Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments									4
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
Other revenue	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
Total assets	182,165				180,644				182,165	180,644	178,869
Net financing revenue and other interest income after the provision for credit losses									3,300	3,600	3,500
Operating Segments | Automotive Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									4,284	4,141	3,769
Other revenue									204	249	269
Total net revenue									4,488	4,390	4,038
Provision for credit losses									1,236	962	920
Total noninterest expense									1,967	1,810	1,750
Income from continuing operations before income tax expense									1,285	1,618	1,368
Total assets	104,794				113,863				104,794	113,863	117,304
Operating Segments | Insurance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									42	54	54
Other revenue									1,334	1,274	981
Total net revenue									1,376	1,328	1,035
Provision for credit losses									0	0	0
Total noninterest expense									1,092	1,013	955
Income from continuing operations before income tax expense									284	315	80
Total assets	9,137				8,547				9,137	8,547	7,734
Operating Segments | Mortgage Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									118	171	179
Other revenue									102	22	7
Total net revenue									220	193	186
Provision for credit losses									7	5	1
Total noninterest expense									160	148	140
Income from continuing operations before income tax expense									53	40	45
Total assets	14,889				16,279				14,889	16,279	15,211
Operating Segments | Corporate Finance operations
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									299	239	204
Other revenue									45	45	38
Total net revenue									344	284	242
Provision for credit losses									149	36	12
Total noninterest expense									107	95	86
Income from continuing operations before income tax expense									88	153	144
Total assets	6,108				5,787				6,108	5,787	4,670
Corporate and Other
Segment Reporting Information [Line Items]
Net financing revenue and other interest income									(40)	28	184
Other revenue									298	171	119
Total net revenue									258	199	303
Provision for credit losses									47	(5)	(15)
Total noninterest expense									507	363	333
Income from continuing operations before income tax expense									(296)	(159)	(15)
Total assets	$ 47,237				$ 36,168				$ 47,237	$ 36,168	$ 33,950