v3.26.1
Shareholder Report
 6 Months Ended
Jun. 30, 2026
USD ($)
Holding
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name BlackRock Funds III
Entity Central Index Key 0000893818
Entity Investment Company Type N-1A
Document Period End Date Jun. 30, 2026
BlackRock Cash Funds: Institutional - SL Agency Shares  
Shareholder Report [Line Items]  
Fund Name BlackRock Cash Funds: Institutional
Class Name SL Agency Shares
Trading Symbol BISXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about BlackRock Cash Funds: Institutional (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the Money Market Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (888) 204-3956.
Additional Information Phone Number (888) 204-3956
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
SL Agency Shares
$
5
(a)
 0.09
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 5 [1]
Expense Ratio, Percent 0.09% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future results.
Updated Performance Information Location [Text Block] Visit blackrock.com for more recent yield information.
Net Assets $ 107,880,346,482
Holdings Count | Holding 1
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
107,880,346,482
Number of Portfolio Holdings 1
Current seven-day yields as of June 30, 2026 (The 7-Day SEC Yield may differ from the 7-Day Yield shown due to
the fact that the 7-Day SEC Yield excludes distributed capital gains.)
  
7-Day SEC Yield 3.82
%
7-Day Yield 3.82
%
BlackRock Cash Funds: Treasury - Institutional Shares  
Shareholder Report [Line Items]  
Fund Name BlackRock Cash Funds: Treasury
Class Name Institutional Shares
Trading Symbol BRIXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about BlackRock Cash Funds: Treasury (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the Treasury Money Market Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio.You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (888) 204-3956.
Additional Information Phone Number (888) 204-3956
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Institutional Shares
$
6
(a)
 0.12
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 6 [1]
Expense Ratio, Percent 0.12% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future results.
Updated Performance Information Location [Text Block] Visit blackrock.com for more recent yield information.
Net Assets $ 20,632,269,684
Holdings Count | Holding 1
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
20,632,269,684
Number of Portfolio Holdings 1
Current seven-day yields as of June 30, 2026 (The 7-Day SEC Yield may differ from the 7-Day Yield shown due to
the fact that the 7-Day SEC Yield excludes distributed capital gains.)
  
7-Day SEC Yield 3.59
%
7-Day Yield 3.59
%
BlackRock Cash Funds - Treasury SL Agency Shares  
Shareholder Report [Line Items]  
Fund Name BlackRock Cash Funds: Treasury
Class Name SL Agency Shares
Trading Symbol XTSLA
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about BlackRock Cash Funds: Treasury (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the Treasury Money Market Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (888) 204-3956.
Additional Information Phone Number (888) 204-3956
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
SL Agency Shares
$
5
(a)
 0.09
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 5 [1]
Expense Ratio, Percent 0.09% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future results.
Updated Performance Information Location [Text Block] Visit blackrock.com for more recent yield information.
Net Assets $ 20,632,269,684
Holdings Count | Holding 1
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
20,632,269,684
Number of Portfolio Holdings 1
Current seven-day yields as of June 30, 2026 (The 7-Day SEC Yield may differ from the 7-Day Yield shown due to
the fact that the 7-Day SEC Yield excludes distributed capital gains.)
  
7-Day SEC Yield 3.62
%
7-Day Yield 3.62
%
BlackRock Diversified Equity Alpha Fund - Institutional Shares  
Shareholder Report [Line Items]  
Fund Name BlackRock Diversified Equity Alpha Fund
Class Name Institutional Shares
Trading Symbol BDVEX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about BlackRock Diversified Equity Alpha Fund (the “Fund”) (formerly known as BlackRock Diversified Equity Fund) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the Diversified Equity Alpha Master Portfolio (the “Master Portfolio”) (formerly known as Diversified Equity Master Portfolio), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 537-4942.
Additional Information Phone Number (800)537-4942
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Institutional Shares
$
20
(a)
 0.39
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 20 [1]
Expense Ratio, Percent 0.39% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 1,313,581,420
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 51.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
1,313,581,420
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 51
%
iShares MSCI Total International Index Fund - Institutional Shares  
Shareholder Report [Line Items]  
Fund Name iShares MSCI Total International Index Fund
Class Name Institutional Shares
Trading Symbol BDOIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares MSCI Total International Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the Total International ex U.S. Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Institutional Shares
$
7
(a)
 0.13
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 7 [1]
Expense Ratio, Percent 0.13% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 2,515,814,503
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 2.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
2,515,814,503
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 2
%
iShares MSCI Total International Index Fund - Investor A Shares  
Shareholder Report [Line Items]  
Fund Name iShares MSCI Total International Index Fund
Class Name Investor A Shares
Trading Symbol BDOAX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares MSCI Total International Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the Total International ex U.S. Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Investor A Shares
$
21
(a)
 0.39
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 21 [1]
Expense Ratio, Percent 0.39% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 2,515,814,503
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 2.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
2,515,814,503
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 2
%
iShares MSCI Total International Index Fund - Class K Shares  
Shareholder Report [Line Items]  
Fund Name iShares MSCI Total International Index Fund
Class Name Class K Shares
Trading Symbol BDOKX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares MSCI Total International Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the Total International ex U.S. Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Class K Shares
$
4
(a)
 0.08
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 4 [1]
Expense Ratio, Percent 0.08% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 2,515,814,503
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 2.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
2,515,814,503
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 2
%
iShares Russell 1000 Large-Cap Index Fund - Institutional Shares  
Shareholder Report [Line Items]  
Fund Name iShares Russell 1000 Large-Cap Index Fund
Class Name Institutional Shares
Trading Symbol BRGNX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares Russell 1000 Large-Cap Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the Large Cap Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Institutional Shares
$
5
(a)
 0.10
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 5 [1]
Expense Ratio, Percent 0.10% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 2,034,134,744
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 8.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
2,034,134,744
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 8
%
iShares Russell 1000 Large-Cap Index Fund - Investor A Shares  
Shareholder Report [Line Items]  
Fund Name iShares Russell 1000 Large-Cap Index Fund
Class Name Investor A Shares
Trading Symbol BRGAX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares Russell 1000 Large-Cap Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the Large Cap Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six mon
t
hs
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Investor A Shares
$
18
(a)
 0.35
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 18 [1]
Expense Ratio, Percent 0.35% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 2,034,134,744
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 8.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
2,034,134,744
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 8
%
iShares Russell 1000 Large-Cap Index Fund - Class K Shares  
Shareholder Report [Line Items]  
Fund Name iShares Russell 1000 Large-Cap Index Fund
Class Name Class K Shares
Trading Symbol BRGKX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares Russell 1000 Large-Cap Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the Large Cap Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six mo
n
ths
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Class K Shares
$
3
(a)
 0.06
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 3 [1]
Expense Ratio, Percent 0.06% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 2,034,134,744
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 8.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
2,034,134,744
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 8
%
iShares S&P 500 Index Fund - Institutional Shares  
Shareholder Report [Line Items]  
Fund Name iShares S&P 500 Index Fund
Class Name Institutional Shares
Trading Symbol BSPIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares S&P 500 Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the S&P 500 Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Institutional Shares
$
5
(a)
 0.10
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 5 [1]
Expense Ratio, Percent 0.10% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 59,061,952,467
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 5.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
59,061,952,467
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 5
%
iShares S&P 500 Index Fund - Service Shares  
Shareholder Report [Line Items]  
Fund Name iShares S&P 500 Index Fund
Class Name Service Shares
Trading Symbol BSPSX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares S&P 500 Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the S&P 500 Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Service Shares
$
11
(a)
 0.22
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 11 [1]
Expense Ratio, Percent 0.22% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 59,061,952,467
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 5.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
59,061,952,467
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 5
%
iShares S&P 500 Index Fund - Investor A Shares  
Shareholder Report [Line Items]  
Fund Name iShares S&P 500 Index Fund
Class Name Investor A Shares
Trading Symbol BSPAX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares S&P 500 Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the S&P 500 Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Investor A Shares
$
18
(a)
 0.35
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 18 [1]
Expense Ratio, Percent 0.35% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 59,061,952,467
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 5.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
59,061,952,467
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 5
%
iShares S&P 500 Index Fund - Investor P Shares  
Shareholder Report [Line Items]  
Fund Name iShares S&P 500 Index Fund
Class Name Investor P Shares
Trading Symbol BSPPX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares S&P 500 Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the S&P 500 Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Investor P Shares
$
18
(a)
 0.35
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 18 [1]
Expense Ratio, Percent 0.35% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 59,061,952,467
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 5.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
59,061,952,467
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 5
%
iShares S&P 500 Index Fund - Class G Shares  
Shareholder Report [Line Items]  
Fund Name iShares S&P 500 Index Fund
Class Name Class G Shares
Trading Symbol BSPGX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares S&P 500 Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the S&P 500 Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Class G Shares
$
1
(a)
 0.01
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 1 [1]
Expense Ratio, Percent 0.01% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 59,061,952,467
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 5.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
59,061,952,467
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 5
%
iShares S&P 500 Index Fund - Class K Shares  
Shareholder Report [Line Items]  
Fund Name iShares S&P 500 Index Fund
Class Name Class K Shares
Trading Symbol WFSPX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares S&P 500 Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the S&P 500 Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio. You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Class K Shares
$
2
(a)
 0.03
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 2 [1]
Expense Ratio, Percent 0.03% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 59,061,952,467
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 5.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
59,061,952,467
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 5
%
iShares U.S. Aggregate Bond Index Fund - Institutional Shares  
Shareholder Report [Line Items]  
Fund Name iShares U.S. Aggregate Bond Index Fund
Class Name Institutional Shares
Trading Symbol BMOIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares U.S. Aggregate Bond Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the U.S. Total Bond Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio.  You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Institutional Shares
$
5
(a)
 0.10
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 5 [1]
Expense Ratio, Percent 0.10% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 3,291,846,117
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 46.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
3,291,846,117
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 46
%
iShares U.S. Aggregate Bond Index Fund - Investor A Shares  
Shareholder Report [Line Items]  
Fund Name iShares U.S. Aggregate Bond Index Fund
Class Name Investor A Shares
Trading Symbol BMOAX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares U.S. Aggregate Bond Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the U.S. Total Bond Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio.  You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Investor A Shares
$
17
(a)
 0.35
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 17 [1]
Expense Ratio, Percent 0.35% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 3,291,846,117
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 46.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
3,291,846,117
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 46
%
iShares U.S. Aggregate Bond Index Fund - Investor P Shares  
Shareholder Report [Line Items]  
Fund Name iShares U.S. Aggregate Bond Index Fund
Class Name Investor P Shares
Trading Symbol BMOPX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares U.S. Aggregate Bond Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the U.S. Total Bond Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio.  You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Investor P Shares
$
17
(a)
 0.35
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 17 [1]
Expense Ratio, Percent 0.35% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 3,291,846,117
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 46.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
3,291,846,117
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 46
%
iShares U.S. Aggregate Bond Index Fund - Class K Shares  
Shareholder Report [Line Items]  
Fund Name iShares U.S. Aggregate Bond Index Fund
Class Name Class K Shares
Trading Symbol WFBIX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about iShares U.S. Aggregate Bond Index Fund (the “Fund”) for the period of January 1, 2026 to June 30, 2026.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] The Fund invests all of its assets in the U.S. Total Bond Index Master Portfolio (the “Master Portfolio”), a series of Master Investment Portfolio.  You can find additional information about the Fund at blackrock.com/fundreports. You can also request this information by contacting us at (800) 441-7762.
Additional Information Phone Number (800) 441-7762
Additional Information Website blackrock.com/fundreports
Expenses [Text Block]
What were the
Fund
costs for the
last six months
?
(based on a hypothetical $10,000 investment)
Class name
Costs of a $10,000
investment
Costs paid as a percentage of a
$10,000 investment
Class K Shares
$
2
(a)
 0.05
%
(a)(b)
(a)
Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
(b)
Annualized.
Expenses Paid, Amount $ 2 [1]
Expense Ratio, Percent 0.05% [1],[2]
Expenses Represent Both Master and Feeder [Text] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio.
Net Assets $ 3,291,846,117
Holdings Count | Holding 1
Investment Company, Portfolio Turnover 46.00%
Additional Fund Statistics [Text Block]
Key Fund statistics
Net Assets
$
3,291,846,117
Number of Portfolio Holdings 1
Portfolio Turnover Rate of the Master Portfolio 46
%
[1] Because the Fund invests all of its assets in the Master Portfolio, the expense example reflects the net expenses of both the Fund and the Master Portfolio. 
[2] Annualized.