Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2015
Accounting Policies [Abstract]
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 processing fees and other revenue in our consolidated statement of income. Investments not meeting the criteria for equity-method treatment 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 with the same counterparty or clearing house and maturity date are recorded on a net basis.
Fee And Net Interest Revenue	Fee and Net Interest Revenue: Fees from investment servicing, investment management, securities finance, trading services and certain types of processing fees and other revenue are recorded in our consolidated statement of income based on estimates or specific contractual terms, including mutually agreed changes to terms, as transactions occur or services are rendered, provided that persuasive evidence exists, the price to the client is fixed or determinable and collectability is reasonably assured. Amounts accrued at period-end are recorded in accrued interest and fees receivable in our consolidated statement of condition. Performance fees generated by our investment management activities are recorded when earned, based on predetermined benchmarks associated with the applicable fund’s performance. Interest revenue on interest-earning assets and interest expense on interest-bearing liabilities are recorded in our consolidated statement of income as components of net interest revenue, and are generally based on the effective yield of the related financial asset or liability.
Recent Accounting Developments	Recent Accounting Developments: In January 2016, the FASB issued an amendment that changes the accounting for equity securities. Under the revised standard, all equity securities will be measured at fair value through earnings with certain exceptions, including investments accounted for under the equity method of accounting. In addition, the FASB clarified guidance related to valuation allowance assessments when recognizing deferred tax assets on unrealized losses on available-for-sale debt securities. The amendments are effective for State Street beginning January 1, 2018. We are currently assessing the potential impact of these amendments on our consolidated financial statements. In September 2015 the FASB issued an amendment that requires an acquirer to recognize purchase price adjustments to provisional amounts in the reporting period in which the adjustments are determined, as opposed to being applied retrospectively at the acquisition date. The amendment, which allows for early adoption, is effective for State Street beginning on January 1, 2016. We did not have any significant acquisitions in 2015 that could have a material impact on future consolidated financial statements. We will assess the amendment's impact in conjunction with new transactions, as applicable. In July 2015, the FASB issued an update to delay the effective date of the new revenue standard by one year. The deferral relates to the FASB standard issued in May 2014, which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. The standard permits the use of either retrospective or cumulative effect transition method. The deferral results in the new revenue standard being effective for State Street beginning on January 1, 2018. We are evaluating the effect on the consolidated financial statements and related disclosures. In May 2015, the FASB issued an amendment that makes certain technical corrections to the FASB Accounting Standards Codification that affect a wide variety of topics to clarify the codification, correct unintended application of the guidance, or make minor improvements that are not expected to have a significant effect on current accounting practice. The amendments that require transition guidance are effective for State Street beginning on January 1, 2016 and all other amendments are effective immediately. Our adoption of this amendment will not have a material impact on our consolidated financial statements. In May 2015, the FASB issued an amendment to U.S. GAAP which removes from the fair value hierarchy, investments for which the practical expedient is used to measure fair value at NAV. Instead, an entity is required to include those investments as a reconciling line item so that the total fair value amount of investments in the disclosure is consistent with the amount on the balance sheet. The amendment is effective for State Street beginning on January 1, 2016 and is required to be applied retrospectively. The adoption of this amendment will have no impact on our consolidated financial statements. In April 2015, the FASB issued an amendment to U.S. GAAP which will assist entities in evaluating the accounting for fees paid by a customer in a cloud computing arrangement. The amendment, which allows for early adoption, is effective for State Street beginning on January 1, 2016. Our adoption of this amendment is not expected to have a material effect on our consolidated financial statements. In April 2015, the FASB issued an amendment to U.S. GAAP that requires debt issuance costs to be presented in the consolidated balance sheet as a direct deduction from the carrying value of the associated debt liability, consistent with the presentation of a debt discount. This amendment, which allows for early adoption, is effective for State Street beginning on January 1, 2016. Our debt issuance costs that are currently classified as deferred credits have a balance of approximately $37 million as of December 31, 2015 and will be reclassified as contra liabilities upon adoption. In August 2015, the FASB issued a related amendment to clarify that debt issuance costs relating to line of credit arrangements may still be presented as an asset, notwithstanding the April 2015 amendment that requires debt issuance costs relating to recognized debt liabilities to be recognized as contra liabilities. Our adoption of this amendment will not have a material effect on our consolidated financial statements. In February 2015, the FASB issued an amendment to U.S. GAAP that updates the consolidation model used to evaluate whether a legal entity is required to be consolidated. The amendment is effective for State Street beginning on January 1, 2016, and may be applied retrospectively or via a modified retrospective approach. The amendment eliminated the indefinite deferral of Accounting Standard Update 2010-10 “Amendments for Certain Investment Funds” for asset management funds with characteristics of an investment company and also eliminated the presumption that a general partner should consolidate a limited partnership. The amendment also changed the consolidation analysis for fee arrangements that meet certain requirements and for related party relationships. Certain money market funds are excluded from the scope of the amendment. Based on our assessment of the amendment, our adoption of this amendment will not have a material effect on our consolidated financial statements. In February 2015, the FASB issued an amendment to U.S. GAAP to remove the concept of "extraordinary items," which are defined as items that are unusual and infrequent in nature. The amendment, which allows for early adoption, is effective for State Street beginning on January 1, 2016. Our adoption of this amendment will not have a material effect on our consolidated financial statements.

Disclosure of accounting policy for cash and cash equivalents, including the policy for determining which items are treated as cash equivalents. Other information that may be disclosed includes (1) the nature of any restrictions on the entity's use of its cash and cash equivalents, (2) whether the entity's cash and cash equivalents are insured or expose the entity to credit risk, (3) the classification of any negative balance accounts (overdrafts), and (4) the carrying basis of cash equivalents (for example, at cost) and whether the carrying amount of cash equivalents approximates fair value.

- Definition

Disclosure of accounting policy for revenue recognition. If the entity has different policies for different types of revenue transactions, the policy for each material type of transaction is generally disclosed. If a sales transaction has multiple element arrangements (for example, delivery of multiple products, services or the rights to use assets) the disclosure may indicate the accounting policy for each unit of accounting as well as how units of accounting are determined and valued. The disclosure may encompass important judgment as to appropriateness of principles related to recognition of revenue. The disclosure also may indicate the entity's treatment of any unearned or deferred revenue that arises from the transaction.

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