Segment Information (Financial Information Operating Segments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net Financing Revenue	$ 1,094	$ 1,067	$ 2,143	$ 2,046
Nonoperating Income (Expense)	364	388	718	784
Revenue, Net	1,458	1,455	2,861	2,830
Provision for Loan Losses Expensed	158	269	419	540
Noninterest Expense	839	810	1,653	1,588
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	461	376	789	702
Assets	171,345	164,345	171,345	164,345	$ 167,148
Interest Income (Expense), after Provision for Loan Loss	936	798	1,724	1,506
Automotive Finance Operations [Member]
Segment Reporting Information [Line Items]
Net Financing Revenue	925	932	1,834	1,824
Nonoperating Income (Expense)	63	107	129	208
Revenue, Net	988	1,039	1,963	2,032
Provision for Loan Losses Expensed	170	266	429	534
Noninterest Expense	436	426	884	863
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	382	347	650	635
Assets	114,915	115,447	114,915	115,447
Insurance Operations [Member]
Segment Reporting Information [Line Items]
Net Financing Revenue	13	14	25	29
Nonoperating Income (Expense)	266	245	512	509
Revenue, Net	279	259	537	538
Provision for Loan Losses Expensed	0	0	0	0
Noninterest Expense	268	280	499	519
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	11	(21)	38	19
Assets	7,634	7,308	7,634	7,308
Mortgage Finance [Member]
Segment Reporting Information [Line Items]
Net Financing Revenue	44	32	87	66
Nonoperating Income (Expense)	2	1	3	1
Revenue, Net	46	33	90	67
Provision for Loan Losses Expensed	0	1	2	2
Noninterest Expense	32	25	66	49
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	14	7	22	16
Assets	13,385	8,902	13,385	8,902
Corporate Finance Operations [Member]
Segment Reporting Information [Line Items]
Net Financing Revenue	57	48	103	82
Nonoperating Income (Expense)	14	10	22	28
Revenue, Net	71	58	125	110
Provision for Loan Losses Expensed	(6)	6	(6)	12
Noninterest Expense	19	17	44	38
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	58	35	87	60
Assets	4,458	3,552	4,458	3,552
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net Financing Revenue	55	41	94	45
Nonoperating Income (Expense)	19	25	52	38
Revenue, Net	74	66	146	83
Provision for Loan Losses Expensed	(6)	(4)	(6)	(8)
Noninterest Expense	84	62	160	119
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(4)	8	(8)	(28)
Assets	$ 30,953	$ 29,136	$ 30,953	$ 29,136