Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Currency translation adjustment	$ 71	$ (39)	$ (80)	$ 51
Net carrying value of software developed	196	184
Vehicle interest income	8	18	13
Advertising expense	111	127	123
Investments in joint ventures	32	36
Retained risks of liability to third parties	422	437
Workers compensation liabilities	66	71
Business Acquisition, Transaction Costs			18
Business Combination, Contingent Consideration, Liability			9
Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability			13
Business Combination, Contingent Consideration, Liability, Favorable Currency Exchange Rate Movements			4
Gain or loss of currency transactions	(3)	6	11
Employee Service Share Based Compensation Incremental Tax Benefit To Be Realized From Exercise Of Stock Awards	56
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		$ 11	$ 43
Increase (Decrease) in Customer Loyalty Program Liability	50
Increase (Decrease) in Customer Loyalty Program Liability (Net of Tax)	$ 40