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 Small_Mid Cap Equity 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 Small/Mid Cap Equity 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 Small/Mid Cap Equity Index Portfolio (the “Small/Mid Cap Equity Index Portfolio”or the “Portfolio”) seeks to provide investment results that, before expenses, correspond generally to the total return of an index that tracks the performance of mid-to small- capitalization exchange traded U.S. equity securities.
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 Small/Mid Cap Equity 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 24% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 24.00%
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. 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 uses a passive management strategy designed to track the performance of the Russell Small Cap Completeness Index (the “Index”). The Index is a float-adjusted, market capitalization index which measures the performance of the Russell 3000® Index companies excluding S&P 500® constituents. The Index is constructed to provide a comprehensive and unbiased barometer of the extended broad market beyond the S&P 500 exposure. The Index is completely reconstituted annually to ensure new and growing equities are included. As of February 28, 2025, the market capitaliza tion of the companies included in the Index ranged from $1.21 million to $122.09 billion. As of February 28, 2025, the Index comprised 2,457 securities. As of February 28, 2025, a significant portion of the Index comprised companies in the information technology, healthcare, industrials, financials and consumer discretionary sectors, although this may change from time to time. 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. Instead, the Portfolio, using a “passive”or “indexing”investment approach, seeks to provide investment results that, before expenses, correspond generally to the total return of the Index. The Portfolio generally intends to invest in all stocks comprising the Index in approximate proportion to their weightings in the Index. However, under various circumstances, it may not be possible or practicable to purchase all stocks in those weightings. In those circumstances, the Portfolio may purchase a sample of the stocks in the Index in proportions expected by SSGA Funds Management, Inc. (“SSGA FM”or the “Adviser”), the investment adviser to the Portfolio, to match generally the performance of the Index as a whole. In addition, from time to time, stocks are added to or removed from the Index. The Portfolio may sell securities that are represented in the Index, or purchase securities that are not yet represented in the Index, in anticipation of their removal from or addition to the Index. Under normal market conditions, the Portfolio invests at least 80% of its net assets (plus borrowings, if any) in stocks in the Index. The Portfolio will provide shareholders with at least sixty (60) days' notice prior to any change in this 80% investment policy. 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. In addition, the Portfolio may invest in equity securities that are not included in the Index, 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 purchase or sell futures contracts on the Index, or options on those futures, in lieu of investing directly in the stocks making up the Index. The Portfolio might do so, for example, in order to increase its investment exposure pending investment of cash in the stocks comprising the Index. Alternatively, the Portfolio might use futures or options on futures to reduce its investment exposure in situations where it intends to sell a portion of the stocks 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 relating to the Index or any securities comprising the Index, 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 sponsored by FTSE Russell (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 that of (i) a broad measure of market performance and (ii) 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 that of (i) a broad measure of market performance and (ii) 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 Portfolio's</span><span style="color:#000000;font-family:Arial;font-size:10.02pt;margin-left:0%;"> 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 Return30.52%Q2 2020Lowest Quarterly Return-27.89%Q1 2020
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 Small_Mid Cap Equity 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 Small_Mid Cap Equity 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 Small_Mid Cap Equity 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 Small_Mid Cap Equity Index Portfolio | Equity Investing Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Equity Investing Risk: The market prices of equity securities owned by the Portfolio may go up or down, sometimes rapidly or unpredictably. The value of a security may decline for a number of reasons that may directly relate to the issuer and also may decline due to general industry or market conditions that are not specifically related to a particular company. In addition, equity markets tend to move in cycles, which may cause stock prices to fall over short or extended periods of time.
State Street Small_Mid Cap Equity Index Portfolio | Small Mid and Micro Capitalization Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Small-, Mid-, and Micro-Capitalization Securities Risk: The securities of small-, mid-, and micro-capitalization companies may be more volatile and may involve more risk than the securities of larger companies. These companies may have limited product lines, markets or financial resources, may lack the competitive strength of larger companies, and may depend on a few key employees. In addition, these companies may have been recently organized and may have little or no track record of success. The securities of smaller companies may trade less frequently and in smaller volumes than more widely held securities. Some securities of smaller issuers may be illiquid or may be restricted as to resale, and their values may have significant volatility. The Portfolio may be unable to liquidate its positions in such securities at any time, or at a favorable price, in order to meet the Portfolio's obligations. Returns on investments in securities of small-, mid-, and micro-capitalization companies could trail the returns on investments in securities of larger companies.
State Street Small_Mid Cap Equity Index Portfolio | Financial Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Financial Sector Risk: Financial services companies are subject to extensive governmental regulation, which may limit both the amounts and types of loans and other financial commitments they can make, the interest rates and fees they can charge, the scope of their activities, the prices they can charge and the amount of capital they must maintain. Profitability is largely dependent on the availability and cost of capital funds and can fluctuate significantly when interest rates change or due to increased competition. In addition, deterioration of the credit markets generally may cause an adverse impact in a broad range of markets, including U.S. and international credit and interbank money markets generally, thereby affecting a wide range of financial institutions and markets. Certain events in the financial sector may cause an unusually high degree of volatility in the financial markets, both domestic and foreign, and cause certain financial services companies to incur large losses. Securities of financial services companies may experience a dramatic decline in value when such companies experience substantial declines in the valuations of their assets, take action to raise capital (such as the issuance of debt or equity securities), or cease operations. Credit losses resulting from financial difficulties of borrowers and financial losses associated with investment activities can negatively impact the sector. Insurance companies may be subject to severe price competition. Adverse economic, business or political developments could adversely affect financial institutions engaged in mortgage finance or other lending or investing activities directly or indirectly connected to the value of real estate.
State Street Small_Mid Cap Equity Index Portfolio | Health Care Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Healthcare Sector Risk: Companies in the healthcare sector are subject to extensive government regulation and their profitability can be significantly affected by restrictions on government reimbursement for medical expenses, rising costs of medical products and services, pricing pressure (including price discounting), limited product lines and an increased emphasis on the delivery of healthcare through outpatient services. Companies in the healthcare sector are heavily dependent on obtaining and defending patents, which may be time consuming and costly, and the expiration of patents may also adversely affect the profitability of these companies. Healthcare companies are also subject to extensive litigation based on product liability and similar claims. In addition, their products can become obsolete due to industry innovation, changes in technologies or other market developments. Many new products in the healthcare sector require significant research and development and may be subject to regulatory approvals, all of which may be time consuming and costly with no guarantee that any product will come to market.
State Street Small_Mid Cap Equity Index Portfolio | Industrial Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Industrial Sector Risk: Stock prices for the types of companies included in this sector are affected by supply and demand both for their specific product or service and for industrial sector products in general. Government regulation, world events and economic conditions, technological developments and liabilities for environmental damage and general civil liabilities will likewise affect the performance of these companies. Aerospace and defense companies, a component of the industrial sector, can be significantly affected by government spending policies because companies involved in this industry rely to a significant extent on U.S. and foreign government demand for their products and services. Thus, the financial condition of, and investor interest in, aerospace and defense companies are heavily influenced by governmental defense spending policies which are typically under pressure from efforts to control the U.S. (and other) government budgets. Transportation stocks, a component of the industrial sector, are cyclical and have occasional sharp price movements which may result from changes in the economy, fuel prices, labor agreements and insurance costs.
State Street Small_Mid Cap Equity Index Portfolio | Information Technology Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Information Technology Sector Risk: Market or economic factors impacting information technology companies could have a major effect on the value of the Portfolio's investments. The value of stocks of information technology companies is particularly vulnerable to rapid changes in technology product cycles, rapid product obsolescence, government regulation and competition, both domestically and internationally, including competition from foreign competitors with lower production costs. Like other technology companies, information technology companies may have limited product lines, markets, financial resources or personnel. Stocks of technology companies and companies that rely heavily on technology, especially those of smaller, less-seasoned companies, tend to be more volatile than the overall market. Information technology companies are heavily dependent on patent and intellectual property rights, the loss or impairment of which may adversely affect profitability. Additionally, companies in the information technology sector may face dramatic and often unpredictable changes in growth rates and competition for the services of qualified personnel.
State Street Small_Mid Cap Equity 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 Small_Mid Cap Equity Index Portfolio | Consumer Discretionary Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Consumer Discretionary Sector Risk: The success of consumer product manufacturers and retailers is tied closely to the performance of the overall global economy, interest rates, competition and consumer confidence. Success depends heavily on disposable household income and consumer spending. Also, companies in the consumer discretionary sector may be subject to severe competition, which may have an adverse impact on their respective profitability. Changes in demographics and consumer tastes can also affect the demand for, and success of, consumer products and services in the marketplace.
State Street Small_Mid Cap Equity 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 Small_Mid Cap Equity 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 Small_Mid Cap Equity 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 Small_Mid Cap Equity 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 Small_Mid Cap Equity 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 Small_Mid Cap Equity 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 Small_Mid Cap Equity 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 Small_Mid Cap Equity Index Portfolio | State Street Small_Mid Cap Equity 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 0.02%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.02%
1 year rr_ExpenseExampleYear01 $ 2
3 years rr_ExpenseExampleYear03 6
5 years rr_ExpenseExampleYear05 11
10 years rr_ExpenseExampleYear10 $ 26
2016 rr_AnnualReturn2016 16.46%
2017 rr_AnnualReturn2017 18.20%
2018 rr_AnnualReturn2018 (9.07%)
2019 rr_AnnualReturn2019 27.98%
2020 rr_AnnualReturn2020 32.46%
2021 rr_AnnualReturn2021 12.50%
2022 rr_AnnualReturn2022 (25.44%)
2023 rr_AnnualReturn2023 25.18%
2024 rr_AnnualReturn2024 17.20%
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 Jun. 30, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 30.52%
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, 2020
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (27.89%)
OneYear rr_AverageAnnualReturnYear01 17.20%
FiveYears rr_AverageAnnualReturnYear05 10.27%
SinceInception rr_AverageAnnualReturnSinceInception 9.99%
Inception Date rr_AverageAnnualReturnInceptionDate Aug. 11, 2015
State Street Small_Mid Cap Equity Index Portfolio | Return After Taxes on Distributions | State Street Small_Mid Cap Equity Index Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 15.24%
FiveYears rr_AverageAnnualReturnYear05 8.52%
SinceInception rr_AverageAnnualReturnSinceInception 8.28%
State Street Small_Mid Cap Equity Index Portfolio | Return After Taxes on Distributions and Sale of Fund Shares | State Street Small_Mid Cap Equity Index Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 10.88%
FiveYears rr_AverageAnnualReturnYear05 7.68%
SinceInception rr_AverageAnnualReturnSinceInception 7.43%
State Street Small_Mid Cap Equity Index Portfolio | S&P 500 Index (reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 25.02%
FiveYears rr_AverageAnnualReturnYear05 14.53%
SinceInception rr_AverageAnnualReturnSinceInception 13.72%
State Street Small_Mid Cap Equity Index Portfolio | Russell Small Cap Completeness Index (reflects no deduction for fees, expenses or taxes)  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 17.14%
FiveYears rr_AverageAnnualReturnYear05 10.27%
SinceInception rr_AverageAnnualReturnSinceInception 9.99%