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Risk Return Abstract rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName STATE STREET INSTITUTIONAL INVESTMENT TRUST
Prospectus Date rr_ProspectusDate Apr. 30, 2025
State StreetGlobal All Cap Equity ex-US 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 Global All Cap Equity ex-U.S. 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 Global All Cap Equity ex-U.S. Index Portfolio (the “Global All Cap Equity ex-U.S. Index Portfolio”or the Portfolio”) seeks to provide investment results that, before fees and expenses, correspond generally to the total return performance of a broad-based index of world (ex-U.S.) equity markets 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 Global All Cap Equity ex-U.S. 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 1% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 1.00%
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 is an “index”fund that seeks to track, before fees and expenses, the total return performance of the MSCI ACWI ex USA Investable Market Index (the “MSCI ACWI ex USA IMI 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. As of February 28, 2025, a significant portion of the Index comprised companies in the financials, industrials and information technology sectors, although this may change from time to time. As of February 28, 2025, a significant portion of the Index comprised companies located in Japan and the United Kingdom and a significant portion of the Index constituents are denominated in the Euro and Yen, although this may change from time to time. In seeking to track the performance of the Index, the Portfolio employs a sampling strategy, which means that the Portfolio is not required to purchase all of the securities represented in the Index. Instead, the Portfolio may purchase a subset of the 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 American Depositary Receipts (“ADRs”) or Global Depositary Receipts (“GDRs”) providing exposure to securities comprising 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. 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 also purchase or sell futures contracts, 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 stocks or other investments. 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, forwards or swap transactions, in lieu of investing directly in the stocks making up the Index. The Portfolio may also, 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 a free float-adjusted market capitalization index that is designed to measure the combined equity market performance of securities, across all market capitalizations, in developed and emerging market countries excluding the United States. All listed equity securities and listed securities that exhibit characteristics of equity securities, except mutual funds, ETFs, equity derivatives, limited partnerships and most investment trusts, are eligible for inclusion. Countries covered in the Index have historically included, among others,Australia, Austria, Belgium, Brazil, Canada, Chile, China, Colombia, Czech Republic, Denmark, Egypt, Finland, France, Germany, Greece, Hong Kong, Hungary, India, Indonesia, Ireland, Israel, Italy, Japan, Korea, Kuwait, Malaysia, Mexico, the Netherlands, New Zealand, Norway, Peru, Philippines, Poland, Portugal, Qatar, Saudi Arabia, Singapore, South Africa, Spain, Sweden, Switzerland, Taiwan, Thailand, Turkey, United Arab Emirates and the United Kingdom. It is not possible to invest directly in the Index.The Index is sponsored by MSCI Inc. (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 Return18.27%Q2 2020Lowest Quarterly Return-24.67%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, such as 401(k) plans or individual retirement accounts (</span><span style="color:#000000;font-family:Arial;font-size:10.02pt;">“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”). Effective October 9, 2019 (the “Benchmark Index Change Date”), the Portfolio's benchmark index changed from the MSCI ACWI ex USA Index (the “Previous Benchmark Index”) to the MSCI ACWI ex USA Investable Market Index, consistent with a change in the Portfolio's principal investment strategy to track the performance of the current index. Performance of the Portfolio prior to the Benchmark Index Change Date is therefore based on the Portfolio's investment strategy to track the Previous Benchmark Index.
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 StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US Index Portfolio | Non U S Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Non-U.S. Securities Risk: Non-U.S. securities (including depositary receipts) are subject to political, regulatory, and economic risks not present in domestic investments. There may be less information publicly available about a non-U.S. entity than about a U.S. entity, and many non-U.S. entities are not subject to accounting, auditing, legal and financial report standards comparable to those in the United States. Further, such entities and/or their securities may be subject to risks associated with currency controls; expropriation; changes in tax policy; greater market volatility; differing securities market structures; higher transaction costs; and various administrative difficulties, such as delays in clearing and settling portfolio transactions or in receiving payment of dividends. Securities traded on foreign markets may be less liquid (harder to sell) than securities traded domestically. Foreign governments may impose restrictions on the repatriation of capital to the U.S. In addition, to the extent the Portfolio buys securities denominated in a foreign currency, there are special risks such as changes in currency exchange rates and the risk that a foreign government could regulate foreign exchange transactions. In addition, to the extent investments are made in a limited number of countries, events in those countries will have a more significant impact on the Portfolio. Investments in depositary receipts may be less liquid and more volatile than the underlying shares in their primary trading market.
State StreetGlobal All Cap Equity ex-US Index Portfolio | Geographic Focus Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Geographic Focus Risk: The performance of a fund that invests significantly in one or more countries or geographic regions will be closely tied to market, currency, economic, political, environmental, or regulatory conditions and developments in the countries or regions in which the fund invests, and may be more volatile than the performance of a fund that does not invest significantly in such countries or regions.China: The Chinese economy is generally considered an emerging market and can be significantly affected by economic and political conditions and policy in China and surrounding Asian countries. A relatively small number of Chinese companies represent a large portion of China's total market and thus may be more sensitive to adverse political or economic circumstances and market movements. The economy of China differs, often unfavorably, from the U.S. economy in such respects as structure, general development, government involvement, wealth distribution, rate of inflation, growth rate, allocation of resources and capital reinvestment, among others. Under China's political and economic system, the central government has historically exercised substantial control over virtually every sector of the Chinese economy through administrative regulation and/or state ownership. The Chinese government may intervene or seek to control the operations, structure, or ownership of Chinese companies, including with respect to foreign investors of such companies. In addition, expropriation, including nationalization, confiscatory taxation, political, economic or social instability or other developments could adversely affect and significantly diminish the values of the Chinese companies in which the Fund invests. The Chinese economy and financial markets have experienced high levels of growth in recent years; any actual or perceived reduction or curtailment in those levels of growth in the future would likely have a substantial adverse impact on the values of Chinese companies. These factors and others could negatively affect the value and liquidity of the Fund. The Fund may invest in shares of Chinese companies traded on stock markets in Mainland China or Hong Kong. These stock markets have recently experienced high levels of volatility, which may continue in the future. The Hong Kong stock market may behave differently from the Mainland China stock market and there may be little to no correlation between the performance of the Hong Kong stock market and the Mainland China stock market. The Fund may gain investment exposure to Chinese companies through variable interest entity (“VIE”) structures. The VIE structure enables foreign investors, such as the Fund, to obtain investment exposure to a Chinese company in situations in which the Chinese government has limited or prohibited non-Chinese ownership of such company. The VIE structure does not involve equity ownership in a China-based company but rather involves claims to the China-based company's profits and control of its assets through contractual arrangements. In addition to the risk of government intervention, investments through a VIE structure are subject to the risk that the China-based company (or its officers, directors, or Chinese equity owners) may breach those contractual arrangements, or Chinese law changes in a way that adversely affects the enforceability of these arrangements, or those contracts are otherwise not enforceable under Chinese law, in which case the Fund may suffer significant losses on its investments through a VIE structure with little or no recourse available. If the Chinese government takes action adversely affecting VIE structures, the market value of the Fund's associated portfolio holdings would likely suffer significant, detrimental, and possibly permanent consequences, which could result in substantial investment losses.Europe: Developed and emerging market countries in Europe will be significantly affected by the fiscal and monetary controls of the Economic and Monetary Union of the European Union (“EMU”). Changes in regulations on trade, decreasing imports or exports, changes in the exchange rate of the euro and recessions among European countries may have a significant adverse effect on the economies of other European countries. In addition, one or more countries may abandon the euro and/or withdraw from the EU. On January 31, 2020, the United Kingdom (“UK”) formally withdrew from the EU (commonly known as “Brexit”). An agreement between the UK and the EU governing their future trade relationship became effective January 1, 2021, but that agreement does not include an agreement on financial services, and it is unlikely that such agreement will be concluded. Moreover, the UK government has started a program of financial services law reform with the ultimate aim of repealing many EU financial services laws that were assimilated into UK law from January 1, 2021, and replacing them with legislation or rules made by the UK government or financial services regulators. Accordingly, uncertainty remains in certain areas as to the future relationship between the UK and the EU. Brexit has already had a significant impact on the UK, Europe, and global economies, and could continue to result in volatility and illiquidity, legal, political, economic and regulatory uncertainties and lower economic growth for these economies that could in turn have an adverse effect on the value of the Portfolio's investments. Any further exits from the EU, or the possibility of such exits, or the abandonment of the euro, may cause additional market disruption globally and introduce new legal and regulatory uncertainties. In addition, a number of countries in Europe have suffered terrorist attacks and additional attacks may occur in the future. Such attacks may cause uncertainty in financial markets and may adversely affect the performance of the issuers to which the Portfolio has exposure.Japan: The growth of Japan's economy has historically lagged that of its Asian neighbors and other major developed economies. The Japanese economy is heavily dependent on international trade and has been adversely affected by trade tariffs, other protectionist measures, competition from emerging economies and the economic conditions of its trading partners. China has become an important trading partner with Japan, yet the countries' political relationship has become strained. Should political tension increase, it could adversely affect the economy, especially the export sector, and destabilize the region as a whole. Japan also remains heavily dependent on oil imports, and higher commodity prices could therefore have a negative impact on the economy. The Japanese yen has fluctuated widely at times and any increase in its value may cause a decline in exports that could weaken the Japanese economy. Japan has, in the past, intervened in the currency markets to attempt to maintain or reduce the value of the yen. Japanese intervention in the currency markets could cause the value of the yen to fluctuate sharply and unpredictably and could cause losses to investors. Japan has an aging workforce and has experienced a significant population decline in recent years. Japan's labor market appears to be undergoing fundamental structural changes, as a labor market traditionally accustomed to lifetime employment adjusts to meet the need for increased labor mobility, which may adversely affect Japan's economic competitiveness. Natural disasters, such as earthquakes, volcanoes, typhoons or tsunamis, could occur in Japan or surrounding areas and could negatively affect the Japanese economy and, in turn, the Portfolio.United Kingdom: The United Kingdom has one of the largest economies in Europe, and the United States and other European countries are substantial trading partners of the United Kingdom. As a result, the British economy may be impacted by changes to the economic condition of the United States and other European countries. The British economy relies heavily on the export of financial services to the United States and other European countries and, therefore, a prolonged slowdown in the financial services sector may have a negative impact on the British economy. Continued governmental involvement or control in certain sectors may stifle competition in certain sectors or cause adverse effects on economic growth. In the past, the UK has been a target of terrorism. Acts of terrorism in the UK or against British interests abroad may cause uncertainty in the British financial markets and adversely affect the performance of the issuers to which the Portfolio has exposure.
State StreetGlobal All Cap Equity ex-US Index Portfolio | Currency Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Currency Risk: The value of the Portfolio's assets may be affected favorably or unfavorably by currency exchange rates, currency exchange control regulations, and delays, restrictions or prohibitions on the repatriation of foreign currencies. Foreign currency exchange rates may have significant volatility, and changes in the values of foreign currencies against the U.S. dollar may result in substantial declines in the values of the Portfolio's assets denominated in foreign currencies.
State StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US Index Portfolio | Depositary Receipts Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Depositary Receipts Risk: Investments in depositary receipts may be less liquid and more volatile than the underlying securities in their primary trading market. If a depositary receipt is denominated in a different currency than its underlying securities, the Portfolio will be subject to the currency risk of both the investment in the depositary receipt and the underlying security. Holders of depositary receipts may have limited or no rights to take action with respect to the underlying securities or to compel the issuer of the receipts to take action. The prices of depositary receipts may differ from the prices of securities upon which they are based. To the extent the Portfolio invests in depositary receipts based on securities included in the Index, such differences in prices may increase index tracking risk.
State StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US Index Portfolio | Emerging Markets Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Emerging Markets Risk: Risks of investing in emerging markets include, among others, greater political and economic instability, greater volatility in currency exchange rates, less developed securities markets, increased potential for market manipulation, possible trade barriers, currency transfer restrictions, a more limited number of potential buyers and issuers, an emerging market country's dependence on revenue from particular commodities or international aid, less governmental supervision and regulation, unavailability of currency hedging techniques, differences in auditing and financial reporting standards, less stringent investor protection and disclosure standards, less developed public health systems, and less developed legal systems. There is also the potential for unfavorable action such as expropriation, nationalization, embargoes, and acts of war. The securities of emerging market companies may trade less frequently and in smaller volumes than more widely held securities. Market disruptions or substantial market corrections may limit very significantly the liquidity of securities of certain companies in a particular country or geographic region, or of all companies in the country or region. 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. These risks are generally greater for investments in frontier market countries, which typically have smaller economies or less developed capital markets than traditional emerging market countries.
State StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US Index Portfolio | Industrial Sector Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Industrial Sector Risk: Industrial companies are affected by supply and demand both for their specific product or service and for industrial sector products and services in general. Government regulation, world events, exchange rates 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 securities, another 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 StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US Index Portfolio | Large Capitalization Securities Risk [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Large-Capitalization Securities Risk: Returns on investments in securities of large companies could trail the returns on investments in securities of smaller and mid-sized companies. Larger companies may be unable to respond as quickly as smaller and mid-sized companies to competitive challenges or to changes in business, product, financial, or other market conditions. Larger companies may not be able to maintain growth at the high rates that may be achieved by well-managed smaller and mid-sized companies.
State StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US Index Portfolio | Special Risk Considerations of Investing in China [Member]  
Risk Return Abstract rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock Special Risk Considerations of Investing in China: Investing in securities of Chinese issuers, including by investing in A Shares, involves certain risks and considerations not typically associated with investing in securities of U.S. issuers, including, among others, more frequent trading suspensions and government interventions (including by nationalization of assets or U.S. or foreign government interventions or restrictions that could preclude the Fund from making certain investments or result in the Fund selling investments at disadvantageous times), currency exchange rate fluctuations or blockages, limits on the use of brokers and on foreign ownership, different financial reporting standards, higher dependence on exports and international trade, potential for increased trade tariffs, sanctions, embargoes and other trade limitations, and custody risks associated with programs used to access Chinese securities.Significant portions of the Chinese securities markets may become rapidly illiquid, as Chinese issuers have the ability to suspend the trading of their equity securities, and have shown a willingness to exercise that option in response to market volatility and other events.
State StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US 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 StreetGlobal All Cap Equity ex-US Index Portfolio | State StreetGlobal All Cap Equity ex-US 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 0.04%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.04%
1 year rr_ExpenseExampleYear01 $ 4
3 years rr_ExpenseExampleYear03 13
5 years rr_ExpenseExampleYear05 23
10 years rr_ExpenseExampleYear10 $ 51
2015 rr_AnnualReturn2015 (5.84%)
2016 rr_AnnualReturn2016 5.06%
2017 rr_AnnualReturn2017 27.20%
2018 rr_AnnualReturn2018 (13.99%)
2019 rr_AnnualReturn2019 21.70%
2020 rr_AnnualReturn2020 11.07%
2021 rr_AnnualReturn2021 8.37%
2022 rr_AnnualReturn2022 (16.43%)
2023 rr_AnnualReturn2023 15.68%
2024 rr_AnnualReturn2024 5.00%
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 18.27%
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 (24.67%)
OneYear rr_AverageAnnualReturnYear01 5.00%
FiveYears rr_AverageAnnualReturnYear05 4.09%
TenYears rr_AverageAnnualReturnYear10 4.87%
State StreetGlobal All Cap Equity ex-US Index Portfolio | Return After Taxes on Distributions | State StreetGlobal All Cap Equity ex-US Index Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 3.71%
FiveYears rr_AverageAnnualReturnYear05 3.03%
TenYears rr_AverageAnnualReturnYear10 3.80%
State StreetGlobal All Cap Equity ex-US Index Portfolio | Return After Taxes on Distributions and Sale of Fund Shares | State StreetGlobal All Cap Equity ex-US Index Portfolio  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 2.96%
FiveYears rr_AverageAnnualReturnYear05 2.69%
TenYears rr_AverageAnnualReturnYear10 3.33%
State StreetGlobal All Cap Equity ex-US Index Portfolio | MSCI ACWI ex USA Investable Market Index/MSCI ACWI ex USA Index (reflects no deduction for fees, expenses or taxes other than withholding taxes on reinvested dividends)  
Risk Return Abstract rr_RiskReturnAbstract  
OneYear rr_AverageAnnualReturnYear01 5.23% [1]
FiveYears rr_AverageAnnualReturnYear05 4.12% [1]
TenYears rr_AverageAnnualReturnYear10 4.83% [1]
[1] Returns shown are reflective of the Index for periods beginning on the Benchmark Index Change Date and the Previous Benchmark Index for periods prior to the Benchmark Index Change Date.