Selected Quarterly Financial Data (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Net interest income	$ 584	$ 548	$ 538	$ 504	$ 507	$ 503	$ 540	$ 562	$ 2,174		$ 2,112		$ 2,252
Provision for credit losses	(30)	(105)	219	726					810		195		(73)
Provision for credit losses					(3)	94	46	58	736		195		(73)
Net interest income after provision for credit losses	614	653	319	(222)	510	409	494	504	1,364		1,917		2,325
Total other revenues	328	356	464	387	542	486	436	390	1,535	[1]	1,854	[1]	1,899	[1]
Operating expenses	764	785	727	1,605	871	1,106	774	750	3,881		3,501		3,638
Income (loss) before income tax	178	224	56	(1,440)	181	(211)	156	144	(982)		270		586
Income tax expense (benefit)	30	36	49	(157)	48	39	35	35	(42)		157		266
Net income (loss)	$ 148	$ 188	$ 7	$ (1,283)	$ 133	$ (250)	$ 121	$ 109	$ (940)		$ 113		$ 320

[1] See Note 24, "Business Segments," for a reconciliation of total other revenues on a U.S. GAAP basis to other operating income for each business segment under the Group Reporting Basis.