Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments									4
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
Noninterest expense	880	838	881	830	804	807	839	814	3,429	3,264	3,110
Income (loss) from continuing operations before income tax expense	487	$ 500	$ 494	$ 486	368	$ 465	$ 461	$ 328	1,967	1,622	1,507
Assets	180,644				178,869				180,644	178,869	167,148
Net financing revenue and other interest income after provision for loan losses									3,600	3,500	3,100
Operating Segments | Automotive Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									4,141	3,769	3,713
Other revenue									249	269	355
Total net revenue									4,390	4,038	4,068
Provision for loan losses									962	920	1,134
Noninterest expense									1,810	1,750	1,714
Income (loss) from continuing operations before income tax expense									1,618	1,368	1,220
Assets	113,863				117,304				113,863	117,304	114,089
Operating Segments | Insurance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									54	54	59
Other revenue									1,274	981	1,059
Total net revenue									1,328	1,035	1,118
Provision for loan losses									0	0	0
Noninterest expense									1,013	955	950
Income (loss) from continuing operations before income tax expense									315	80	168
Assets	8,547				7,734				8,547	7,734	7,464
Operating Segments | Mortgage Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									171	179	132
Other revenue									22	7	4
Total net revenue									193	186	136
Provision for loan losses									5	1	8
Noninterest expense									148	140	108
Income (loss) from continuing operations before income tax expense									40	45	20
Assets	16,279				15,211				16,279	15,211	11,708
Operating Segments | Corporate Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									239	204	167
Other revenue									45	38	45
Total net revenue									284	242	212
Provision for loan losses									36	12	22
Noninterest expense									95	86	76
Income (loss) from continuing operations before income tax expense									153	144	114
Assets	5,787				4,670				5,787	4,670	3,979
Corporate and Other
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									28	184	150
Other revenue									171	119	81
Total net revenue									199	303	231
Provision for loan losses									(5)	(15)	(16)
Noninterest expense									363	333	262
Income (loss) from continuing operations before income tax expense									(159)	(15)	(15)
Assets	$ 36,168				$ 33,950				$ 36,168	$ 33,950	$ 29,908