Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments									4
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
Net revenue									5,804	5,765	5,437
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
Assets	178,869				167,148				178,869	167,148	163,728
Net financing revenue and other interest income after provision for loan losses									3,500	3,100	3,000
Corporate and Other
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									184	150	(37)
Other revenue									119	81	162
Net revenue									303	231	125
Provision for loan losses									(15)	(16)	(13)
Noninterest expense									333	262	199
Income from continuing operations before income tax expense									(15)	(15)	(61)
Assets	33,950				29,908				33,950	29,908	28,719
Automotive Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									3,769	3,713	3,665
Other revenue									269	355	306
Net revenue									4,038	4,068	3,971
Provision for loan losses									920	1,134	924
Noninterest expense									1,750	1,714	1,667
Income from continuing operations before income tax expense									1,368	1,220	1,380
Assets	117,304				114,089				117,304	114,089	116,347
Insurance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									54	59	61
Other revenue									981	1,059	1,036
Net revenue									1,035	1,118	1,097
Provision for loan losses									0	0	0
Noninterest expense									955	950	940
Income from continuing operations before income tax expense									80	168	157
Assets	7,734				7,464				7,734	7,464	7,172
Mortgage Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									179	132	97
Other revenue									7	4	0
Net revenue									186	136	97
Provision for loan losses									1	8	(4)
Noninterest expense									140	108	67
Income from continuing operations before income tax expense									45	20	34
Assets	15,211				11,708				15,211	11,708	8,307
Corporate Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									204	167	121
Other revenue									38	45	26
Net revenue									242	212	147
Provision for loan losses									12	22	10
Noninterest expense									86	76	66
Income from continuing operations before income tax expense									144	114	71
Assets	$ 4,670				$ 3,979				$ 4,670	$ 3,979	$ 3,183