Segment Reporting and Geographic Information - Operations Reporting by Revenue (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Total revenue	$ 1,305	$ 1,261	$ 1,191	$ 1,097	$ 1,101	$ 867	$ 836	$ 540	$ 4,854	$ 3,344	$ 1,960
Operating expenses, including leased vehicle expenses									2,009	1,223
Provision for loan losses									604	475
Interest expense									1,426	721	283
Acquisition and integration expenses									0	42	20
Income before income taxes									815	883	744
Operating Segments [Member] | North America Segment [Member]
Segment Reporting Information [Line Items]
Total revenue									2,909	2,376
Operating expenses, including leased vehicle expenses									1,385	893
Provision for loan losses									472	393
Interest expense									459	360
Acquisition and integration expenses										0
Income before income taxes									593	730
Operating Segments [Member] | International Segment [Member]
Segment Reporting Information [Line Items]
Total revenue									1,945	968
Operating expenses, including leased vehicle expenses									624	330
Provision for loan losses									132	82
Interest expense									954	362
Acquisition and integration expenses										42
Income before income taxes									235	152
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total revenue									56	43
Operating expenses, including leased vehicle expenses									0	0
Provision for loan losses									0	0
Interest expense									69	42
Acquisition and integration expenses										0
Income before income taxes									(13)	1
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Total revenue									(56)	(43)
Operating expenses, including leased vehicle expenses									0	0
Provision for loan losses									0	0
Interest expense									(56)	(43)
Acquisition and integration expenses										0
Income before income taxes									$ 0	$ 0