Class K Shares [Member] Performance Management - Class K - BlackRock Short Obligations Fund	Jul. 31, 2025
Prospectus [Line Items]
Bar Chart and Performance Table [Heading]	Performance Information
Performance Narrative [Text Block]	The information shows you how the performance for the Fund has varied year by year and provides some indication of the risks of investing in the Fund. 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 Bloomberg U.S. Aggregate Bond Index and the ICE BofA 6-Month U.S. Treasury Bill Index. The ICE BofA 6-Month U.S. Treasury Bill Index is relevant to the Fund because it has characteristics similar to the Fund’s investment strategy. 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 Past Does Not Indicate Future [Text]	As with all such investments, past performance (before and after taxes) is not an indication of future results.
Performance Information Illustrates Variability of Returns [Text]	The information shows you how the performance for the Fund has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Additional Market Index [Text]	The table compares the Fund’s performance to that of the Bloomberg U.S. Aggregate Bond Index and the ICE BofA 6-Month U.S. Treasury Bill Index. The ICE BofA 6-Month U.S. Treasury Bill Index is relevant to the Fund because it has characteristics similar to the Fund’s investment strategy.
Bar Chart [Heading]	Class K SharesANNUAL TOTAL RETURNSBlackRock Short Obligations FundAs of 12/31
Bar Chart Closing [Text Block]	During the ten-year period shown in the bar chart, the highest return for a quarter was 1.68% (quarter ended December 31, 2023) and the lowest return for a quarter was -0.50% (quarter ended March 31, 2022). The year-to-date return as of September 30, 2025 was 3.63%.
Performance Table Heading	For the periods ended 12/31/24Average Annual Total Returns
Performance Table Narrative	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.
Performance Table Uses Highest Federal Rate	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	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 Availability Website Address [Text]	www.blackrock.com/cash
Performance Availability Phone [Text]	(800) 882-0052
Class K Shares
Prospectus [Line Items]
Year to Date Return, Label [Optional Text]	year-to-date return
Bar Chart, Year to Date Return	3.63%
Bar Chart, Year to Date Return, Date	Sep. 30, 2025
Highest Quarterly Return, Label [Optional Text]	highest return
Highest Quarterly Return	1.68%
Highest Quarterly Return, Date	Dec. 31, 2023
Lowest Quarterly Return, Label [Optional Text]	lowest return
Lowest Quarterly Return	(0.50%)
Lowest Quarterly Return, Date	Mar. 31, 2022

Class K Shares [Member] Performance Management - Class K - BlackRock Short Obligations Fund

Jul. 31, 2025

Bar Chart and Performance Table [Heading]

Performance Information

The information shows you how the performance for the Fund has varied year by year and provides some indication of the risks of investing in the Fund. 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 Bloomberg U.S. Aggregate Bond Index and the ICE BofA 6-Month U.S. Treasury Bill Index. The ICE BofA 6-Month U.S. Treasury Bill Index is relevant to the Fund because it has characteristics similar to the Fund’s investment strategy. 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 Past Does Not Indicate Future [Text]

As with all such investments, past performance (before and after taxes) is not an indication of future results.

Performance Information Illustrates Variability of Returns [Text]

The information shows you how the performance for the Fund has varied year by year and provides some indication of the risks of investing in the Fund.

Performance Additional Market Index [Text]

The table compares the Fund’s performance to that of the Bloomberg U.S. Aggregate Bond Index and the ICE BofA 6-Month U.S. Treasury Bill Index. The ICE BofA 6-Month U.S. Treasury Bill Index is relevant to the Fund because it has characteristics similar to the Fund’s investment strategy.

Bar Chart [Heading]

Class K SharesANNUAL TOTAL RETURNSBlackRock Short Obligations FundAs of 12/31

Bar Chart Closing [Text Block]

During the ten-year period shown in the bar chart, the highest return for a quarter was 1.68% (quarter ended December 31, 2023) and the lowest return for a quarter was -0.50% (quarter ended March 31, 2022). The year-to-date return as of September 30, 2025 was 3.63%.

Performance Table Heading

For the periods ended 12/31/24Average Annual Total Returns

Performance Table Narrative

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.

Performance Table Uses Highest Federal Rate

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

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 Availability Website Address [Text]

www.blackrock.com/cash

Performance Availability Phone [Text]

(800) 882-0052

Class K Shares

Prospectus [Line Items]

Year to Date Return, Label [Optional Text]

year-to-date return

Bar Chart, Year to Date Return

3.63%

Bar Chart, Year to Date Return, Date

Sep. 30, 2025