Segment Reporting and Geographic Information - Geographic Information (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
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 3,252	$ 3,161	$ 2,990	$ 2,748	$ 2,554	$ 2,360	$ 2,138	$ 1,931	$ 12,151	$ 8,983	$ 5,867
Long-Lived Assets	43,141				34,556				43,141	34,556	20,250
Leased vehicles, net	42,882				34,342				42,882	34,342	20,100
U.S. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									10,424	7,440	4,324
Long-Lived Assets	40,674				32,506				40,674	32,506	18,501
Brazil [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									589	652	757
Long-Lived Assets	3				3				3	3	3
Other Countries [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,138	891	786
Long-Lived Assets	$ 2,464				$ 2,047				$ 2,464	$ 2,047	$ 1,746