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Class K | BlackRock Short Obligations Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Fund Overview  Key Facts About BlackRock Short Obligations Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
The investment objective of BlackRock Short Obligations Fund (“Short Obligations Fund” or the “Fund”), a series of BlackRock FundsSM (the “Trust”), is to seek current income consistent with preservation of capital.
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 Class K Shares of Short Obligations 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 Advisors, LLC (“BlackRock”) 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. For the fiscal year ended July 31, 2021, the Fund’s portfolio turnover rate was 67% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 67.00%
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 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 market conditions, Short Obligations Fund will invest in U.S. dollar-denominated investment grade and short-term fixed and floating rate debt securities maturing in three years or less (with certain exceptions) and will maintain a dollar-weighted average maturity of 180 days or less and a dollar-weighted average life of 365 days or less.
To achieve its investment objective, the Fund may invest in corporate securities, mortgage- and asset-backed securities, and money market instruments, including government, U.S. and foreign bank and commercial obligations, obligations issued by or on behalf of states, territories and possessions of the United States, the District of Columbia and their respective authorities, agencies, instrumentalities and political subdivisions and derivative securities such as beneficial interests in municipal trust certificates and partnership trusts, and repurchase agreements.
The Fund may invest in variable and floating rate instruments and when-issued and delayed delivery securities.
The Fund invests a significant portion of its assets in securities issued by financial services companies, including banks, broker-dealers and insurance companies, and repurchase agreements secured by such obligations.
Investment grade securities purchased by the Fund (or the issuers of such securities) will carry a rating of BBB-, or equivalent, or higher by at least one nationally recognized statistical rating organization (“NRSRO”) and short-term investments (or the issuers of such securities) will carry a rating in the highest two rating categories of at least one NRSRO (e.g., A-2, P-2 or F2 or better by Standard & Poor’s Ratings Services, Moody’s Investors Service, Inc., or Fitch Ratings Inc., respectively), or if such investments are unrated, determined to be of comparable quality by BlackRock, at the time of investment.
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 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. 
■ 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.
■ Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these securities to fall.
■ Financial Services Industry Risk — Because the Fund invests a significant portion of its assets in the financial services industry, the Fund will be more susceptible to any economic, business, political or other developments which generally affect this industry sector. As a result, the Fund will be exposed to a large extent to the risks associated with that industry, such as government regulation, the availability and cost of capital funds, consolidation and general economic conditions. Financial services companies are also exposed to losses if borrowers and other counterparties experience financial problems and/or cannot repay their obligations.
  When interest rates go up, the value of securities issued by many types of financial services companies generally goes down. In many countries, financial services and the companies that provide them are regulated by governmental entities, which can increase costs for new services or products and make it difficult to pass increased costs on to consumers. In certain areas, deregulation of financial services companies has resulted in increased competition and reduced profitability for certain companies.
  The profitability of many types of financial services companies may be adversely affected in certain market cycles, including periods of rising interest rates, which may restrict the availability and increase the cost of capital, and declining economic conditions, which may cause credit losses due to financial difficulties of borrowers. Because many types of financial services companies are vulnerable to these economic cycles, the Fund’s investments may lose value during such periods.
■ Foreign Exposure Risk — Securities issued or supported by foreign entities, including foreign banks and corporations, may involve additional risks and considerations. Extensive public information about the foreign issuer may not be available, and unfavorable political, economic or governmental developments in the foreign country involved could affect the payment of principal and interest.
■ Interest Rate Risk — Interest rate risk is the risk that the value of a debt security may fall when interest rates rise. In general, the market price of debt securities with longer maturities will go up or down more in response to changes in interest rates than the market price of shorter-term securities. Due to fluctuations in interest rates, the market 
  value of such securities may vary during the period shareholders own shares of the Fund. Very low or negative interest rates may magnify interest rate risk. During periods of very low or negative interest rates, the Fund may be unable to maintain positive returns or pay dividends to Fund shareholders. Changing interest rates, including rates that fall below zero, may have unpredictable effects on markets, may result in heightened market volatility and may detract from the Fund’s ability to achieve its investment objective.
■ 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. Because the Fund invests in short-term instruments these events have caused some instruments to have declining yields, which may impair the results of the Fund if these conditions persisted. 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.
■ 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.
■ 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.
■ 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.
■ Repurchase Agreements Risk— If the other party to a repurchase agreement 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 and the market value of the security declines, the Fund may lose money. 
■ Treasury Obligations Risk — Direct obligations of the U.S. Treasury have historically involved little risk of loss of principal if held to maturity. However, due to fluctuations in interest rates, the market value of such securities may vary during the period shareholders own shares of the Fund.
■ U.S. Government Obligations Risk — Certain securities in which the Fund may invest, including securities issued by certain U.S. Government agencies and U.S. Government sponsored enterprises, are not guaranteed by the U.S. Government or supported by the full faith and credit of the United States.
■ Variable and Floating Rate Instrument Risk — Variable and floating rate securities provide for periodic adjustment in the interest rate paid on the securities. These securities may be subject to greater illiquidity risk than other fixed income securities, meaning the absence of an active market for these securities could make it difficult for the Fund to dispose of them at any given time.
■ When-Issued and Delayed Settlement Transactions Risk — When-issued and delayed delivery securities involve the risk that the security the Fund buys will lose value prior to its delivery. There also is the risk that the security will not be issued or that the other party to the transaction will not meet its obligation. If this occurs, the Fund may lose both the investment opportunity for the assets it set aside to pay for the security and any gain in the security’s price. 
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 performance for the 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 Class K Shares of the Fund for the complete calendar years since the commencement of the Fund’s operations. Prior to September 1, 2015, Class K Shares of the Fund were designated BlackRock Shares. The table compares the Fund’s performance to that of the ICE BofA 6-Month U.S. Treasury Bill Index. To the extent that dividends and distributions have been paid by the Fund, the performance 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 BlackRock 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 www.blackrock.com/cash 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 for the 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 www.blackrock.com/cash
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 BlackRock Short Obligations Fund As of 12/31
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
During the periods shown in the bar chart, the highest return for a quarter was 1.55% (quarter ended June 30, 2020) and the lowest return for a quarter was -0.31% (quarter ended March 31, 2020). The year-to-date return as of September 30, 2021 was 0.07%.
Performance Table Heading rr_PerformanceTableHeading For the periods ended 12/31/20 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 Short Obligations Fund | Class K Shares  
Risk/Return: rr_RiskReturnAbstract  
Management Fee rr_ManagementFeesOverAssets 0.24% [1]
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.07%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.31%
Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.01%) [1],[2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 0.30% [1],[2]
1 Year rr_ExpenseExampleYear01 $ 31
3 Years rr_ExpenseExampleYear03 99
5 Years rr_ExpenseExampleYear05 173
10 Years rr_ExpenseExampleYear10 $ 392
2013 rr_AnnualReturn2013 0.61%
2014 rr_AnnualReturn2014 0.50%
2015 rr_AnnualReturn2015 0.52%
2016 rr_AnnualReturn2016 1.34%
2017 rr_AnnualReturn2017 1.40%
2018 rr_AnnualReturn2018 1.97%
2019 rr_AnnualReturn2019 2.99%
2020 rr_AnnualReturn2020 1.56%
Year to Date Return, Label rr_YearToDateReturnLabel year-to-date return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Sep. 30, 2021
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 0.07%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 1.55%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2020
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (0.31%)
1 Year rr_AverageAnnualReturnYear01 1.56%
5 Years rr_AverageAnnualReturnYear05 1.85%
Since Inception rr_AverageAnnualReturnSinceInception 1.34%
Inception Date rr_AverageAnnualReturnInceptionDate Nov. 15, 2012
Class K | BlackRock Short Obligations Fund | Return After Taxes on Distributions | Class K Shares  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.08%
5 Years rr_AverageAnnualReturnYear05 1.14%
Since Inception rr_AverageAnnualReturnSinceInception 0.82%
Inception Date rr_AverageAnnualReturnInceptionDate Nov. 15, 2012
Class K | BlackRock Short Obligations Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class K Shares  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 0.92%
5 Years rr_AverageAnnualReturnYear05 1.11%
Since Inception rr_AverageAnnualReturnSinceInception 0.80%
Inception Date rr_AverageAnnualReturnInceptionDate Nov. 15, 2012
Class K | BlackRock Short Obligations Fund | ICE BofA 6-Month U.S. Treasury Bill Index (Reflects no deduction for fees, expenses or taxes)  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 1.05%
5 Years rr_AverageAnnualReturnYear05 1.43%
Since Inception rr_AverageAnnualReturnSinceInception 0.95%
[1] As described in the “Management of the Fund” section of the Fund’s prospectus beginning on page 25, 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 June 30, 2023. In addition, BlackRock has contractually agreed to waive its management fees by the amount of investment advisory fees the Fund pays to BlackRock indirectly through its investment in money market funds managed by BlackRock 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 25, BlackRock has 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 (excluding Dividend Expense, Interest Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) to 0.30% of average daily net assets through June 30, 2023. 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.