Label	Element	Value
Investor A, Institutional | iShares Municipal Bond Index Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About iShares Municipal Bond Index Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of iShares Municipal 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 the aggregate, as represented by the ICE BofA US Municipal Securities Index (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, hold and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to your financial professional or your selected securities dealer, broker, investment adviser, service provider or industry professional (including BlackRock Fund Advisors (“BFA”) and its affiliates) (each, a “Financial Intermediary”), which are not reflected in the table and example below.
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, 2023
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 8% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	8.00%
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 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	The Fund seeks to track the investment results of the Underlying Index, which tracks the performance of U.S. dollar denominated investment grade tax-exempt debt publicly issued by U.S. states and territories, and their political subdivisions, in the U.S. domestic market. Qualifying securities must have at least one year remaining term to final maturity, at least 18 months to final maturity at the time of issuance, a fixed coupon schedule and an investment grade rating (based on an average of Moody’s, S&P and Fitch). Minimum size requirements vary based on the initial term to final maturity at time of issuance. The Underlying Index contains 1,562 issuers as of March 31, 2021 and was created on December 31, 1988. BFA uses a representative sampling indexing strategy to manage the Fund. “Representative sampling” is an indexing strategy that involves investing in a representative sample of securities that collectively has an investment profile similar to that of the Underlying Index. The securities selected are expected to have, in the aggregate, investment characteristics (based on factors such as market capitalization and industry weightings), fundamental characteristics (such as return variability and yield) and liquidity measures similar to those of the Underlying Index. The Fund may or may not hold all of the securities in the Underlying Index. The Fund generally invests at least 80% of its assets, plus the amount of any borrowings for investment purposes, in securities or other financial instruments that are components of or have economic characteristics similar to the securities in the Underlying Index. The Fund may invest a portion of the remainder of its assets in securities not included in the Underlying Index, but which BFA believes will help the Fund track the Underlying Index.
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 order of the below risk factors does not indicate the significance of any particular risk factor. ■ Asset Class Risk — Securities and other assets or financial instruments in the Underlying Index or in the Fund’s portfolio may underperform in comparison to the general financial markets, a particular financial market or other asset classes. ■ 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. ■ Income Risk— The Fund’s income may decline due to a decline in inflation, deflation or changes in inflation expectations. ■ 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 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. ■ 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. ■ Indexed Securities Risk — Indexed securities provide a potential return based on a particular index of value or interest rates. The Fund’s return on these securities will be subject to risk with respect to the value of the particular index. These securities are subject to leverage risk and correlation risk. Certain indexed securities have greater sensitivity to changes in interest rates or index levels than other securities, and the Fund’s investment in such instruments may decline significantly in value if interest rates or index levels move in a way Fund management does not anticipate. ■ Management Risk — As the Fund may not fully replicate the Underlying Index, it is subject to the risk that BFA’s investment strategy may not produce the intended results. ■ 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. ■ Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:   General Obligation Bonds Risks — Timely payments depend on the issuer’s credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.   Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.   Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment.   Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.   Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.   Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.   Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers’ bond counsel and, in the case of derivative securities, sponsors’ counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities. ■ 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. ■ 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. ■ Tracking Error Risk — The Fund may be subject to tracking error, which is the divergence of the Fund’s performance from that of the Underlying Index. Tracking error may occur because of differences between the securities and other instruments held in the Fund’s portfolio and those included in the Underlying Index, pricing differences (including, as applicable, differences between a security’s price at the local market close and the Fund’s valuation of a security at the time of calculation of the Fund’s net asset value), differences in transaction costs, the Fund’s holding of uninvested cash, differences in timing of the accrual of or the valuation of dividends or other distributions, interest, the requirements to maintain pass-through tax treatment, portfolio transactions carried out to minimize the distribution of capital gains to shareholders, changes to the Underlying Index and the cost to the Fund of complying with various new or existing regulatory requirements. These risks may be heightened during times of increased market volatility or other unusual market conditions. In addition, tracking error may result because the Fund incurs fees and expenses, while the 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	The Fund adopted the performance of State Farm Tax Advantaged Bond Fund (the “Predecessor Fund”), a series of State Farm Mutual Fund Trust, as a result of a reorganization (the “Reorganization”) that was completed on November 19, 2018 in which the Fund acquired all of the assets and assumed certain stated liabilities of the Predecessor Fund. The returns presented for the Fund for the periods prior to November 19, 2018 reflect the performance of the Predecessor Fund. Performance shown below for the periods prior to November 19, 2018 is based on the investment objective and investment strategies utilized by the Predecessor Fund, which differed from those of the Fund. The Predecessor Fund was managed by a different investment adviser and had a different portfolio management team from that of the Fund. The returns presented for Investor A Shares of the Fund for the periods prior to November 19, 2018 reflect the performance of Premier Shares of the Predecessor Fund. The performance information for Institutional Shares prior to November 19, 2018, the commencement of operations of Institutional Shares, shows the performance of the Fund’s Investor A Shares, which adopted the performance of the Premier Shares of the Predecessor Fund. The performance of the Fund’s Institutional Shares would be substantially similar to Investor A Shares because Institutional Shares and Investor A Shares invest in the same portfolio of securities and performance would only differ to the extent that Institutional Shares and Investor A Shares have different expenses. The actual returns of Institutional Shares would have been higher than those of the Investor A Shares because Institutional Shares have lower expenses than the Investor A Shares. 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 Underlying 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 investment adviser to the Fund, the investment adviser to the Predecessor Fund and their affiliates had not waived or reimbursed certain Predecessor Fund or 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 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	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 iShares Municipal Bond Index Fund 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 3.68% (quarter ended September 30, 2011) and the lowest return for a quarter was -3.78% (quarter ended June 30, 2013).
Performance Table Heading	rr_PerformanceTableHeading	For the periods ended 12/31/20Average 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 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 | iShares Municipal Bond Index Fund | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.10%	[1]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other Expenses	rr_OtherExpensesOverAssets	0.15%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.50%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	none	[1],[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.50%	[1],[2]
1 Year	rr_ExpenseExampleYear01	$ 51
3 Years	rr_ExpenseExampleYear03	160
5 Years	rr_ExpenseExampleYear05	280
10 Years	rr_ExpenseExampleYear10	$ 628
2011	rr_AnnualReturn2011	10.70%
2012	rr_AnnualReturn2012	4.98%
2013	rr_AnnualReturn2013	(3.46%)
2014	rr_AnnualReturn2014	8.76%
2015	rr_AnnualReturn2015	2.80%
2016	rr_AnnualReturn2016	0.48%
2017	rr_AnnualReturn2017	3.83%
2018	rr_AnnualReturn2018	1.24%
2019	rr_AnnualReturn2019	7.24%
2020	rr_AnnualReturn2020	4.70%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2011
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	3.68%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2013
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(3.78%)
1 Year	rr_AverageAnnualReturnYear01	4.70%
5 Years	rr_AverageAnnualReturnYear05	3.46%
10 Years	rr_AverageAnnualReturnYear10	4.05%
Investor A, Institutional | iShares Municipal Bond Index Fund | Institutional Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.10%	[1]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.13%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.23%
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	none	[1],[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.23%	[1],[2]
1 Year	rr_ExpenseExampleYear01	$ 24
3 Years	rr_ExpenseExampleYear03	74
5 Years	rr_ExpenseExampleYear05	130
10 Years	rr_ExpenseExampleYear10	$ 293
1 Year	rr_AverageAnnualReturnYear01	4.97%
5 Years	rr_AverageAnnualReturnYear05	3.57%
10 Years	rr_AverageAnnualReturnYear10	4.10%
Investor A, Institutional | iShares Municipal Bond Index Fund | Return After Taxes on Distributions | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	4.66%
5 Years	rr_AverageAnnualReturnYear05	3.37%
10 Years	rr_AverageAnnualReturnYear10	4.00%
Investor A, Institutional | iShares Municipal Bond Index Fund | Return After Taxes on Distributions and Sale of Fund Shares | Investor A Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	3.73%
5 Years	rr_AverageAnnualReturnYear05	3.22%
10 Years	rr_AverageAnnualReturnYear10	3.76%
Investor A, Institutional | iShares Municipal Bond Index Fund | ICE BofA US Municipal Securities Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	5.26%
5 Years	rr_AverageAnnualReturnYear05	3.94%
10 Years	rr_AverageAnnualReturnYear10	4.79%

[1] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 32, BFA 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 BFA or its affiliates that have a contractual management fee, through June 30, 2023. In addition, BFA has contractually agreed to waive its management fees by the amount of investment advisory fees the Fund pays to BFA indirectly through its investment in money market funds managed by BFA or its affiliates, through June 30, 2023. The contractual agreements 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] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 32, BFA 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 0.50% of average daily net assets (for Investor A Shares) and 0.25% of average daily net assets (for Institutional Shares) through June 30, 2023. The Fund may have to repay some of these waivers and/or reimbursements to BFA in the following two years, and such repayment arrangement will terminate on November 16, 2025. 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.