Asset Quality - Changes in Liability for Credit Losses on Lending Related Commitments (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at the end of the prior period	$ 68	$ 198	$ 64	$ 68	$ 64	$ 64
Liability for credit losses on contingent guarantees at the end of the prior period							$ 0	$ 7	$ 0	$ 0
Balance at beginning of period	68	208	65	208	65	64	$ 198	68	$ 64	$ 64
Provision (credit) for losses on off balance sheet exposures		10	1	67	1
Balance at end of period		$ 208	$ 65	208	$ 65	68
Impact of ASC 326 Adoption
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Cumulative effect from changes in accounting principle						66
As Reported Under ASC 326
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at the end of the prior period	141			141
Balance at beginning of period	141			$ 141		141		$ 141
Balance at end of period						$ 141
Other Financial Assets | Impact of ASC 326 Adoption
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Cumulative effect from changes in accounting principle	$ 4