Basis of Presentation and Accounting Policies - Narrative (Details) - USD ($) $ in Millions			6 Months Ended
	Jan. 01, 2020		Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to amortized cost basis of PCD loans	$ 4
Noncredit discount on PCD loans	15
Cumulative effect from changes in accounting principle			$ 17,542		$ 17,038
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member
Change in Accounting Principle, Accounting Standards Update, Transition Option Elected [Extensible List]			key:AccountingStandardsUpdate201613ProspectiveMember
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from changes in accounting principle			$ 12,154	$ 12,174	12,469	$ 12,005	$ 11,771	$ 11,556
Retained Earnings | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred taxes recognized on adoption	71
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from changes in accounting principle	$ (230)	[1]			$ 230

[1] Includes the impact of implementing ASU 2016-13, Financial Instruments - Credit Losses (ASC 326): Measurement of Credit Losses on Financial Instruments.