Asset Quality - Changes in Liability for Credit Losses on Lending Related Commitments (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at the end of the prior period	$ 68	$ 152	$ 198	$ 197	$ 68	$ 197	$ 68	$ 68
Liability for credit losses on contingent guarantees at the end of the prior period				0	0			0	$ 7
Balance at beginning of period		152	208	152	198	152	208	197	68
Provision (credit) for losses on off balance sheet exposures		0	10			(45)	67
Balance at end of period		152	208	152	198	152	208	197	68
Impact of ASC 326 Adoption
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Cumulative effect from changes in accounting principle				0	0			0	66
Adjusted balance
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]
Balance at the end of the prior period	141	$ 152	$ 198	197	141	$ 197	$ 141	141
Balance at beginning of period				152	198			197	141
Balance at end of period				$ 152	$ 198			$ 197	$ 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