Segment Reporting and Geographic Information - Operations Reporting by Revenue (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]
Total revenue	$ 3,252	$ 3,161	$ 2,990	$ 2,748	$ 2,554	$ 2,360	$ 2,138	$ 1,931	$ 12,151	$ 8,983	$ 5,867
Operating expenses									1,390	1,250	1,051
Leased vehicle expenses									6,415	4,506	2,190
Provision for loan losses (Note 4)									757	644	603
Interest expense									2,566	1,972	1,460
Equity income									173	151	116
Income from continuing operations before income taxes	$ 450	$ 186	$ 270	$ 179	$ 242	$ 134	$ 143	$ 138	1,196	762	679
Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Total revenue									10,999	7,948	4,777
Operating expenses									1,050	891	735
Leased vehicle expenses									6,391	4,499	2,190
Provision for loan losses (Note 4)									623	566	466
Interest expense									2,032	1,481	907
Equity income									0	0	0
Income from continuing operations before income taxes									903	511	479
Operating Segments [Member] | International [Member]
Segment Reporting Information [Line Items]
Total revenue									1,152	1,035	1,090
Operating expenses									340	359	316
Leased vehicle expenses									24	7	0
Provision for loan losses (Note 4)									134	78	137
Interest expense									534	491	553
Equity income									173	151	116
Income from continuing operations before income taxes									$ 293	$ 251	$ 200