Quarterly Financial Statements (unaudited) (Details) - USD ($) $ / shares in Units, $ 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 financing revenue and other interest income	$ 1,303	$ 1,200	$ 1,054	$ 1,146	$ 1,156	$ 1,188	$ 1,157	$ 1,132	$ 4,703	$ 4,633	$ 4,390
Noninterest Income	678	484	555	266	487	413	395	466	1,983	1,761	1,414
Total net revenue	1,981	1,684	1,609	1,412	1,643	1,601	1,552	1,598	6,686	6,394	5,804
Provision for credit losses	102	147	287	903	276	263	177	282	1,439	998	918
Total noninterest expense	1,023	905	985	920	880	838	881	830	3,833	3,429	3,264
Income from continuing operations before income tax expense	856	632	337	(411)	487	500	494	486	1,414	1,967	1,622
Income tax (benefit) expense from continuing operations	169	156	95	(92)	106	119	(90)	111	328	246	359
Net income from continuing operations	687	476	242	(319)	381	381	584	375	1,086	1,721	1,263
Loss from discontinued operations, net of tax	0	0	(1)	0	(3)	0	(2)	(1)	(1)	(6)	0
Net income	$ 687	$ 476	$ 241	$ (319)	$ 378	$ 381	$ 582	$ 374	$ 1,085	$ 1,715	$ 1,263
Net (loss) income from continuing operations, basic earnings per common share	$ 1.83	$ 1.27	$ 0.65	$ (0.85)	$ 1.00	$ 0.98	$ 1.47	$ 0.93	$ 2.89	$ 4.38	$ 2.97
Net (loss) income, basic earnings per common share	1.83	1.27	0.64	(0.85)	0.99	0.97	1.46	0.93	2.89	4.36	2.97
Net (loss) income from continuing operations, diluted earnings per common share	1.82	1.26	0.64	(0.85)	0.99	0.97	1.46	0.92	2.88	4.35	2.95
Net (loss) income, diluted earnings per common share	1.82	1.26	0.64	(0.85)	0.99	0.97	1.46	0.92	2.88	4.34	2.95
Cash dividends declared per common share	$ 0.19	$ 0.19	$ 0.19	$ 0.19	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.76	$ 0.68	$ 0.56