Quarterly Financial Statements (unaudited) Quarterly Financial Statements (unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Statements (unaudited) [Abstract]
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
Income Tax Expense (Benefit)	231	115	122	113	134	130	56	150	581	470	496
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent	179	280	254	213	246	261	357	247	926	1,111	897
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	2	2	(2)	1	2	(52)	3	3	3	(44)	392
Net Income (Loss) Attributable to Parent	$ 181	$ 282	$ 252	$ 214	$ 248	$ 209	$ 360	$ 250	$ 929	$ 1,067	$ 1,289
Income (Loss) from Continuing Operations, Per Basic Share	$ 0.40	$ 0.62	$ 0.55	$ 0.46	$ 0.52	$ 0.54	$ 0.70	$ 0.48	$ 2.04	$ 2.25	$ (3.47)
Earnings Per Share, Basic	0.41	0.63	0.55	0.46	0.53	0.43	0.71	0.49	2.05	2.15	(2.66)
Income (Loss) from Continuing Operations, Per Diluted Share	0.40	0.62	0.55	0.46	0.52	0.54	0.70	0.48	2.03	2.24	(3.47)
Earnings Per Share, Diluted	0.41	0.63	0.55	0.46	0.52	0.43	0.71	0.49	2.04	2.15	(2.66)
Common Stock, Dividends, Per Share, Declared	$ 0.12	$ 0.12	$ 0.08	$ 0.08	$ 0.08	$ 0.08	$ 0.00	$ 0.00	$ 0.40	$ 0.16	$ 0.00
Net impacts as a result of the Tax Act [Member]
Tax Act Impacts [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount									$ 119	$ 0	$ 0