Leases (Lessor) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Lessor, Lease, Description [Line Items]
Total lease revenues	$ 2,967	$ 1,504	$ 6,651	$ 3,963
Avis
Lessor, Lease, Description [Line Items]
Total lease revenues	1,573	824	3,464	2,181
Budget
Lessor, Lease, Description [Line Items]
Total lease revenues	1,185	532	2,676	1,401
Other
Lessor, Lease, Description [Line Items]
Total lease revenues	209	148	511	381
Americas
Lessor, Lease, Description [Line Items]
Total lease revenues	2,391	1,099	5,417	2,896
Europe, Middle East and Africa
Lessor, Lease, Description [Line Items]
Total lease revenues	508	350	968	843
Asia and Australasia
Lessor, Lease, Description [Line Items]
Total lease revenues	$ 68	$ 55	$ 266	$ 224