called a “master/feeder” structure. The Fund’s investment results will correspond directly to the investment results of the Master Portfolio. For simplicity, this prospectus uses the terms “ACWI ex-US Index Fund” or “Fund” to include the Master Portfolio.
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 ACWI ex-US Index 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. Concentration Risk — To the extent the Fund concentrates in a particular industry, it may be more susceptible to economic conditions and risks affecting that industry. Convertible Securities Risk — The market value of a convertible security performs like that of a regular debt security; that is, if market interest rates rise, the value of a convertible security usually falls. In addition, convertible securities are subject to the risk that the issuer will not be able to pay interest or dividends when due, and their market value may change based on changes in the issuer’s credit rating or the market’s perception of the issuer’s creditworthiness. Since it derives a portion of its value from the common stock into which it may be converted, a convertible security is also subject to the same types of market and issuer risks that apply to the underlying common stock. 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. Equity Securities Risk — Stock markets are volatile. The price of equity securities fluctuates based on changes in a company’s financial condition and overall market and economic conditions. 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. — The European financial markets have recently experienced volatility and adverse trends due to concerns about economic downturns in, or rising government debt levels of, several European countries. These events may spread to other countries in Europe, including countries that do not use the Euro. These events may affect the value and liquidity of certain of the Fund’s investments. Futures Risk — The Fund’s use of futures may reduce the Fund’s returns. In these transactions, the Fund is subject to liquidity risk and correlation risk (i.e., that fluctuations in a future’s value may not correlate perfectly with the change in market value of the instruments held by the Fund). Index Fund Risk — An index fund has operating and other expenses while an index does not. As a result, while the Fund will attempt to track the MSCI ACWI ex-US Index as closely as possible, it will tend to underperform the 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. 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 indexes or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. Mid-Cap Securities Risk — The securities of mid-cap companies generally trade in lower volumes and are generally subject to greater and less predictable price changes than the securities of larger capitalization companies. Non-Diversification Risk — The Fund is a non-diversified fund. Because the Fund may invest in securities of a smaller number of issuers, it may be more exposed to the risks associated with and developments affecting an individual issuer than a fund that invests more widely. 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. Small Cap and Emerging Growth Securities Risk — Small cap or emerging growth companies may have limited product lines or markets. They may be less financially secure than larger, more established companies. They may depend on a more limited management group than larger capitalized companies. Tracking Error Risk — The performance of the Fund may diverge from that of the MSCI ACWI ex-US Index.
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.
Risk Nondiversified Status [Text]	rr_RiskNondiversifiedStatus	Non-Diversification Risk — The Fund is a non-diversified fund. Because the Fund may invest in securities of a smaller number of issuers, it may be more exposed to the risks associated with and developments affecting an individual issuer than a fund that invests more widely.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance Information
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The information shows you how the performance of ACWI ex-US Index Fund has varied for the periods since inception and provides some indication of the risks of investing in the Fund. The bar chart shows the returns for Investor A Shares of the Fund for the one complete calendar year since the commencement of the Fund’s operations. The table compares the Fund’s performance to that of the MSCI ACWI ex-US Index. 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 BFA, BAL and their 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 can be obtained by visiting http://www.blackrock.com/funds 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 performance of ACWI ex-US Index Fund has varied for the periods since inception 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/funds
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 BlackRock ACWI ex-US Index Fund 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 period shown in the bar chart, the highest return for a quarter was 11.27% (quarter ended March 31, 2012) and the lowest return for a quarter was –7.76% (quarter ended June 30, 2012). The year-to-date return for the quarter ended March 31, 2013 is 2.33%.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/12 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 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 Institutional Shares will vary.
Investor A, Institutional | BlackRock ACWI ex-US Index Fund | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.15%	[1]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%	[1]
Administration Fees	rr_Component1OtherExpensesOverAssets	0.10%	[1]
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.90%	[1],[2],[3]
Other Expenses	rr_OtherExpensesOverAssets	1.00%	[1],[2],[3]
Acquired Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.02%	[1],[3]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.42%	[1],[3]
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.75%)	[1],[4]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.67%	[1],[4]
1 Year	rr_ExpenseExampleYear01	68
3 Years	rr_ExpenseExampleYear03	375
5 Years	rr_ExpenseExampleYear05	705
10 Years	rr_ExpenseExampleYear10	1,637
2012	rr_AnnualReturn2012	16.31%
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Mar. 31, 2013
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	2.33%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Mar. 31, 2012
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	11.27%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2012
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(7.76%)
1 Year	rr_AverageAnnualReturnYear01	16.31%
Since Inception	rr_AverageAnnualReturnSinceInception	(3.18%)
Inception Date	rr_AverageAnnualReturnInceptionDate	Jun. 30, 2011
Investor A, Institutional | BlackRock ACWI ex-US Index Fund | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.15%	[1]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none	[1]
Administration Fees	rr_Component1OtherExpensesOverAssets	0.10%	[1]
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.45%	[1],[2],[3]
Other Expenses	rr_OtherExpensesOverAssets	0.55%	[1],[2],[3]
Acquired Fund Fees and Expenses	rr_AcquiredFundFeesAndExpensesOverAssets	0.02%	[1],[3]
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.72%	[1],[3]
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.30%)	[1],[4]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.42%	[1],[4]
1 Year	rr_ExpenseExampleYear01	43
3 Years	rr_ExpenseExampleYear03	200
5 Years	rr_ExpenseExampleYear05	371
10 Years	rr_ExpenseExampleYear10	866
1 Year	rr_AverageAnnualReturnYear01	16.61%
Since Inception	rr_AverageAnnualReturnSinceInception	(2.97%)
Inception Date	rr_AverageAnnualReturnInceptionDate	Jun. 30, 2011
Investor A, Institutional | BlackRock ACWI ex-US Index Fund | Return After Taxes on Distributions | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	15.57%
Since Inception	rr_AverageAnnualReturnSinceInception	(3.56%)
Investor A, Institutional | BlackRock ACWI ex-US Index Fund | Return After Taxes on Distributions and Sale of Shares | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	11.53%
Since Inception	rr_AverageAnnualReturnSinceInception	(2.64%)
Investor A, Institutional | BlackRock ACWI ex-US Index Fund | MSCI ACWI ex-US Index (reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	16.83%
Since Inception	rr_AverageAnnualReturnSinceInception	(1.92%)

[1] The fees and expenses shown in the table and the example that follows include both the expenses of the Fund and its share of the allocated expenses of ACWI ex-US Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio (“MIP”). The management fees are paid by the Master Portfolio.

[2] Other Expenses and Miscellaneous Other Expenses have been restated to reflect current fees.

[3] The Total Annual Fund Operating Expenses do not correlate to the ratio of expenses to average net assets given in the Fund’s most recent annual report which does not include the Acquired Fund Fees and Expenses or the restatement of Other Expenses and Miscellaneous Other Expenses to reflect current fees.

[4] As described in the “Management of the Funds” section of the Fund’s prospectus on pages 30-34, BlackRock Fund Advisors (“BFA”), the investment adviser for the Master Portfolio, and BlackRock Advisors, LLC (“BAL”), the administrator for the Fund, have contractually agreed to waive and/or reimburse fees and/or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements of the Fund (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) as a percentage of average daily net assets to 0.65% for Investor A Shares and 0.40% for Institutional Shares until May 1, 2014. The Fund may have to repay some of these waivers and reimbursements to BFA and BAL in the following two years. This agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of the Trust or of MIP, as applicable, or by a vote of a majority of the outstanding voting securities of the Fund or the Master Portfolio, as applicable.