Segment and Geographic Information (Financial Information Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net Financing Revenue	$ 1,094	$ 1,081	$ 1,067	$ 979	$ 976	$ 996	$ 984	$ 951	$ 4,221	$ 3,907	$ 3,719
Nonoperating Income (Expense)	379	381	388	396	392	388	374	376	1,544	1,530	1,142
Revenue, Net	1,473	1,462	1,455	1,375	1,368	1,384	1,358	1,327	5,765	5,437	4,861
Provision for Loan Losses Expensed	294	314	269	271	267	258	172	220	1,148	917	707
Noninterest Expense	769	753	810	778	721	735	773	710	3,110	2,939	2,761
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	410	$ 395	$ 376	$ 326	380	$ 391	$ 413	$ 397	1,507	1,581	1,393
Assets	167,148				163,728				167,148	163,728	158,581
Interest Income (Expense), after Provision for Loan Loss									3,073	2,990	3,000
Automotive Finance Operations [Member]
Segment Reporting Information [Line Items]
Net Financing Revenue									3,713	3,665	3,429
Nonoperating Income (Expense)									355	306	235
Revenue, Net									4,068	3,971	3,664
Provision for Loan Losses Expensed									1,134	924	696
Noninterest Expense									1,714	1,667	1,633
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									1,220	1,380	1,335
Assets	114,089				116,347				114,089	116,347	115,636
Insurance Operations Member
Segment Reporting Information [Line Items]
Net Financing Revenue									59	61	57
Nonoperating Income (Expense)									1,059	1,036	1,033
Revenue, Net									1,118	1,097	1,090
Provision for Loan Losses Expensed									0	0	0
Noninterest Expense									950	940	879
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									168	157	211
Assets	7,464				7,172				7,464	7,172	7,053
Mortgage Finance Operations [Member]
Segment Reporting Information [Line Items]
Net Financing Revenue									132	97	57
Nonoperating Income (Expense)									4	0	0
Revenue, Net									136	97	57
Provision for Loan Losses Expensed									8	(4)	7
Noninterest Expense									108	67	39
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									20	34	11
Assets	11,708				8,307				11,708	8,307	6,461
Corporate Finance Operations [Member]
Segment Reporting Information [Line Items]
Net Financing Revenue									167	121	89
Nonoperating Income (Expense)									45	26	25
Revenue, Net									212	147	114
Provision for Loan Losses Expensed									22	10	9
Noninterest Expense									76	66	55
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									114	71	50
Assets	3,979				3,183				3,979	3,183	2,677
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net Financing Revenue									150	(37)	87
Nonoperating Income (Expense)									81	162	(151)
Revenue, Net									231	125	(64)
Provision for Loan Losses Expensed									(16)	(13)	(5)
Noninterest Expense									262	199	155
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									(15)	(61)	(214)
Assets	$ 29,908				$ 28,719				$ 29,908	$ 28,719	$ 26,754