Allowance for Credit Losses Allowance for Credit Losses - Change in Liability for Off-balance Sheet Credit Exposures (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period	$ 158	$ 96
Provision charged (credited) to income	213	(7)
Balance at end of period	371	89
Accounting Standards Update 2016-13 [Member]
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Cumulative effect adjustment to initially apply new accounting guidance for measuring credit losses			$ 54	$ 0
As Previously Reported [Member]
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at beginning of period	$ 104	$ 96