Summary of Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 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.
Consolidation	The consolidated financial statements accompanying these condensed notes are unaudited. In the opinion of management, all adjustments, consisting only of normal recurring adjustments, which are necessary for a fair statement of the consolidated results of operations in these financial statements, have been made. Certain previously reported amounts presented in this Form 10-Q have been reclassified to conform to current-period presentation. Events occurring subsequent to the date of our consolidated statement of condition were evaluated for potential recognition or disclosure in our consolidated financial statements through the date we filed this Form 10-Q with the SEC.
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. These accounting estimates reflect the best judgment of management, but actual results could differ. Our consolidated statement of condition as of December 31, 2020 included in the accompanying consolidated financial statements was derived from the audited financial statements as of that date, but does not include all notes required by U.S. GAAP for a complete set of consolidated financial statements. In September 2021, we announced that we had entered into a definitive agreement to acquire the BBH Investor Services business for $3.5 billion in cash.
Recent Accounting Developments	Recent Accounting Developments We did not adopt any new accounting standards in the third quarter of 2021 that had a material impact to our financial statements. Additionally, we continue to evaluate accounting standards that were recently issued but not yet adopted as of September 30, 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. For information about our valuation techniques for financial assets and financial liabilities measured at fair value and the fair value hierarchy, refer to pages 135 to 142 in Note 2 to the consolidated financial statements included under Item 8, Financial Statements and Supplementary Data, in our 2020 Form 10-K.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 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.

Consolidation

The consolidated financial statements accompanying these condensed notes are unaudited. In the opinion of management, all adjustments, consisting only of normal recurring adjustments, which are necessary for a fair statement of the consolidated results of operations in these financial statements, have been made. Certain previously reported amounts presented in this Form 10-Q have been reclassified to conform to current-period presentation. Events occurring subsequent to the date of our consolidated statement of condition were evaluated for potential recognition or disclosure in our consolidated financial statements through the date we filed this Form 10-Q with the SEC.

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. These accounting estimates reflect the best judgment of management, but actual results could differ.

Our consolidated statement of condition as of December 31, 2020 included in the accompanying consolidated financial statements was derived from the audited financial statements as of that date, but does not include all notes required by U.S. GAAP for a complete set of consolidated financial statements.

In September 2021, we announced that we had entered into a definitive agreement to acquire the BBH Investor Services business for $3.5 billion in cash.

Recent Accounting Developments

We did not adopt any new accounting standards in the third quarter of 2021 that had a material impact to our financial statements. Additionally, we continue to evaluate accounting standards that were recently issued but not yet adopted as of September 30, 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. For information about our valuation techniques for financial assets and financial liabilities measured at fair value and the fair value hierarchy, refer to pages 135 to 142 in Note 2 to the consolidated financial statements included under Item 8, Financial Statements and Supplementary Data, in our 2020 Form 10-K.