v3.25.1
Label Element Value
Risk Return Abstract rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName STATE STREET INSTITUTIONAL INVESTMENT TRUST
Prospectus Date rr_ProspectusDate Apr. 30, 2025
State Street Aggregate Bond Index Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;text-transform:uppercase;">State Street Aggregate Bond Index Portfolio</span>
Objective [Heading] rr_ObjectiveHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Investment Objective</span>
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The State Street Aggregate Bond Index Portfolio (the “Aggregate Bond Index Portfolio”or the “Portfolio”) seeks to provide investment results that, before fees and expenses, correspond generally to the price and yield performance of an index that tracks the U.S. dollar denominated investment grade bond market over the long term.
Expense [Heading] rr_ExpenseHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Fees and Expenses of the Portfolio</span>
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The tables below describe the fees and expenses that you may pay if you buy, hold, and sell shares of the Aggregate Bond Index Portfolio (“Portfolio Shares”). You may pay other fees, such as brokerage commissions and other fees to financial intermediaries which are not reflected in the tables and examples below. The Portfolio's shares are offered exclusively to investors that pay fees to SSGA Funds Management, Inc. (“SSGA FM”or the “Adviser”), the Portfolio's investment adviser, or its affiliates; the Portfolio pays no management fee to SSGA FM, as shown in the table below.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Shareholder Fees </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">(fees paid directly from your investment)</span>
Operating Expenses Caption [Text] rr_OperatingExpensesCaption <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Annual Fund Operating Expenses </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">(expenses that you pay each year as a percentage of the value of your investment)</span>
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Portfolio Turnover:</span>
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock The Portfolio 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 Portfolio Shares are held in a taxable account. These costs, which are not reflected in Annual Fund Operating Expenses or in the Example, affect the Portfolio's performance. During the most recent fiscal year, the Portfolio's portfolio turnover rate was 16% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 16.00%
Expense Example [Heading] rr_ExpenseExampleHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Example:</span>
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This Example is intended to help you compare the cost of investing in the Portfolio with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Portfolio for the time periods indicated, and then sell or hold all of your Portfolio Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Portfolio's operating expenses remain the same. The calculation of costs for the one-year period takes into account the effect of any current contractual fee waivers and/or reimbursements; and the calculation of costs for the remaining periods takes such fee waivers and/or reimbursements into account only for the first year of each such period. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Principal Investment Strategies</span>
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Portfolio is an “index”fund that seeks to track, before fees and expenses, the total return performance of the Bloomberg U.S. Aggregate Bond Index (the “U.S. Aggregate Bond Index”or sometimes referred to in context as the “Index”) over the long term. As an “index”fund, the Portfolio is not managed according to traditional methods of “active”investment management, which involve the buying and selling of securities based upon economic, financial and market analysis and investment judgment. In seeking to track the performance of the Index, the Portfolio employs a sampling strategy, which means that the Portfolio will not typically purchase all of the securities represented in the Index. Instead, the Portfolio may purchase a subset of the securities in the Index, or securities the Adviser considers to be comparable to securities in the Index, in an effort to hold a portfolio of securities with generally the same risk and return characteristics of the Index. The number of holdings in the Portfolio will be based on a number of factors, including asset size of the Portfolio. SSGA Funds Management, Inc. (“SSGA FM”or the “Adviser”), the investment adviser to the Portfolio, generally expects the Portfolio to hold fewer than the total number of securities in the Index, but reserves the right to hold as many securities as it believes necessary to achieve the Portfolio's investment objective. Under normal circumstances, the Portfolio generally invests substantially all, but at least 80%, of its net assets (plus borrowings, if any) in securities comprising the Index or in securities that the Adviser determines have economic characteristics that are comparable to the economic characteristics of securities that comprise the Index. The notional value of the Portfolio's investments in derivatives or other synthetic instruments that provide exposures comparable, in the judgment of the Adviser, to investments in the Index may be counted toward satisfaction of this 80% policy. The Portfolio will provide shareholders with at least sixty (60) days' notice prior to any change in this 80% investment policy. The Portfolio may also invest in other debt securities, cash and cash equivalents or money market instruments, such as repurchase agreements and money market funds (including money market funds advised by the Adviser). The Portfolio may at times purchase or sell futures contracts on fixed-income securities, or options on those futures, in lieu of investing directly in fixed-income securities themselves. The Portfolio may also purchase or sell futures contracts and related options on the Index (or other fixed-income securities indices). The Portfolio might do so, for example, in order to adjust the interest-rate sensitivity of the Portfolio to bring the characteristics of the Portfolio more closely in line with those of the Index. It might also do so to increase its investment exposure pending investment of cash in bonds or other investments or to reduce its investment exposure in situations where it intends to sell a portion of the securities in its portfolio but the sale has not yet been completed. The Portfolio may also enter into other derivatives transactions, including the use of options or swap transactions, in lieu of investing directly in the stocks making up the Index. The Portfolio may, to the extent permitted by applicable law, invest in shares of other mutual funds whose investment objectives and policies are similar to those of the Portfolio (including funds advised by the Adviser). The Index is designed to measure the performance of the U.S. dollar denominated investment grade bond market, which includes investment grade (must be Baa3/BBB-/BBB- or higher using the middle rating of Moody's Investors Service, Inc., Standard & Poor's, and Fitch Inc.) government bonds, investment grade corporate bonds, mortgage pass-through securities, commercial mortgage backed securities and other asset backed securities that are publicly for sale in the United States. The securities in the Index must have at least 1 year remaining to maturity and must have $300 million or more of outstanding face value. Asset backed securities must have a minimum deal size of $500 million and a minimum tranche size of $25 million. For commercial mortgage backed securities, the original aggregate transaction must have a minimum deal size of $500 million, and a minimum tranche size of $25 million; the aggregate outstanding transaction sizes must be at least $300 million to remain in the Index. In addition, the securities must be U.S. dollar denominated, fixed rate, non-convertible, and taxable. Certain types of securities, such as flower bonds, targeted investor notes, and state and local government series bonds are excluded from the Index. Also excluded from the Index are structured notes with embedded swaps or other special features, private placements and floating rate securities. The Index is market capitalization weighted and the securities in the Index are updated on the last business day of each month. It is not possible to invest directly in the Index. The Portfolio expects typically to invest a significant portion of its assets in U.S. agency mortgage pass-through securities up to a total weight that is comparable to that of the Index. Most transactions in mortgage pass-through securities occur through standardized contracts for future delivery in which the exact mortgage pools to be delivered are not specified until a few days prior to settlement, referred to as a “to-be-announced transaction”or “TBA Transaction.”In a TBA Transaction, the buyer and seller agree upon general trade parameters such as agency, settlement date, par amount and price. The actual pools delivered generally are determined two days prior to the settlement date; however, it is not anticipated that the Portfolio will receive pools, but instead will participate in rolling TBA Transactions. The Portfolio expects to enter into such contracts on a regular basis. The Index is sponsored by Bloomberg Index Services Limited (the “Index Provider”) which is not affiliated with the Portfolio or the Adviser. The Index Provider determines the composition of the Index, relative weightings of the securities in the Index and publishes information regarding the market value of the Index.
Risk [Heading] rr_RiskHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Principal Risks of investing in the Portfolio</span>
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Performance</span>
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The bar chart and table below provide some indication of the risks of investing in the Portfolio by illustrating the variability of the Portfolio's returns from year-to-year and by showing how the Portfolio's average annual returns for the periods indicated compared with those of the Index. The Portfolio's past performance does not necessarily indicate how the Portfolio will perform in the future. Current performance information for the Portfolio is available toll free by calling (800) 997-7327 or by visiting our website at www.ssga.com.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns <span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">The bar chart and table below provide some indication of the risks of investing in the Portfolio by illustrating the variability of the Portfolio's returns from year-to-year and by showing how the Portfolio's average annual returns for the periods indicated compared with those of the Index. </span>
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone <span style="color:#000000;font-family:Arial;font-size:10.02pt;">(800) 997-7327</span>
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress <span style="color:#000000;font-family:Arial;font-size:10.02pt;">www.ssga.com</span>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture <span style="color:#000000;font-family:Arial;font-size:10.02pt;">The </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">Portfolio's past performance does not necessarily indicate how the Portfolio will perform in the future.</span>
Bar Chart [Heading] rr_BarChartHeading <span style="color:#000000;font-family:Arial;font-size:10.02pt;font-weight:bold;">Annual Total Returns </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">(years ended 12/31)</span>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock ReturnsQuarter/YearHighest Quarterly Return6.70%Q4 2023Lowest Quarterly Return-5.97%Q1 2022
Performance Table Heading rr_PerformanceTableHeading <span style="font-family:Arial;font-size:10.02pt;font-weight:bold;">Average Annual Total Returns </span><span style="font-family:Arial;font-size:10.02pt;">(for periods ended 12/31/24)</span>
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate <span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">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. </span>
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred <span style="color:#000000;font-family:Arial;font-size:10.02pt;">After-tax returns depend on an investor's tax situation and may differ from those </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;">shown, and after-tax returns are not relevant to investors who hold their shares through tax-advantaged arrangements, </span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">such as 401(k) plans or individual retirement accounts (“IRAs”).</span>
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. After-tax returns depend on an investor's tax situation and may differ from those shown, and after-tax returns are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (“IRAs”).
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock The Portfolio will make updated performance information, including its current net asset value, available at the Portfolio's website:  www.ssga.com.
State Street Aggregate Bond Index Portfolio | Risk Lose Money [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock You could lose money by investing in the Portfolio.
State Street Aggregate Bond Index Portfolio | Risk Not Insured Depository Institution [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock An investment in the Portfolio is subject to investment risks, including possible loss of principal, is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation (“FDIC”) or any other government agency.
State Street Aggregate Bond Index Portfolio | Market Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Market Risk: The Portfolio's investments are subject to changes in general economic conditions, general market fluctuations and the risks inherent in investment in securities markets. Investment markets can be volatile and prices of investments can change substantially due to various factors including, but not limited to, economic growth or recession, changes in interest rates, inflation, changes in the actual or perceived creditworthiness of issuers, and general market liquidity. The Portfolio is subject to the risk that geopolitical events will disrupt securities markets and adversely affect global economies and markets. Local, regional or global events such as war, military conflicts, acts of terrorism, trade policy changes or disputes, the threat or actual imposition of tariffs, natural disasters, the spread of infectious illness or other public health issues, or other events could have a significant impact on the Portfolio and its investments.
State Street Aggregate Bond Index Portfolio | Debt Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Debt Securities Risk: The values of debt securities may increase or decrease as a result of the following: market fluctuations, changes in interest rates, actual or perceived inability or unwillingness of issuers, guarantors or liquidity providers to make scheduled principal or interest payments, or illiquidity in debt securities markets. To the extent that interest rates rise, certain underlying obligations may be paid off substantially slower than originally anticipated and the value of those securities may fall sharply. A rising interest rate environment may cause the value of the Portfolio's fixed income securities to decrease, an adverse impact on the liquidity of the Portfolio's fixed income securities, and increased volatility of the fixed income markets. During periods when interest rates are at low levels, the Portfolio's yield can be low, and the Portfolio may have a negative yield (i.e., it may lose money on an operating basis). To the extent that interest rates fall, certain underlying obligations may be paid off substantially faster than originally anticipated. If the principal on a debt obligation is prepaid before expected, the prepayments of principal may have to be reinvested in obligations paying interest at lower rates. During periods of falling interest rates, the income received by the Portfolio may decline. Changes in interest rates will likely have a greater effect on the values of debt securities of longer durations. Returns on investments in debt securities could trail the returns on other investment options, including investments in equity securities. High levels of inflation and/or a significantly changing interest rate environment can lead to heightened levels of volatility and reduced liquidity.
State Street Aggregate Bond Index Portfolio | Mortgage Related and Other Asset Backed Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Mortgage-Related and Other Asset-Backed Securities Risk: Investments in mortgage-related and other asset-backed securities are subject to the risk of significant credit downgrades, illiquidity, and defaults to a greater extent than many other types of fixed-income investments. The liquidity of mortgage-related and asset-backed securities may change over time. During periods of falling interest rates, mortgage- and asset-backed securities may be called or prepaid, which may result in the Portfolio having to reinvest proceeds in other investments at a lower interest rate. During periods of rising interest rates, the average life of mortgage- and asset-backed securities may extend, which may lock in a below-market interest rate, increase the security's duration and interest rate sensitivity, and reduce the value of the security. Enforcing rights against the underlying assets or collateral may be difficult, and the underlying assets or collateral may be insufficient if the issuer defaults.
State Street Aggregate Bond Index Portfolio | US Government Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock U.S. Government Securities Risk: Certain U.S. government securities are supported by the full faith and credit of the United States; others are supported by the right of the issuer to borrow from the U.S. Treasury; others are supported by the discretionary authority of the U.S. government to purchase the agency's obligations; and still others are supported only by the credit of the issuing agency, instrumentality, or enterprise. Although U.S. government-sponsored enterprises such as the Federal Home Loan Mortgage Corporation (“Freddie Mac”) and the Federal National Mortgage Association (“Fannie Mae”) may be chartered or sponsored by Congress, they are not funded by Congressional appropriations, and their securities are not issued by the U.S. Treasury, are not supported by the full faith and credit of the U.S. government, and involve increased credit risks.
State Street Aggregate Bond Index Portfolio | Indexing Strategy Index Tracking Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Indexing Strategy/Index Tracking Risk: The Portfolio is managed with an indexing investment strategy, attempting to track the performance of an unmanaged index of securities, regardless of the current or projected performance of the Index or of the actual securities comprising the Index. This differs from an actively-managed fund, which typically seeks to outperform a benchmark index. As a result, the Portfolio's performance may be less favorable than that of a portfolio managed using an active investment strategy. The structure and composition of the Index will affect the performance, volatility, and risk of the Index and, consequently, the performance, volatility, and risk of the Portfolio. While the Adviser seeks to track the performance of the Index (i.e., achieve a high degree of correlation with the Index), the Portfolio's return may not match the return of the Index. The Portfolio incurs a number of operating expenses not applicable to the Index, and incurs costs in buying and selling securities. In addition, the Portfolio may not be fully invested at times, generally as a result of cash flows into or out of the Portfolio or reserves of cash held by the Portfolio to meet redemptions. The Adviser may attempt to replicate the Index return by investing in fewer than all of the securities in the Index, or in some securities not included in the Index, potentially increasing the risk of divergence between the Portfolio's return and that of the Index.
State Street Aggregate Bond Index Portfolio | Counterparty Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Counterparty Risk: The Portfolio will be subject to credit risk with respect to the counterparties with which the Portfolio enters into derivatives contracts, repurchase agreements, reverse repurchase agreements, and other transactions. If a counterparty fails to meet its contractual obligations, the Portfolio may be unable to terminate or realize any gain on the investment or transaction, or to recover collateral posted to the counterparty, resulting in a loss to the Portfolio. If the Portfolio holds collateral posted by its counterparty, it may be delayed or prevented from realizing on the collateral in the event of a bankruptcy or insolvency proceeding relating to the counterparty.
State Street Aggregate Bond Index Portfolio | Derivatives Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Derivatives Risk: Derivative transactions can create investment leverage and may have significant volatility. It is possible that a derivative transaction will result in a much greater loss than the principal amount invested, that changes in the value of a derivative transaction may not correlate perfectly with the underlying asset, and that the Portfolio may not be able to close out a derivative transaction at a favorable time or price. The counterparty to a derivatives contract may be unable or unwilling to make timely settlement payments, return the Portfolio's margin, or otherwise honor its obligations. A derivatives transaction may not behave in the manner anticipated by the Adviser or may not have the effect on the Portfolio anticipated by the Adviser.
State Street Aggregate Bond Index Portfolio | Income Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Income Risk: The Portfolio's income may decline due to falling interest rates or other factors. Issuers of securities held by the Portfolio may call or redeem the securities during periods of falling interest rates, and the Portfolio would likely be required to reinvest in securities paying lower interest rates. If an obligation held by the Portfolio is prepaid, the Portfolio may have to reinvest the prepayment in other obligations paying income at lower rates.
State Street Aggregate Bond Index Portfolio | Large Transactions Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Large Transactions Risk: To the extent a large proportion of the shares of the Portfolio are held by a small number of shareholders (or a single shareholder), including funds or accounts over which the Adviser has investment discretion, the Portfolio is subject to the risk that these shareholders will purchase or redeem Portfolio Shares in large amounts rapidly or unexpectedly, including as a result of an asset allocation decision made by the Adviser. In addition, a large number of shareholders collectively may purchase or redeem Portfolio Shares in large amounts rapidly or unexpectedly (collectively, such transactions are referred to as “large shareholder transactions”). Large shareholder transactions could adversely affect the ability of the Portfolio to conduct its investment program.
State Street Aggregate Bond Index Portfolio | Liquidity Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Liquidity Risk: Lack of a ready market, stressed market conditions, or restrictions on resale may limit the ability of the Portfolio to sell a security at an advantageous time or price or at all. Illiquid investments may trade at a discount from comparable, more liquid investments and may be subject to wide fluctuations in market value. Illiquidity of the Portfolio's holdings may also limit the ability of the Portfolio to obtain cash to meet redemptions on a timely basis. In addition, the Portfolio, due to limitations on investments in any illiquid investments and/or the difficulty in purchasing and selling such investments, may be unable to achieve its desired level of exposure to a certain market or sector.
State Street Aggregate Bond Index Portfolio | Risk of Investment in Other Pools [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Risk of Investment in Other Pools: If the Portfolio invests in another pooled investment vehicle, it is exposed to the risk that the other pool will not perform as expected and is exposed indirectly to all of the risks applicable to an investment in such other pool. The investment policies of the other pool may not be the same as those of the Portfolio; as a result, an investment in the other pool may be subject to additional or different risks than those to which the Portfolio is typically subject. The Portfolio bears its proportionate share of the fees and expenses of any pool in which it invests. The Adviser or an affiliate may serve as investment adviser to a pool in which the Portfolio may invest, leading to potential conflicts of interest. It is possible that other clients of the Adviser or its affiliates will purchase or sell interests in a pool sponsored or managed by the Adviser or its affiliates at prices and at times more favorable than those at which the Portfolio does so.
State Street Aggregate Bond Index Portfolio | Unconstrained Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Unconstrained Sector Risk: The Portfolio may invest a substantial portion of its assets within one or more economic sectors or industries, which may change from time to time. Greater investment focus on one or more sectors or industries increases the potential for volatility and the risk that events negatively affecting such sectors or industries could reduce returns, potentially causing the value of the Portfolio's Shares to decrease, perhaps significantly.
State Street Aggregate Bond Index Portfolio | Valuation Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Valuation Risk: Certain portfolio holdings may be valued on the basis of factors other than market quotations. This may occur more often in times of market turmoil or reduced liquidity. There are multiple methods that can be used to value a portfolio holding when market quotations are not readily available. The value established for any portfolio holding at a point in time might differ from what would be produced using a different methodology or if it had been priced using market quotations. Portfolio holdings that are valued using techniques other than market quotations, including “fair valued”securities, may be subject to greater fluctuation in their valuations from one day to the next than if market quotations were used. In addition, there is no assurance that the Portfolio could sell or close out a portfolio position for the value established for it at any time, and it is possible that the Portfolio would incur a loss because a portfolio position is sold or closed out at a discount to the valuation established by the Portfolio at that time. Investors who purchase or redeem Portfolio Shares on days when the Portfolio is holding fair-valued investments may receive fewer or more shares or lower or higher redemption proceeds than they would have received if the Portfolio had not fair-valued the holding(s) or had used a different valuation methodology.
State Street Aggregate Bond Index Portfolio | When Issued TBA and Delayed Delivery Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock When-Issued, TBA and Delayed Delivery Securities Risk: The Portfolio may purchase securities on a when-issued, to-be-announced (“TBA”) or delayed delivery basis and may purchase securities on a forward commitment basis. The purchase price of the securities is typically fixed at the time of the commitment, but delivery and payment can take place a month or more after the date of the commitment. At the time of delivery of the securities, the value may be more or less than the purchase or sale price. Purchase of securities on a when-issued, TBA, delayed delivery, or forward commitment basis may give rise to investment leverage, and may result in increased volatility of the Portfolio's net asset value. Default by, or bankruptcy of, a counterparty to a when-issued, TBA or delayed delivery transaction would expose the Portfolio to possible losses because of an adverse market action, expenses or delays in connection with the purchase or sale of the pools specified in such transaction. The Financial Industry Regulatory Authority, Inc. (“FINRA”) imposes mandatory margin requirements for certain types of when-issued, delayed delivery or forward commitment transactions (including TBA transactions), with limited exceptions. Such transactions require mandatory collateralization which may increase the cost of such transactions and impose added operational complexity.
State Street Aggregate Bond Index Portfolio | State Street Aggregate Bond Index Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as a percentage of the lower of the sale proceeds or the original offering price) rr_MaximumDeferredSalesChargeOverOther none
Management Fee rr_ManagementFeesOverAssets none
Distribution and/or Shareholder Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.021%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.002%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.023%
Less Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.002%) [1]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 0.021%
1 year rr_ExpenseExampleYear01 $ 2
3 years rr_ExpenseExampleYear03 7
5 years rr_ExpenseExampleYear05 13
10 years rr_ExpenseExampleYear10 $ 29
2015 rr_AnnualReturn2015 0.65%
2016 rr_AnnualReturn2016 2.39%
2017 rr_AnnualReturn2017 3.38%
2018 rr_AnnualReturn2018 (0.12%)
2019 rr_AnnualReturn2019 8.69%
2020 rr_AnnualReturn2020 7.86%
2021 rr_AnnualReturn2021 (1.72%)
2022 rr_AnnualReturn2022 (13.30%)
2023 rr_AnnualReturn2023 5.61%
2024 rr_AnnualReturn2024 1.34%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel <span style="font-family:Arial;font-size:9.02pt;font-weight:bold;margin-left:0.0pt;">Highest Quarterly Return</span>
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2023
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 6.70%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel <span style="font-family:Arial;font-size:9.02pt;font-weight:bold;margin-left:0.0pt;">Lowest Quarterly Return</span>
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2022
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (5.97%)
OneYear rr_AverageAnnualReturnYear01 1.34%
FiveYears rr_AverageAnnualReturnYear05 (0.33%)
TenYears rr_AverageAnnualReturnYear10 1.30%
State Street Aggregate Bond Index Portfolio | Return After Taxes on Distributions | State Street Aggregate Bond Index Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 (0.18%)
FiveYears rr_AverageAnnualReturnYear05 (1.46%)
TenYears rr_AverageAnnualReturnYear10 0.15%
State Street Aggregate Bond Index Portfolio | Return After Taxes on Distributions and Sale of Fund Shares | State Street Aggregate Bond Index Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 0.79%
FiveYears rr_AverageAnnualReturnYear05 (0.70%)
TenYears rr_AverageAnnualReturnYear10 0.51%
State Street Aggregate Bond Index Portfolio | Bloomberg U.S. Aggregate Bond Index (reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 1.25%
FiveYears rr_AverageAnnualReturnYear05 (0.33%)
TenYears rr_AverageAnnualReturnYear10 1.35%
[1] SSGA FM is contractually obligated to waive up to the portion of the management fee and/or expenses attributable to acquired fund fees and expenses in connection with the Portfolio's investments in acquired funds held for other than cash management purposes. This fee waiver and/or expense reimbursement may only be terminated with approval of the Portfolio's Board of Trustees.