Guarantor Consolidating Financial Statements - Guarantor Income Statements (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Income Statements, Captions [Line Items]
Finance charge income									$ 3,381	$ 3,475	$ 2,563
Leased vehicle income									2,807	1,090	595
Other income									266	289	186
Total revenue	$ 1,878	$ 1,707	$ 1,515	$ 1,354	$ 1,305	$ 1,261	$ 1,191	$ 1,097	6,454	4,854	3,344
Salaries and benefits									726	614	448
Other operating expenses									567	548	322
Total operating expenses									1,293	1,162	770
Leased vehicle expenses									2,200	847	453
Provision for loan losses									624	604	475
Interest expense									1,616	1,426	721
Acquisition and integration expenses									0	0	42
Total costs and expenses									5,733	4,039	2,461
Equity income									116	0	0
Income before income taxes	167	231	225	214	120	208	265	222	837	815	883
Income tax (benefit) provision									191	278	317
Net income	$ 131	$ 179	$ 186	$ 150	$ 59	$ 158	$ 175	$ 145	646	537	566
Comprehensive (loss) income									(25)	93	580
Parent Company [Member]
Condensed Income Statements, Captions [Line Items]
Finance charge income									0	0	0
Leased vehicle income									0	0	0
Other income									13	68	56
Total revenue									13	68	56
Salaries and benefits									0	0	0
Other operating expenses									64	159	(100)
Total operating expenses									64	159	(100)
Leased vehicle expenses									0	0	0
Provision for loan losses									0	0	0
Interest expense									488	232	180
Acquisition and integration expenses											0
Total costs and expenses									552	391	80
Equity income									941	757	584
Income before income taxes									402	434	560
Income tax (benefit) provision									(244)	(103)	(6)
Net income									646	537	566
Comprehensive (loss) income									(25)	93	580
Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Finance charge income									403	194	140
Leased vehicle income									0	0	0
Other income									505	432	433
Total revenue									908	626	573
Salaries and benefits									332	249	216
Other operating expenses									105	(17)	228
Total operating expenses									437	232	444
Leased vehicle expenses									0	0	0
Provision for loan losses									398	334	239
Interest expense									18	23	37
Acquisition and integration expenses											0
Total costs and expenses									853	589	720
Equity income									579	523	551
Income before income taxes									634	560	404
Income tax (benefit) provision									25	12	(42)
Net income									609	548	446
Comprehensive (loss) income									498	491	449
Non-Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Finance charge income									2,978	3,281	2,423
Leased vehicle income									2,807	1,090	595
Other income									139	178	105
Total revenue									5,924	4,549	3,123
Salaries and benefits									394	365	232
Other operating expenses									649	657	441
Total operating expenses									1,043	1,022	673
Leased vehicle expenses									2,200	847	453
Provision for loan losses									226	270	236
Interest expense									1,250	1,309	665
Acquisition and integration expenses											42
Total costs and expenses									4,719	3,448	2,069
Equity income									116	0	0
Income before income taxes									1,321	1,101	1,054
Income tax (benefit) provision									410	369	365
Net income									911	732	689
Comprehensive (loss) income									225	298	700
Consolidation, Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Finance charge income									0	0	0
Leased vehicle income									0	0	0
Other income									(391)	(389)	(408)
Total revenue									(391)	(389)	(408)
Salaries and benefits									0	0	0
Other operating expenses									(251)	(251)	(247)
Total operating expenses									(251)	(251)	(247)
Leased vehicle expenses									0	0	0
Provision for loan losses									0	0	0
Interest expense									(140)	(138)	(161)
Acquisition and integration expenses											0
Total costs and expenses									(391)	(389)	(408)
Equity income									(1,520)	(1,280)	(1,135)
Income before income taxes									(1,520)	(1,280)	(1,135)
Income tax (benefit) provision									0	0	0
Net income									(1,520)	(1,280)	(1,135)
Comprehensive (loss) income									$ (723)	$ (789)	$ (1,149)