v3.26.1
Shareholder Report
 6 Months Ended
Mar. 31, 2026
USD ($)
Holding
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name STATE STREET INSTITUTIONAL INVESTMENT TRUST
Entity Central Index Key 0001107414
Entity Investment Company Type N-1A
Document Period End Date Mar. 31, 2026
Shareholder Report Annual or Semi-Annual Semi-Annual Shareholder Report
C000226667  
Shareholder Report [Line Items]  
Fund Name State Street U.S. Core Equity Fund
Trading Symbol SSAQX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about the State Street U.S. Core Equity Fund (the "Fund") for the period of October 1, 2025 through March 31, 2026.
Additional Information [Text Block] You can find additional information about the Fund, including the Prospectus, Statement of Additional Information, financial statements and other information at www.ssga.com/us/en/institutional/fund-finder?tab=documents&type=mf. You can also request this information about the Fund by contacting us at 1-800-242-0134.
Additional Information Phone Number 1-800-242-0134
Additional Information Website www.ssga.com/us/en/institutional/fund-finder?tab=documents&type=mf
Expenses [Text Block]

What were the Fund costs for the last six months? (based on a hypothetical $10,000 Investment)

Table Summary
Fund Name
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
State Street U.S. Core Equity Fund
$7
0.14%Footnote Reference*
Footnote Description
Footnote*
 Annualized.
Expenses Paid, Amount $ 7
Expense Ratio, Percent 0.14% [1]
AssetsNet $ 3,745,676,496
Holdings Count | Holding 98
InvestmentCompanyPortfolioTurnover 21.00%
Additional Fund Statistics [Text Block]

Key Fund Statistics as of 3/31/2026

  • Total Net Assets$3,745,676,496
  • Number of Portfolio Holdings98
  • Portfolio Turnover Rate21%
Holdings [Text Block]

Top Ten Industries

Table Summary
Industries
%
Semiconductors
15.0%
Interactive Media & Services
9.4%
Systems Software
6.7%
Technology Hardware, Storage & Peripherals
5.6%
Broadline Retail
4.7%
Pharmaceuticals
4.2%
Diversified Banks
3.4%
Financial Exchanges & Data
3.3%
Integrated Oil & Gas
3.0%
Transaction & Payment Processing Services
2.4%
Largest Holdings [Text Block]

Top Ten Holdings

Table Summary
Holdings
%
NVIDIA Corp.
9.1%
Microsoft Corp.
5.8%
Apple, Inc.
5.6%
Alphabet, Inc., Class A
5.4%
Amazon.com, Inc.
4.7%
Broadcom, Inc.
3.4%
Meta Platforms, Inc., Class A
3.1%
Exxon Mobil Corp.
1.9%
JPMorgan Chase & Co.
1.9%
Eli Lilly & Co.
1.8%
C000226666  
Shareholder Report [Line Items]  
Fund Name State Street Income Fund
Trading Symbol SSASX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains important information about the State Street Income Fund (the "Fund") for the period of October 1, 2025 through March 31, 2026.
Additional Information [Text Block] You can find additional information about the Fund, including the Prospectus, Statement of Additional Information, financial statements and other information at www.ssga.com/us/en/institutional/fund-finder?tab=documents&type=mf. You can also request this information about the Fund by contacting us at 1-800-242-0134.
Additional Information Phone Number 1-800-242-0134
Additional Information Website www.ssga.com/us/en/institutional/fund-finder?tab=documents&type=mf
Expenses [Text Block]

What were the Fund costs for the last six months? (based on a hypothetical $10,000 Investment)

Table Summary
Fund Name
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
State Street Income Fund
$11
0.22%Footnote Reference*
Footnote Description
Footnote*
 Annualized.
Expenses Paid, Amount $ 11
Expense Ratio, Percent 0.22% [2]
AssetsNet $ 278,197,882
Holdings Count | Holding 1,202
InvestmentCompanyPortfolioTurnover 24.00%
Additional Fund Statistics [Text Block]

Key Fund Statistics as of 3/31/2026

  • Total Net Assets$278,197,882
  • Number of Portfolio Holdings1,202
  • Portfolio Turnover Rate24%
Holdings [Text Block]

Top Security Types

Table Summary
Assets
%
Short-Term Investments
41.2%
U.S. Treasuries
34.3%
Corporate Notes
25.2%
Agency Mortgage Backed
24.1%
Non-Agency Collateralized Mortgage Obligations
11.1%
Asset Backed
1.6%
Agency Collateralized Mortgage Obligations
0.8%
Municipal Bonds and Notes
0.2%
Largest Holdings [Text Block]

Top Ten Holdings

Table Summary
Holdings
%
Uniform Mortgage-Backed Security, TBA, 2.50%, due 04/01/56
5.4%
Uniform Mortgage-Backed Security, TBA, 6.00%, due 04/01/56
4.9%
U.S. Treasury Notes, 3.88%, due 07/31/27
4.6%
U.S. Treasury Notes, 4.63%, due 05/31/31
2.8%
U.S. Treasury Notes, 3.50%, due 10/31/27
2.8%
Uniform Mortgage-Backed Security, TBA, 2.00%, due 04/01/56
2.2%
U.S. Treasury Bonds, 4.75%, due 08/15/55
2.2%
U.S. Treasury Notes, 4.00%, due 07/31/29
2.2%
U.S. Treasury Notes, 3.88%, due 08/15/33
2.1%
U.S. Treasury Bonds, 3.00%, due 08/15/48
2.0%
[1] Annualized.
[2] Annualized.