Summary of Significant Accounting Policies Revenue (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
Disaggregation of Revenue [Line Items]
Revenues	$ 2,050	$ 2,778	$ 2,328	$ 1,968	$ 2,019	$ 2,752	$ 2,238	$ 1,839	$ 9,124	$ 8,848	$ 8,659
Americas
Disaggregation of Revenue [Line Items]
Revenues									6,186
Europe, Middle east, Africa [Member]
Disaggregation of Revenue [Line Items]
Revenues									2,314
Asia and Australasia [Member]
Disaggregation of Revenue [Line Items]
Revenues									624
Avis [Member]
Disaggregation of Revenue [Line Items]
Revenues									5,266
Budget [Member]
Disaggregation of Revenue [Line Items]
Revenues									3,057
Other Brands [Member]
Disaggregation of Revenue [Line Items]
Revenues									$ 801