Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Statements [Abstract]
Net financing revenue and other interest income	$ 1,140	$ 1,107	$ 1,094	$ 1,049	$ 1,094	$ 1,081	$ 1,067	$ 979	$ 4,390	$ 4,221	$ 3,907
Other revenue	298	398	364	354	379	381	388	396	1,414	1,544	1,530
Total net revenue	1,438	1,505	1,458	1,403	1,473	1,462	1,455	1,375
Provision for loan losses	266	233	158	261	294	314	269	271	918	1,148	917
Noninterest expense	804	807	839	814	769	753	810	778	3,264	3,110	2,939
Income from continuing operations before income tax expense	368	465	461	328	410	395	376	326	1,622	1,507	1,581
Income tax expense from continuing operations	79	91	113	76	231	115	122	113	359	581	470
Net income from continuing operations	289	374	348	252	179	280	254	213	1,263	926	1,111
Income (loss) from discontinued operations, net of tax	1	0	1	(2)	2	2	(2)	1	0	3	(44)
Net income	$ 290	$ 374	$ 349	$ 250	$ 181	$ 282	$ 252	$ 214	$ 1,263	$ 929	$ 1,067
Net income from continuing operations, basic earnings per common share	$ 0.70	$ 0.89	$ 0.81	$ 0.58	$ 0.40	$ 0.62	$ 0.55	$ 0.46	$ 2.97	$ 2.04	$ 2.25
Net income, basic earnings per common share	0.70	0.89	0.81	0.57	0.41	0.63	0.55	0.46	2.97	2.05	2.15
Net income from continuing operations, diluted earnings per common share	0.70	0.88	0.80	0.57	0.40	0.62	0.55	0.46	2.95	2.03	2.24
Net income, diluted earnings per common share	0.70	0.88	0.81	0.57	0.41	0.63	0.55	0.46	2.95	2.04	2.15
Cash dividends declared per common share	$ 0.15	$ 0.15	$ 0.13	$ 0.13	$ 0.12	$ 0.12	$ 0.08	$ 0.08	$ 0.56	$ 0.40	$ 0.16
Tax Act Impacts [Line Items]
Change in enacted tax rate, amount									$ (23)	$ 119	$ 0
Net impacts as a result of the Tax Act
Tax Act Impacts [Line Items]
Change in enacted tax rate, amount										$ 119