Summary of Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accounting and financial reporting policies of State Street Corporation conform to U.S. GAAP. State Street Corporation, the Parent Company, is a financial holding company headquartered in Boston, Massachusetts. Unless otherwise indicated or unless the context requires otherwise, all references in these notes to consolidated financial statements to “State Street,” “we,” “us,” “our” or similar references mean State Street Corporation and its subsidiaries on a consolidated basis, including our principal banking subsidiary, State Street Bank. We have two lines of business: Investment Servicing, through State Street Institutional Services, State Street Global Markets, State Street Digital and CRD, provides services for institutional clients, including mutual funds, collective investment funds and other investment pools, corporate and public retirement plans, insurance companies, investment managers, foundations and endowments worldwide. Products include: custody; product accounting; daily pricing and administration; master trust and master custody; depotbank services (a fund oversight role created by non-U.S. regulation); record-keeping; cash management; foreign exchange, brokerage and other trading services; securities finance and enhanced custody products; deposit and short-term investment facilities; loans and lease financing; investment manager and alternative investment manager operations outsourcing; performance, risk and compliance analytics; and financial data management to support institutional investors. Included within our Investment Servicing line of business is CRD, which we acquired in 2018. The Charles River Investment Management System is a technology offering which is designed to automate and simplify the institutional investment process across asset classes, from portfolio management and risk analytics through trading and post-trade settlement, with integrated compliance and managed data throughout. With the acquisition of CRD, we took the first step in building our front-to-back platform, State Street Alpha. Today our State Street Alpha platform combines portfolio management, trading and execution, analytics and compliance tools, and advanced data aggregation and integration with other industry platforms and providers. In 2021, we further expanded our technology offering with the acquisition of Mercatus, Inc., enabling the launch of Alpha for Private Markets. Investment Management, through State Street Global Advisors, provides a broad range of investment management strategies and products for our clients. Our investment management strategies and products span the risk/reward spectrum for equity, fixed income and cash assets, including core and enhanced indexing, multi-asset strategies, active quantitative and fundamental active capabilities and alternative investment strategies. Our AUM is currently primarily weighted to indexed strategies. In addition, we provide a breadth of services and solutions, including ESG investing, defined benefit and defined contribution and Global Fiduciary Solutions (formerly Outsourced Chief Investment Officer). State Street Global Advisors is also a provider of ETFs, including the SPDR® ETF brand.
Consolidation	Consolidation Our consolidated financial statements include the accounts of the Parent Company and its majority- and wholly-owned and otherwise controlled subsidiaries, including State Street Bank. All material inter-company transactions and balances have been eliminated. Certain previously reported amounts have been reclassified to conform to current-year presentation. We consolidate subsidiaries in which we exercise control. Investments in unconsolidated subsidiaries, recorded in other assets, generally are accounted for under the equity method of accounting if we have the ability to exercise significant influence over the operations of the investee. For investments accounted for under the equity method, our share of income or loss is recorded in software and processing fees in our consolidated statement of income. Investments not meeting the criteria for equity-method treatment are measured at fair value through earnings, except for investments where a fair market value is not readily available, which are accounted for under the cost method of accounting.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions in the application of certain of our significant accounting policies that may materially affect the reported amounts of assets, liabilities, equity, revenue and expenses. As a result of unanticipated events or circumstances, actual results could differ from those estimates.
Foreign Currency Translation	Foreign Currency Translation The assets and liabilities of our operations with functional currencies other than the U.S. dollar are translated at month-end exchange rates, and revenue and expenses are translated at rates that approximate average monthly exchange rates. Gains or losses from the translation of the net assets of subsidiaries with functional currencies other than the U.S. dollar, net of related taxes, are recorded in AOCI, a component of shareholders’ equity.
Cash and Cash Equivalents	Cash and Cash Equivalents For purposes of the consolidated statement of cash flows, cash and cash equivalents are defined as cash and due from banks.
Interest-Bearing Deposits with Banks	Interest-Bearing Deposits with Banks Interest-bearing deposits with banks generally consist of highly liquid, short-term investments maintained at the Federal Reserve Bank and other non-U.S. central banks with original maturities at the time of purchase of one month or less.
Securities Purchased Under Resale Agreements and Securities Sold Under Repurchase Agreements	Securities Purchased Under Resale Agreements and Securities Sold Under Repurchase Agreements Securities purchased under resale agreements and sold under repurchase agreements are treated as collateralized financing transactions, and are recorded in our consolidated statement of condition at the amounts at which the securities will be subsequently resold or repurchased, plus accrued interest. Our policy is to take possession or control of securities underlying resale agreements either directly or through agent banks, allowing borrowers the right of collateral substitution and/or short-notice termination. We revalue these securities daily to determine if additional collateral is necessary from the borrower to protect us against credit exposure. We can use these securities as collateral for repurchase agreements. For securities sold under repurchase agreements collateralized by our investment securities portfolio, the dollar value of the securities remains in investment securities in our consolidated statement of condition. Where a master netting agreement exists or both parties are members of a common clearing organization, resale and repurchase agreements are recorded on a net basis when specific netting criteria are met.
Fee and Net Interest Income	Fee and Net Interest Income The majority of fees from investment servicing, investment management, securities finance, trading services and certain types of software and processing fees are recorded in our consolidated statement of income based on the consideration specified in contracts with our customers, and excludes taxes collected from customers subsequently remitted to governmental authorities. We recognize revenue as the services are performed or at a point in time depending on the nature of the services provided. Payments made to third party service providers are generally recognized on a gross basis when we control those services and are deemed to be the principal. Additional information about revenue from contracts with customers is provided in Note 25. Interest income on interest-earning assets and interest expense on interest-bearing liabilities are recorded in our consolidated statement of income as components of NII, and are generally based on the effective yield of the related financial asset or liability.
Recent Accounting Developments	Recent Accounting Developments The FASB issued ASU 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting, which was effective as of March 12, 2020. The guidance (1) provides temporary optional expedients and exceptions to the existing guidance on contract modifications and hedge accounting in relation to the transition from LIBOR and other rates impacted by reference rate reform to alternative reference rates; and (2) allows a one-time election to transfer to AFS or trading HTM debt securities that reference an interest rate affected by reference rate reform. In January 2021, the FASB issued ASU No. 2021-01, Reference Rate Reform (Topic 848): Reference Rate Reform - Scope, which clarifies that the scope of the initial accounting relief includes derivative instruments that do not reference a rate that is expected to be discontinued but that use an interest rate for margining, discounting or contract price alignment that is modified as a result of reference rate reform. In the fourth quarter of 2021, we made the one-time election to reclassify to AFS certain HTM securities that reference an interest rate affected by reference rate reform. Securities with a book value of $438 million referencing LIBOR and other eligible reference rates were reclassified from HTM to AFS. We recognized a $72 million unrealized gain in AOCI on the reclassification of such securities and a $58 million realized gain on securities reclassified and subsequently sold. We also elected to apply certain optional expedients related to hedge accounting and contract modification with no significant impact on the 2021 results. Additionally, we continue to evaluate accounting standards that were recently issued but not yet adopted as of December 31, 2021; none are expected to have a material impact to our financial statements.
Fair Value Measurements	Fair Value Measurements We carry trading account assets and liabilities, AFS debt securities, certain equity securities and various types of derivative financial instruments, at fair value in our consolidated statement of condition on a recurring basis. Changes in the fair values of these financial assets and liabilities are recorded either as components of our consolidated statement of income or as components of AOCI within shareholders' equity in our consolidated statement of condition. We measure fair value for the above-described financial assets and liabilities in conformity with U.S. GAAP that governs the measurement of the fair value of financial instruments. Management believes that its valuation techniques and underlying assumptions used to measure fair value conform to the provisions of U.S. GAAP. We categorize the financial assets and liabilities that we carry at fair value based on a prescribed three-level valuation hierarchy. The hierarchy gives the highest priority to quoted prices in active markets for identical assets or liabilities (level 1) and the lowest priority to valuation methods using significant unobservable inputs (level 3). If the inputs used to measure a financial asset or liability cross different levels of the hierarchy, categorization is based on the lowest-level input that is significant to the fair-value measurement. Management's assessment of the significance of a particular input to the overall fair-value measurement of a financial asset or liability requires judgment, and considers factors specific to that asset or liability. The three levels of the valuation hierarchy are described below. Level 1. Financial assets and liabilities with values based on unadjusted quoted prices for identical assets or liabilities in an active market. Our level 1 financial assets and liabilities primarily include positions in U.S. government securities and highly liquid U.S. and non-U.S. government fixed-income securities. Our level 1 financial assets also include actively traded exchange- traded equity securities. Level 2. Financial assets and liabilities with values based on quoted prices for similar assets and liabilities in active markets, and inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the asset or liability. Level 2 inputs include the following: ▪Quoted prices for similar assets or liabilities in active markets; ▪Quoted prices for identical or similar assets or liabilities in non-active markets; ▪Pricing models whose inputs are observable for substantially the full term of the asset or liability; and ▪Pricing models whose inputs are derived principally from, or corroborated by, observable market information through correlation or other means for substantially the full term of the asset or liability. Our level 2 financial assets and liabilities primarily include non-U.S. debt securities carried in trading account assets and various types of fixed-income AFS investment securities, as well as various types of foreign exchange and interest rate derivative instruments. Fair value for our AFS investment securities categorized in level 2 is measured primarily using information obtained from independent third parties. This third-party information is subject to review by management as part of a validation process, which includes obtaining an understanding of the underlying assumptions and the level of market participant information used to support those assumptions. In addition, management compares significant assumptions used by third parties to available market information. Such information may include known trades or, to the extent that trading activity is limited, comparisons to market research information pertaining to credit expectations, execution prices and the timing of cash flows and, where information is available, back- testing. Derivative instruments categorized in level 2 predominantly represent foreign exchange contracts used in our trading activities, for which fair value is measured using discounted cash-flow techniques, with inputs consisting of observable spot and forward points, as well as observable interest rate curves. With respect to derivative instruments, we evaluate the impact on valuation of the credit risk of our counterparties. We consider factors such as the likelihood of default by our counterparties, our current and potential future net exposures and remaining maturities in determining the fair value. Valuation adjustments associated with derivative instruments were not material to those instruments for the years ended December 31, 2021 and 2020. Level 3. Financial assets and liabilities with values based on prices or valuation techniques that require inputs that are both unobservable in the market and significant to the overall measurement of fair value. These inputs reflect management's judgment about the assumptions that a market participant would use in pricing the financial asset or liability, and are based on the best available information, some of which may be internally developed. The following provides a more detailed discussion of our financial assets and liabilities that we may categorize in level 3 and the related valuation methodology. •The fair value of our investment securities categorized in level 3 is measured using information obtained from third-party sources, typically non-binding broker/dealer quotes, or through the use of internally-developed pricing models. Management has evaluated its methodologies used to measure fair value and has considered the level of observable market information to be insufficient to categorize the securities in level 2. •The fair value of certain foreign exchange contracts, primarily options, is measured using an option-pricing model. Because of a limited number of observable transactions, certain model inputs are not observable, such as implied volatility surface, but are derived from observable market information. Our level 3 financial assets and liabilities are similar in structure and profile to our level 1 and level 2 financial instruments, but they trade in less liquid markets, and the measurement of their fair value is therefore less observable.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2021

The accounting and financial reporting policies of State Street Corporation conform to U.S. GAAP. State Street Corporation, the Parent Company, is a financial holding company headquartered in Boston, Massachusetts. Unless otherwise indicated or unless the context requires otherwise, all references in these notes to consolidated financial statements to “State Street,” “we,” “us,” “our” or similar references mean State Street Corporation and its subsidiaries on a consolidated basis, including our principal banking subsidiary, State Street Bank.

We have two lines of business:

Investment Servicing, through State Street Institutional Services, State Street Global Markets, State Street Digital and CRD, provides services for institutional clients, including mutual funds, collective investment funds and other investment pools, corporate and public retirement plans, insurance companies, investment managers, foundations and endowments worldwide. Products include: custody; product accounting; daily pricing and administration; master trust and master custody; depotbank services (a fund oversight role created by non-U.S. regulation); record-keeping; cash management; foreign exchange, brokerage and other trading services; securities finance and enhanced custody products; deposit and short-term investment facilities; loans and lease financing; investment manager and alternative investment manager operations outsourcing; performance, risk and compliance analytics; and financial data management to support institutional investors.

Included within our Investment Servicing line of business is CRD, which we acquired in 2018. The Charles River Investment Management System is a technology offering which is designed to automate and simplify the institutional investment process across asset classes, from portfolio management and risk analytics through trading and post-trade settlement, with integrated compliance and managed data throughout. With the acquisition of CRD, we took the first step in building our front-to-back platform, State Street Alpha. Today our State Street Alpha platform combines portfolio management, trading and execution, analytics and compliance tools, and advanced data aggregation and integration with other industry platforms and providers. In 2021, we further expanded our technology offering with the acquisition of Mercatus, Inc., enabling the launch of Alpha for Private Markets.

Investment Management, through State Street Global Advisors, provides a broad range of investment management strategies and products for our clients. Our investment management strategies

and products span the risk/reward spectrum for equity, fixed income and cash assets, including core and enhanced indexing, multi-asset strategies, active quantitative and fundamental active capabilities and alternative investment strategies. Our AUM is currently primarily weighted to indexed strategies. In addition, we provide a breadth of services and solutions, including ESG investing, defined benefit and defined contribution and Global Fiduciary Solutions (formerly Outsourced Chief Investment Officer). State Street Global Advisors is also a provider of ETFs, including the SPDR® ETF brand.

Consolidation

Our consolidated financial statements include the accounts of the Parent Company and its majority- and wholly-owned and otherwise controlled subsidiaries, including State Street Bank. All material inter-company transactions and balances have been eliminated. Certain previously reported amounts have been reclassified to conform to current-year presentation.

We consolidate subsidiaries in which we exercise control. Investments in unconsolidated subsidiaries, recorded in other assets, generally are accounted for under the equity method of accounting if we have the ability to exercise significant influence over the operations of the investee. For investments accounted for under the equity method, our share of income or loss is recorded in software and processing fees in our consolidated statement of income. Investments not meeting the criteria for equity-method treatment are measured at fair value through earnings, except for investments where a fair market value is not readily available, which are accounted for under the cost method of accounting.

Use of Estimates

The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions in the application of certain of our significant accounting policies that may materially affect the reported amounts of assets, liabilities, equity, revenue and expenses. As a result of unanticipated events or circumstances, actual results could differ from those estimates.

Foreign Currency Translation

The assets and liabilities of our operations with functional currencies other than the U.S. dollar are translated at month-end exchange rates, and revenue and expenses are translated at rates that approximate average monthly exchange rates. Gains or losses from the translation of the net assets of subsidiaries with functional currencies other than the U.S. dollar, net of related taxes, are recorded in AOCI, a component of shareholders’ equity.

Cash and Cash Equivalents

For purposes of the consolidated statement of cash flows, cash and cash equivalents are defined as cash and due from banks.

Interest-Bearing Deposits with Banks

Interest-bearing deposits with banks generally consist of highly liquid, short-term investments maintained at the Federal Reserve Bank and other non-U.S. central banks with original maturities at the time of purchase of one month or less.

Securities Purchased Under Resale Agreements and Securities Sold Under Repurchase Agreements

Securities purchased under resale agreements and sold under repurchase agreements are treated as collateralized financing transactions, and are recorded in our consolidated statement of condition at the amounts at which the securities will be subsequently resold or repurchased, plus accrued interest. Our policy is to take possession or control of securities underlying resale agreements either directly or through agent banks, allowing borrowers the right of collateral substitution and/or short-notice termination. We revalue these securities daily to determine if additional collateral is necessary from the borrower to protect us against credit exposure. We can use these securities as collateral for repurchase agreements.

For securities sold under repurchase agreements collateralized by our investment securities portfolio, the dollar value of the securities remains in investment securities in our consolidated statement of condition. Where a master netting agreement exists or both parties are members of a common clearing organization, resale and repurchase agreements are recorded on a net basis when specific netting criteria are met.

Fee and Net Interest Income

The majority of fees from investment servicing, investment management, securities finance, trading services and certain types of software and processing fees are recorded in our consolidated statement of income based on the consideration specified in contracts with our customers, and excludes taxes collected from customers subsequently remitted to governmental authorities. We recognize revenue as the services are performed or at a point in time depending on the nature of the services provided. Payments made to third party service providers are generally recognized on a gross basis when we control those services and are deemed to be the principal. Additional information about revenue from contracts with customers is provided in Note 25.

Interest income on interest-earning assets and interest expense on interest-bearing liabilities are

recorded in our consolidated statement of income as components of NII, and are generally based on the effective yield of the related financial asset or liability.

Recent Accounting Developments

The FASB issued ASU 2020-04, Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting, which was effective as of March 12, 2020. The guidance (1) provides temporary optional expedients and exceptions to the existing guidance on contract modifications and hedge accounting in relation to the transition from LIBOR and other rates impacted by reference rate reform to alternative reference rates; and (2) allows a one-time election to transfer to AFS or trading HTM debt securities that reference an interest rate affected by reference rate reform. In January 2021, the FASB issued ASU No. 2021-01, Reference Rate Reform (Topic 848): Reference Rate Reform - Scope, which clarifies that the scope of the initial accounting relief includes derivative instruments that do not reference a rate that is expected to be discontinued but that use an interest rate for margining, discounting or contract price alignment that is modified as a result of reference rate reform.

In the fourth quarter of 2021, we made the one-time election to reclassify to AFS certain HTM securities that reference an interest rate affected by reference rate reform. Securities with a book value of $438 million referencing LIBOR and other eligible reference rates were reclassified from HTM to AFS. We recognized a $72 million unrealized gain in AOCI on the reclassification of such securities and a $58 million realized gain on securities reclassified and subsequently sold.

We also elected to apply certain optional expedients related to hedge accounting and contract modification with no significant impact on the 2021 results.

Additionally, we continue to evaluate accounting standards that were recently issued but not yet adopted as of December 31, 2021; none are expected to have a material impact to our financial statements.

Fair Value Measurements

We carry trading account assets and liabilities, AFS debt securities, certain equity securities and various types of derivative financial instruments, at fair value in our consolidated statement of condition on a recurring basis. Changes in the fair values of these financial assets and liabilities are recorded either as components of our consolidated statement of income or as components of AOCI within shareholders' equity in our consolidated statement of condition.

We measure fair value for the above-described financial assets and liabilities in conformity with U.S. GAAP that governs the measurement of the fair value

of financial instruments. Management believes that its valuation techniques and underlying assumptions used to measure fair value conform to the provisions of U.S. GAAP. We categorize the financial assets and liabilities that we carry at fair value based on a prescribed three-level valuation hierarchy. The hierarchy gives the highest priority to quoted prices in active markets for identical assets or liabilities (level 1) and the lowest priority to valuation methods using significant unobservable inputs (level 3). If the inputs used to measure a financial asset or liability cross different levels of the hierarchy, categorization is based on the lowest-level input that is significant to the fair-value measurement. Management's assessment of the significance of a particular input to the overall fair-value measurement of a financial asset or liability requires judgment, and considers factors specific to that asset or liability. The three levels of the valuation hierarchy are described below.

Level 1. Financial assets and liabilities with values based on unadjusted quoted prices for identical assets or liabilities in an active market. Our level 1 financial assets and liabilities primarily include positions in U.S. government securities and highly liquid U.S. and non-U.S. government fixed-income securities. Our level 1 financial assets also include actively traded exchange- traded equity securities.

Level 2. Financial assets and liabilities with values based on quoted prices for similar assets and liabilities in active markets, and inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the asset or liability. Level 2 inputs include the following:

▪Quoted prices for similar assets or liabilities in active markets;

▪Quoted prices for identical or similar assets or liabilities in non-active markets;

▪Pricing models whose inputs are observable for substantially the full term of the asset or liability; and

▪Pricing models whose inputs are derived principally from, or corroborated by, observable market information through correlation or other means for substantially the full term of the asset or liability.

Our level 2 financial assets and liabilities primarily include non-U.S. debt securities carried in trading account assets and various types of fixed-income AFS investment securities, as well as various types of foreign exchange and interest rate derivative instruments.

Fair value for our AFS investment securities categorized in level 2 is measured primarily using information obtained from independent third parties.

This third-party information is subject to review by management as part of a validation process, which includes obtaining an understanding of the underlying assumptions and the level of market participant information used to support those assumptions. In addition, management compares significant assumptions used by third parties to available market information. Such information may include known trades or, to the extent that trading activity is limited, comparisons to market research information pertaining to credit expectations, execution prices and the timing of cash flows and, where information is available, back- testing.

Derivative instruments categorized in level 2 predominantly represent foreign exchange contracts used in our trading activities, for which fair value is measured using discounted cash-flow techniques, with inputs consisting of observable spot and forward points, as well as observable interest rate curves. With respect to derivative instruments, we evaluate the impact on valuation of the credit risk of our counterparties. We consider factors such as the likelihood of default by our counterparties, our current and potential future net exposures and remaining maturities in determining the fair value. Valuation adjustments associated with derivative instruments were not material to those instruments for the years ended December 31, 2021 and 2020.

Level 3. Financial assets and liabilities with values based on prices or valuation techniques that require inputs that are both unobservable in the market and significant to the overall measurement of fair value. These inputs reflect management's judgment about the assumptions that a market

participant would use in pricing the financial asset or liability, and are based on the best available information, some of which may be internally developed. The following provides a more detailed discussion of our financial assets and liabilities that we may categorize in level 3 and the related valuation methodology.

•The fair value of our investment securities categorized in level 3 is measured using information obtained from third-party sources, typically non-binding broker/dealer quotes, or through the use of internally-developed pricing models. Management has evaluated its methodologies used to measure fair value and has considered the level of observable market information to be insufficient to categorize the securities in level 2.

•The fair value of certain foreign exchange contracts, primarily options, is measured using an option-pricing model. Because of a limited number of observable transactions, certain model inputs are not observable, such as implied volatility surface, but are derived from observable market information.

Our level 3 financial assets and liabilities are similar in structure and profile to our level 1 and level 2 financial instruments, but they trade in less liquid markets, and the measurement of their fair value is therefore less observable.