Summary of Significant Accounting Policies Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 5,402	$ 9,172	$ 9,124
Avis [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,961	5,250
Budget [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,908	3,179
Other Brands [Member]
Disaggregation of Revenue [Line Items]
Revenues	533	743
Americas [Member]
Disaggregation of Revenue [Line Items]
Revenues	3,965	6,352
Europe, Middle east, Africa [Member]
Disaggregation of Revenue [Line Items]
Revenues	1,127	2,222
Asia and Australasia [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 310	$ 598