Allowance for Credit Losses - Change in Liability for Off-balance Sheet Credit Exposures (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period		$ 237	$ 158
Provision charged (credited) to income		(82)	213
Balance at end of period		155
Accounting Standards Update 2016-13 [Member]
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period	[1]	0	54
As Previously Reported [Member]
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period		$ 237	104
Balance at end of period			$ 371

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