Quarterly Financial Statements (unaudited) Quarterly Financial Statements (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Net financing revenue and other interest income	$ 1,156	$ 1,188	$ 1,157	$ 1,132	$ 1,140	$ 1,107	$ 1,094	$ 1,049	$ 4,633	$ 4,390	$ 4,221
Other revenue	487	413	395	466	298	398	364	354	1,761	1,414	1,544
Total net revenue	1,643	1,601	1,552	1,598	1,438	1,505	1,458	1,403	6,394	5,804	5,765
Provision for loan losses	276	263	177	282	266	233	158	261	998	918	1,148
Total noninterest expense	880	838	881	830	804	807	839	814	3,429	3,264	3,110
Income from continuing operations before income tax expense	487	500	494	486	368	465	461	328	1,967	1,622	1,507
Income tax expense from continuing operations	106	119	(90)	111	79	91	113	76	246	359	581
Net income from continuing operations	381	381	584	375	289	374	348	252	1,721	1,263	926
Loss from discontinued operations, net of tax	(3)	0	(2)	(1)	1	0	1	(2)	(6)	0	3
Net income	$ 378	$ 381	$ 582	$ 374	$ 290	$ 374	$ 349	$ 250	$ 1,715	$ 1,263	$ 929
Net income from continuing operations, basic earnings per common share	$ 1.00	$ 0.98	$ 1.47	$ 0.93	$ 0.70	$ 0.89	$ 0.81	$ 0.58	$ 4.38	$ 2.97	$ 2.04
Earnings Per Share, Basic	0.99	0.97	1.46	0.93	0.70	0.89	0.81	0.57	4.36	2.97	2.05
Net income from continuing operations, diluted earnings per common share	0.99	0.97	1.46	0.92	0.70	0.88	0.80	0.57	4.35	2.95	2.03
Earnings Per Share, Diluted	0.99	0.97	1.46	0.92	0.70	0.88	0.81	0.57	4.34	2.95	2.04
Cash dividends declared per common share	$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.15	$ 0.15	$ 0.13	$ 0.13	$ 0.68	$ 0.56	$ 0.40