Allowance for Credit Losses - Change in Liability for Off-balance Sheet Credit Exposures (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021		Sep. 30, 2020
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period	$ 128	$ 253	$ 237		$ 158
Provision charged (credited) to income	(18)	(28)	(127)		67
Balance at end of period	110		110
Accounting Standards Update 2016-13 [Member]
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period	0	0	0	[1]	54	[1]
As Previously Reported [Member]
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period	$ 128	253	$ 237		104
Balance at end of period		$ 225			$ 225

[1] See Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," in our 2020 Form 10-K for additional discussion.