Basis of Presentation and Accounting Policies - Narrative (Details) - USD ($) $ in Millions			9 Months Ended	12 Months Ended
		Jan. 01, 2020	Sep. 30, 2020	Dec. 31, 2019	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 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			$ (17,722)	$ (17,038)
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect from changes in accounting principle			$ (12,375)	$ (12,469)	$ (12,154)	$ (12,209)	$ (12,005)	$ (11,556)
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	[1]			230
Deferred taxes recognized on adoption				$ 71

[1] Includes the impact of implementing ASU 2016-13, Financial Instruments - Credit Losses (ASC 326): Measurement of Credit Losses on Financial Instruments. See Note 1 (“Basis of Presentation and Accounting Policies”) for more information on our adoption of this guidance and the impact to our results of operations.