Recent Accounting Pronouncements	9 Months Ended
Recent Accounting Pronouncements	Sep. 30, 2020
Accounting Changes and Error Corrections [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements On January 1, 2020, Northern Trust adopted Accounting Standards Update (ASU) No. 2016-13, “Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments” (ASU 2016-13). ASU 2016-13 significantly changed the way impairment of financial instruments is recognized by requiring immediate recognition of estimated credit losses expected to occur over the remaining life of financial instruments. The main provisions of ASU 2016-13 include (1) replacing the “incurred loss” approach under current GAAP with an “expected loss” model for instruments measured at amortized cost, (2) requiring entities to record an allowance for available for sale debt securities rather than reduce the carrying amount of the investments, as is required by the other-than-temporary-impairment model under legacy GAAP, and (3) a simplified accounting model for purchased credit-impaired debt securities and loans. Upon adoption of ASU 2016-13, Northern Trust recorded a $13.7 million increase in the allowance for credit losses with a corresponding cumulative effect adjustment to decrease retained earnings by $10.1 million, net of income taxes, on January 1, 2020. Northern Trust did not restate comparative periods for the effects of applying ASU 2016-13. There was no significant impact to Northern Trust’s consolidated results of operations. Please refer to Note 7 — Allowance for Credit Losses for further information. On January 1, 2020, Northern Trust adopted ASU No. 2017-04, “Intangibles—Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment” (ASU 2017-04). ASU 2017-04 amends the subsequent measurement of goodwill whereby Step 2 from the goodwill impairment test is eliminated. As a result, the goodwill impairment test is performed by comparing the fair value of a reporting unit to its carrying value and an impairment charge should be recognized for the amount by which the carrying amount exceeds the reporting unit’s fair value, not to exceed the total amount of goodwill allocated to that reporting unit. Upon adoption of ASU 2017-04, there was no significant impact to Northern Trust’s consolidated financial condition or results of operations. On January 1, 2020, Northern Trust adopted ASU No. 2018-13, “Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement” (ASU 2018-13). The primary objective of ASU 2018-13 is to improve the effectiveness of disclosures in the notes to financial statements. Upon adoption of ASU 2018-13, there was no significant impact to Northern Trust’s consolidated financial condition or results of operations. On January 1, 2020, Northern Trust adopted ASU No. 2018-15, “Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract (a consensus of the FASB Emerging Issues Task Force)” (ASU 2018-15). ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal use software license). Upon adoption of ASU 2018-15, there was no significant impact to Northern Trust’s consolidated financial condition or results of operations. On January 1, 2020, Northern Trust adopted ASU No. 2018-17, “Consolidation (Topic 810): Targeted Improvements to Related Party Guidance for Variable Interest Entities” (ASU 2018-17). ASU 2018-17 requires that indirect interests held through related parties in common control arrangements should be considered on a proportional basis (rather than as the equivalent of a direct interest in its entirety) for determining whether fees paid to decision makers and service providers are variable interests. Upon adoption of ASU 2018-17, there was no significant impact to Northern Trust’s consolidated financial condition or results of operations. On April 1, 2020, Northern Trust adopted ASU No. 2020-04, “Reference Rate Reform (Topic 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting” (ASU 2020-04). The global transition toward alternative reference rates and away from referencing the London Interbank Offered Rate (LIBOR) and other interbank offered rates (Reference Rate Reform) is expected to have a significant impact on the volume of contract modifications, hedge accounting, and other transactions that reference LIBOR or another reference rate expected to be discontinued because of Reference Rate Reform. ASU 2020-04 provides temporary optional expedients and exceptions for applying GAAP to contract modifications, hedging relationships, and other transactions affected by Reference Rate Reform if certain criteria are met. The main provisions of ASU 2020-04 provide the following optional expedients: (1) simplification of the accounting evaluations under current GAAP for contract modifications, including loan, debt, lease and other contracts with potential embedded derivatives, if qualifying criteria are met (2) preservation of hedging relationships without dedesignation upon certain changes to the critical terms of an existing hedging relationship due to Reference Rate Reform and other optional hedge accounting relief provisions and (3) a one-time election to sell or transfer, or both sell and transfer, debt securities classified as held to maturity that reference a rate affected by Reference Rate Reform and are classified as held to maturity before January 1, 2020. The optional expedients in ASU 2020-04 for contract modifications and hedging relationships are applied prospectively, while the one-time election to sell or transfer, or both sell and transfer debt securities classified as held to maturity may be made at any time after March 12, 2020. The optional expedients and exceptions provided by ASU 2020-04 do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022, except for hedging relationships existing as of December 31, 2022 for which an entity has elected certain optional expedients and which are retained through the end of the hedging relationship. Upon adoption of ASU 2020-04, there was no significant impact on Northern Trust’s consolidated financial condition or results of operations. Northern Trust expects to elect the optional expedients provided in ASU 2020-04 and does not expect a significant impact on Northern Trust’s consolidated financial condition or results of operations as a result of electing such expedients.