Segment Reporting and Geographic Information - Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 3,599	$ 3,518	$ 3,488	$ 3,411	$ 3,252	$ 3,161	$ 2,990	$ 2,748	$ 14,016	$ 12,151	$ 8,983
Long-Lived Assets	43,810				43,141				43,810	43,141	34,556
Leased vehicles, net	43,559				42,882				43,559	42,882	34,300
U.S. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									12,158	10,424	7,440
Long-Lived Assets	41,320				40,674				41,320	40,674	32,506
Non-US [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,858	1,727	1,543
Long-Lived Assets	$ 2,490				$ 2,467				$ 2,490	$ 2,467	$ 2,050