Allowance for Credit Losses - Change in Liability for Off-balance Sheet Credit Exposures (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period	$ 158	$ 96	$ 106
Provision charged (credited) to income	79	8	(10)
Balance at end of period	237	158	96
Accounting Standards Update 2016-13 [Member]
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period	54
Balance at end of period		54
As Previously Reported [Member]
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period	$ 104	96	106
Balance at end of period		$ 104	$ 96