Performance Management - BlackRock U.S. Mortgage Portfolio	Apr. 30, 2025
Prospectus [Line Items]
Bar Chart and Performance Table [Heading]	Performance Information
Performance Narrative [Text Block]	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 Bloomberg U.S. Aggregate Bond Index and a customized weighted index comprised of 50% Bloomberg ABS Index/50% Bloomberg Non-Agency Investment Grade CMBS Index (the “Customized Reference Benchmark”). The Customized Reference Benchmark is relevant to the Fund because it has characteristics similar to the Fund’s investment strategy. The Fund acquired all of the assets, subject to the liabilities, of BlackRock U.S. Mortgage Portfolio, a series of Managed Account Series (the “Predecessor Fund”), in a reorganization on September 17, 2018 (the “Reorganization”). The Fund adopted the performance of the Predecessor Fund as a result of the Reorganization on September 17, 2018. The performance information below is based on the performance of the Predecessor Fund for periods prior to the date of the Reorganization. The Predecessor Fund had the same investment objectives, strategies and policies, portfolio management team and contractual arrangements, including the same contractual fees and expenses, as the Fund as of the date of the Reorganization. The Fund’s and Predecessor Fund’s total returns prior to August 5, 2025 as reflected in the bar chart and table are the returns of the Fund and the Predecessor Fund, as applicable, when each followed a different investment objective and different investment strategies and process under the name “BlackRock U.S. Mortgage Portfolio.” Effective August 5, 2025, the Fund has changed the benchmark against which it measures its performance from the Bloomberg U.S. Mortgage-Backed Securities Index to the Customized Reference Benchmark. Fund management believes the Customized Reference Benchmark more accurately reflects the investment strategy of the Fund. The returns for Institutional Shares are based on the performance of its predecessor class. 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. 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 the Fund’s investment manager 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 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 Fund’s performance 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 a customized weighted index comprised of 50% Bloomberg ABS Index/50% Bloomberg Non-Agency Investment Grade CMBS Index (the “Customized Reference Benchmark”). The Customized Reference Benchmark is relevant to the Fund because it has characteristics similar to the Fund’s investment strategy.
Bar Chart Does Not Reflect Sales Loads [Text]	Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart [Heading]	Institutional Shares ANNUAL TOTAL RETURNS BlackRock Securitized Income Fund As 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 6.06% (quarter ended December 31, 2023) and the lowest return for a quarter was -5.87% (quarter ended March 31, 2022). The year-to-date return as of June 30, 2025 was 4.45%.
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. After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A and Investor C Shares will vary.
Performance Table Does Reflect Sales Loads	However, the table includes all applicable fees and sales charges.
Performance Table Market Index Changed	Effective August 5, 2025, the Fund has changed the benchmark against which it measures its performance from the Bloomberg U.S. Mortgage-Backed Securities Index to the Customized Reference Benchmark. Fund management believes the Customized Reference Benchmark more accurately reflects the investment strategy of the Fund.
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 Table One Class of after Tax Shown [Text]	After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A and Investor C Shares will vary.
Performance Availability Website Address [Text]	www.blackrock.com
Performance Availability Phone [Text]	800-882-0052
Institutional Shares
Prospectus [Line Items]
Year to Date Return, Label [Optional Text]	year-to-date return
Bar Chart, Year to Date Return	4.45%
Bar Chart, Year to Date Return, Date	Jun. 30, 2025
Highest Quarterly Return, Label [Optional Text]	highest return
Highest Quarterly Return	6.06%
Highest Quarterly Return, Date	Dec. 31, 2023
Lowest Quarterly Return, Label [Optional Text]	lowest return
Lowest Quarterly Return	(5.87%)
Lowest Quarterly Return, Date	Mar. 31, 2022

Performance Management - BlackRock U.S. Mortgage Portfolio

Apr. 30, 2025

Bar Chart and Performance Table [Heading]

Performance Information

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 Bloomberg U.S. Aggregate Bond Index and a customized weighted index comprised of 50% Bloomberg ABS Index/50% Bloomberg Non-Agency Investment Grade CMBS Index (the “Customized Reference Benchmark”). The Customized Reference Benchmark is relevant to the Fund because it has characteristics similar to the Fund’s investment strategy. The Fund acquired all of the assets, subject to the liabilities, of BlackRock U.S. Mortgage Portfolio, a series of Managed Account Series (the “Predecessor Fund”), in a reorganization on September 17, 2018 (the “Reorganization”). The Fund adopted the performance of the Predecessor Fund as a result of the Reorganization on September 17, 2018. The performance information below is based on the performance of the Predecessor Fund for periods prior to the date of the Reorganization. The Predecessor Fund had the same investment objectives, strategies and policies, portfolio management team and contractual arrangements, including the same contractual fees and expenses, as the Fund as of the date of the Reorganization. The Fund’s and Predecessor Fund’s total returns prior to August 5, 2025 as reflected in the bar chart and table are the returns of the Fund and the Predecessor Fund, as applicable, when each followed a different investment objective and different investment strategies and process under the name “BlackRock U.S. Mortgage Portfolio.” Effective August 5, 2025, the Fund has changed the benchmark against which it measures its performance from the Bloomberg U.S. Mortgage-Backed Securities Index to the Customized Reference Benchmark. Fund management believes the Customized Reference Benchmark more accurately reflects the investment strategy of the Fund. The returns for Institutional Shares are based on the performance of its predecessor class. 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. 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 the Fund’s investment manager 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 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 Fund’s performance 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 a customized weighted index comprised of 50% Bloomberg ABS Index/50% Bloomberg Non-Agency Investment Grade CMBS Index (the “Customized Reference Benchmark”). The Customized Reference Benchmark is relevant to the Fund because it has characteristics similar to the Fund’s investment strategy.

Bar Chart Does Not Reflect Sales Loads [Text]

Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.

Bar Chart [Heading]

Institutional Shares ANNUAL TOTAL RETURNS BlackRock Securitized Income Fund As 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 6.06% (quarter ended December 31, 2023) and the lowest return for a quarter was -5.87% (quarter ended March 31, 2022). The year-to-date return as of June 30, 2025 was 4.45%.

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. After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A and Investor C Shares will vary.

Performance Table Does Reflect Sales Loads

However, the table includes all applicable fees and sales charges.

Performance Table Market Index Changed

Effective August 5, 2025, the Fund has changed the benchmark against which it measures its performance from the Bloomberg U.S. Mortgage-Backed Securities Index to the Customized Reference Benchmark. Fund management believes the Customized Reference Benchmark more accurately reflects the investment strategy of the Fund.

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 Table One Class of after Tax Shown [Text]

After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A and Investor C Shares will vary.

Performance Availability Website Address [Text]

www.blackrock.com

Performance Availability Phone [Text]

800-882-0052

Institutional Shares

Prospectus [Line Items]

Year to Date Return, Label [Optional Text]

year-to-date return

Bar Chart, Year to Date Return

4.45%