Revenues - Disaggregation of Revenue Stream (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Gross fee revenues	$ 845	$ 759
Contract investment amortization	(18)	(11)
Net fee revenues	827	748
Owned, leased, and other revenue	406	428
Cost reimbursement revenue	3,773	3,736
Revenues	5,006	4,912
Segments
Disaggregation of Revenue [Line Items]
Gross fee revenues	734	679
Contract investment amortization	(18)	(11)
Net fee revenues	716	668
Owned, leased, and other revenue	393	418
Cost reimbursement revenue	3,753	3,648
Revenues	4,862	4,734
Segments | North American Full-Service
Disaggregation of Revenue [Line Items]
Gross fee revenues	299	289
Contract investment amortization	(11)	(6)
Net fee revenues	288	283
Owned, leased, and other revenue	155	197
Cost reimbursement revenue	2,856	2,760
Revenues	3,299	3,240
Segments | North American Limited-Service
Disaggregation of Revenue [Line Items]
Gross fee revenues	196	181
Contract investment amortization	(3)	(3)
Net fee revenues	193	178
Owned, leased, and other revenue	33	31
Cost reimbursement revenue	535	532
Revenues	761	741
Segments | Asia Pacific
Disaggregation of Revenue [Line Items]
Gross fee revenues	117	97
Contract investment amortization	0	0
Net fee revenues	117	97
Owned, leased, and other revenue	47	41
Cost reimbursement revenue	111	103
Revenues	275	241
Segments | Other International
Disaggregation of Revenue [Line Items]
Gross fee revenues	122	112
Contract investment amortization	(4)	(2)
Net fee revenues	118	110
Owned, leased, and other revenue	158	149
Cost reimbursement revenue	251	253
Revenues	527	512
Unallocated corporate
Disaggregation of Revenue [Line Items]
Revenues	$ 144	$ 178