Recent Accounting Pronouncements (Policies)	9 Months Ended
Sep. 30, 2025
Accounting Changes and Error Corrections [Abstract]
Basis of Presentation	The consolidated financial statements include the accounts of Northern Trust Corporation (Corporation) and its wholly-owned subsidiary, The Northern Trust Company (Bank), and various other wholly-owned subsidiaries of the Corporation and Bank. Throughout the notes to the consolidated financial statements, the term “Northern Trust” refers to the Corporation and its subsidiaries. Intercompany balances and transactions have been eliminated in consolidation. The consolidated financial statements, as of and for the periods ended September 30, 2025 and 2024, have not been audited by the Corporation’s independent registered public accounting firm. In the opinion of management, all accounting entries and adjustments, including normal recurring accruals, necessary for a fair presentation of the financial position and the results of operations for the interim periods have been made. The accounting and financial reporting policies of Northern Trust conform to U.S. generally accepted accounting principles (GAAP) and reporting practices prescribed for the banking industry.
Recent Accounting Pronouncements	On January 1, 2025, Northern Trust adopted ASU No. 2023-08, “Intangibles—Goodwill and Other—Crypto Assets (Subtopic 350-60): Accounting for and Disclosure of Crypto Assets” (ASU 2023-08). ASU 2023-08 requires entities to subsequently measure certain crypto assets at fair value, with changes in fair value recorded in net income in each reporting period, and present crypto assets separately from other intangible assets on the face of the balance sheet and changes in fair value of crypto assets separately from changes in the carrying amount of other intangible assets on the statement of income. ASU 2023-08 also requires enhanced disclosures about in-scope crypto assets and respective activities. As Northern Trust does not hold crypto assets, upon adoption of ASU 2023-08, there was no impact on the consolidated balance sheets or consolidated statements of income.
Fair Value Measurements	Fair Value Hierarchy. The following describes the hierarchy of valuation inputs (Levels 1, 2, and 3) used to measure fair value and the primary valuation methodologies used by Northern Trust for financial instruments measured at fair value on a recurring basis. Observable inputs reflect market data obtained from sources independent of the reporting entity; unobservable inputs reflect the entity’s own assumptions about how market participants would value an asset or liability based on the best information available. GAAP requires an entity measuring fair value to maximize the use of observable inputs and minimize the use of unobservable inputs and establishes a fair value hierarchy of inputs. Financial instruments are categorized within the hierarchy based on the lowest level input that is significant to their valuation. No transfers into or out of Level 3 occurred during the nine months ended September 30, 2025 or the year ended December 31, 2024. Level 1 — Quoted, active market prices for identical assets or liabilities. Northern Trust’s Level 1 assets are comprised primarily of AFS investments in U.S. Treasury securities. Level 2 — Observable inputs other than Level 1 prices, such as quoted active market prices for similar assets or liabilities, quoted prices for identical or similar assets in inactive markets, and model-derived valuations in which all significant inputs are observable in active markets. Northern Trust’s Level 2 assets include AFS debt securities, the fair values of which are determined predominantly by external pricing vendors. Prices received from vendors are compared to other vendor and third-party prices. If a security price obtained from a pricing vendor is determined to exceed predetermined tolerance levels that are assigned based on an asset type’s characteristics, the exception is researched and, if the price is not able to be validated, an alternate pricing vendor is utilized, consistent with Northern Trust’s pricing source hierarchy. As of September 30, 2025, Northern Trust’s AFS debt securities portfolio included 980 Level 2 debt securities with an aggregate market value of $24.7 billion, all valued by external pricing vendors. As of December 31, 2024, Northern Trust’s AFS debt securities portfolio included 940 Level 2 debt securities with an aggregate market value of $21.6 billion, all valued by external pricing vendors. Level 2 assets and liabilities also include derivative contracts which are valued internally using widely accepted income-based models that incorporate inputs readily observable in actively quoted markets and reflect the contractual terms of the contracts. Observable inputs include foreign exchange rates and interest rates for foreign exchange contracts; interest rates for interest rate swap contracts and forward contracts; and interest rates and volatility inputs for interest rate option contracts. Northern Trust evaluates the impact of counterparty credit risk and its own credit risk on the valuation of its derivative instruments. Factors considered include the likelihood of default by Northern Trust and its counterparties, the remaining maturities of the instruments, net exposures after giving effect to master netting arrangements or similar agreements, available collateral, and other credit enhancements in determining the appropriate fair value of derivative instruments. The resulting valuation adjustments have not been considered material. Level 3 — Valuation techniques in which one or more significant inputs are unobservable in the marketplace. Northern Trust’s Level 3 liabilities consist of swaps that Northern Trust entered into with the purchaser of 1.1 million and 1.0 million shares of Visa Class B common shares previously held by Northern Trust and sold in June 2016 and 2015, respectively. Pursuant to the swaps, Northern Trust retains the risks associated with the ultimate conversion of the Visa Class B common shares into Visa Class A common shares, such that the counterparty will be compensated for any dilutive adjustments to the conversion ratio and Northern Trust will be compensated for any anti-dilutive adjustments to the ratio. The swaps also require periodic payments from Northern Trust to the counterparty calculated by reference to the market price of Visa Class A common shares and a fixed rate of interest. The fair value of the swaps is determined using a discounted cash flow methodology. The significant unobservable inputs used in the fair value measurement are Northern Trust’s own assumptions about estimated changes in the conversion rate of the Visa Class B common shares into Visa Class A common shares, the date on which such conversion is expected to occur and the estimated appreciation of the Visa Class A common share price. See “Visa Class B Common Shares and Makewhole Agreement” under Note 20—Commitments and Contingent Liabilities for further information. Northern Trust believes its valuation methods for its assets and liabilities carried at fair value are appropriate; however, the use of different methodologies or assumptions, particularly as applied to Level 3 assets and liabilities, could have a material effect on the computation of their estimated fair values.
Revenue from Contract with Clients	Trust, Investment, and Other Servicing Fees. Custody and Fund Administration income is comprised of revenues received from our core asset servicing business for providing custody, fund administration, and middle-office-related services, primarily to Asset Servicing clients. Investment Management and Advisory income contains revenue received from providing asset management and related services to Asset Servicing and Wealth Management clients and to Northern Trust sponsored funds. Securities Lending income represents revenues generated from securities lending arrangements that Northern Trust enters into as agent, mainly with Asset Servicing clients. Other income largely consists of revenues received from providing employee benefit, investment risk and analytic and other services to Asset Servicing and Wealth Management clients. Other Noninterest Income. Treasury Management income represents revenues received from providing cash and liquidity management services to Asset Servicing and Wealth Management clients. The portion of Security Commissions and Trading Income that relates to revenue from contracts with clients is primarily comprised of commissions earned from providing securities brokerage services to Asset Servicing and Wealth Management clients. The portion of Other Operating Income that relates to revenue from contracts with clients is mainly comprised of service fees for banking-related services provided to Wealth Management and Asset Servicing clients. Performance Obligations. Clients are typically charged monthly or quarterly in arrears based on the fee arrangement agreed to with each client; payment terms will vary depending on the client and services offered. Substantially all revenues generated from contracts with clients for asset servicing, asset management, securities lending, treasury management and banking-related services are recognized on an accrual basis, over the period in which services are provided. The nature of Northern Trust’s performance obligations is to provide a series of distinct services in which the client simultaneously receives and consumes the benefits of the promised services as they are performed. Fee arrangements are mainly comprised of variable amounts based on market value of client assets managed and serviced, transaction volumes, number of accounts, and securities lending volume and spreads. Revenue is recognized using the output method in an amount that reflects the consideration to which Northern Trust expects to be entitled in exchange for providing each month or quarter of service. For contracts with multiple performance obligations, revenue is allocated to each performance obligation based on the price agreed to with the client, representing its relative standalone selling price. Security brokerage revenue is primarily represented by securities commissions received in exchange for providing trade execution related services. Control is transferred at a point in time, on the trade date of the transaction, and fees are typically variable based on transaction volumes and security types. Northern Trust’s contracts with its clients are typically open-ended arrangements and are therefore considered to have an original duration of less than one year. Northern Trust has elected the practical expedient to not disclose the value of remaining performance obligations for contracts with an original expected duration of one year or less.

Recent Accounting Pronouncements (Policies)

9 Months Ended

Sep. 30, 2025

Accounting Changes and Error Corrections [Abstract]

The consolidated financial statements include the accounts of Northern Trust Corporation (Corporation) and its wholly-owned subsidiary, The Northern Trust Company (Bank), and various other wholly-owned subsidiaries of the Corporation and Bank. Throughout the notes to the consolidated financial statements, the term “Northern Trust” refers to the Corporation and its subsidiaries. Intercompany balances and transactions have been eliminated in consolidation. The consolidated financial statements, as of and for the periods ended September 30, 2025 and 2024, have not been audited by the Corporation’s independent registered public accounting firm. In the opinion of management, all accounting entries and adjustments, including normal recurring accruals, necessary for a fair presentation of the financial position and the results of operations for the interim periods have been made. The accounting and financial reporting policies of Northern Trust conform to U.S. generally accepted accounting principles (GAAP) and reporting practices prescribed for the banking industry.

Recent Accounting Pronouncements

On January 1, 2025, Northern Trust adopted ASU No. 2023-08, “Intangibles—Goodwill and Other—Crypto Assets (Subtopic 350-60): Accounting for and Disclosure of Crypto Assets” (ASU 2023-08). ASU 2023-08 requires entities to subsequently measure certain crypto assets at fair value, with changes in fair value recorded in net income in each reporting period, and present crypto assets separately from other intangible assets on the face of the balance sheet and changes in fair value of crypto assets separately from changes in the carrying amount of other intangible assets on the statement of income. ASU 2023-08 also requires enhanced disclosures about in-scope crypto assets and respective activities. As Northern Trust does not hold crypto assets, upon adoption of ASU 2023-08, there was no impact on the consolidated balance sheets or consolidated statements of income.

Fair Value Measurements

Fair Value Hierarchy. The following describes the hierarchy of valuation inputs (Levels 1, 2, and 3) used to measure fair value and the primary valuation methodologies used by Northern Trust for financial instruments measured at fair value on a recurring basis. Observable inputs reflect market data obtained from sources independent of the reporting entity; unobservable inputs reflect the entity’s own assumptions about how market participants would value an asset or liability based on the best information available. GAAP requires an entity measuring fair value to maximize the use of observable inputs and minimize the use of unobservable inputs and establishes a fair value hierarchy of inputs. Financial instruments are categorized within the hierarchy based on the lowest level input that is significant to their valuation. No transfers into or out of Level 3 occurred during the nine months ended September 30, 2025 or the year ended December 31, 2024.

Level 1 — Quoted, active market prices for identical assets or liabilities.

Northern Trust’s Level 1 assets are comprised primarily of AFS investments in U.S. Treasury securities.

Level 2 — Observable inputs other than Level 1 prices, such as quoted active market prices for similar assets or liabilities, quoted prices for identical or similar assets in inactive markets, and model-derived valuations in which all significant inputs are observable in active markets.

Northern Trust’s Level 2 assets include AFS debt securities, the fair values of which are determined predominantly by external pricing vendors. Prices received from vendors are compared to other vendor and third-party prices. If a security price obtained from a pricing vendor is determined to exceed predetermined tolerance levels that are assigned based on an asset type’s characteristics, the exception is researched and, if the price is not able to be validated, an alternate pricing vendor is utilized, consistent with Northern Trust’s pricing source hierarchy. As of September 30, 2025, Northern Trust’s AFS debt securities portfolio included 980 Level 2 debt securities with an aggregate market value of $24.7 billion, all valued by external pricing vendors. As of December 31, 2024, Northern Trust’s AFS debt securities portfolio included 940 Level 2 debt securities with an aggregate market value of $21.6 billion, all valued by external pricing vendors.

Level 2 assets and liabilities also include derivative contracts which are valued internally using widely accepted income-based models that incorporate inputs readily observable in actively quoted markets and reflect the contractual terms of the contracts. Observable inputs include foreign exchange rates and interest rates for foreign exchange contracts; interest rates for interest rate swap contracts and forward contracts; and interest rates and volatility inputs for interest rate option contracts. Northern Trust evaluates the impact of counterparty credit risk and its own credit risk on the valuation of its derivative instruments. Factors considered include the likelihood of default by Northern Trust and its counterparties, the remaining maturities of the instruments, net exposures after giving effect to master netting arrangements or similar agreements, available collateral, and other credit enhancements in determining the appropriate fair value of derivative instruments. The resulting valuation adjustments have not been considered material.

Level 3 — Valuation techniques in which one or more significant inputs are unobservable in the marketplace.

Northern Trust’s Level 3 liabilities consist of swaps that Northern Trust entered into with the purchaser of 1.1 million and 1.0 million shares of Visa Class B common shares previously held by Northern Trust and sold in June 2016 and 2015, respectively. Pursuant to the swaps, Northern Trust retains the risks associated with the ultimate conversion of the Visa Class B common shares into Visa Class A common shares, such that the counterparty will be compensated for any dilutive adjustments to the conversion ratio and Northern Trust will be compensated for any anti-dilutive adjustments to the ratio. The swaps also require periodic payments from Northern Trust to the counterparty calculated by reference to the market price of Visa Class A common shares and a fixed rate of interest. The fair value of the swaps is determined using a discounted cash flow methodology. The significant unobservable inputs used in the fair value measurement are Northern Trust’s own assumptions about estimated changes in the conversion rate of the Visa Class B common shares into Visa Class A common shares, the date on which such conversion is expected to occur and the estimated appreciation of the Visa Class A common share price. See “Visa Class B Common Shares and Makewhole Agreement” under Note 20—Commitments and Contingent Liabilities for further information.

Northern Trust believes its valuation methods for its assets and liabilities carried at fair value are appropriate; however, the use of different methodologies or assumptions, particularly as applied to Level 3 assets and liabilities, could have a material effect on the computation of their estimated fair values.

Revenue from Contract with Clients

Trust, Investment, and Other Servicing Fees. Custody and Fund Administration income is comprised of revenues received from our core asset servicing business for providing custody, fund administration, and middle-office-related services, primarily to Asset Servicing clients. Investment Management and Advisory income contains revenue received from providing asset management and related services to Asset Servicing and Wealth Management clients and to Northern Trust sponsored funds. Securities Lending income represents revenues generated from securities lending arrangements that Northern Trust enters into as agent, mainly with Asset Servicing clients. Other income largely consists of revenues received from providing employee benefit, investment risk and analytic and other services to Asset Servicing and Wealth Management clients.

Other Noninterest Income. Treasury Management income represents revenues received from providing cash and liquidity management services to Asset Servicing and Wealth Management clients. The portion of Security Commissions and Trading Income that relates to revenue from contracts with clients is primarily comprised of commissions earned from providing securities brokerage services to Asset Servicing and Wealth Management clients. The portion of Other Operating Income that relates to revenue from contracts with clients is mainly comprised of service fees for banking-related services provided to Wealth Management and Asset Servicing clients.

Performance Obligations. Clients are typically charged monthly or quarterly in arrears based on the fee arrangement agreed to with each client; payment terms will vary depending on the client and services offered.

Substantially all revenues generated from contracts with clients for asset servicing, asset management, securities lending, treasury management and banking-related services are recognized on an accrual basis, over the period in which services are provided. The nature of Northern Trust’s performance obligations is to provide a series of distinct services in which the client simultaneously receives and consumes the benefits of the promised services as they are performed. Fee arrangements are mainly comprised of variable amounts based on market value of client assets managed and serviced, transaction volumes, number of accounts, and securities lending volume and spreads. Revenue is recognized using the output method in an amount that reflects the consideration to which Northern Trust expects to be entitled in exchange for providing each month or quarter of service. For contracts with multiple performance obligations, revenue is allocated to each performance obligation based on the price agreed to with the client, representing its relative standalone selling price.

Security brokerage revenue is primarily represented by securities commissions received in exchange for providing trade execution related services. Control is transferred at a point in time, on the trade date of the transaction, and fees are typically variable based on transaction volumes and security types.

Northern Trust’s contracts with its clients are typically open-ended arrangements and are therefore considered to have an original duration of less than one year. Northern Trust has elected the practical expedient to not disclose the value of remaining performance obligations for contracts with an original expected duration of one year or less.