Summary of Significant Accounting Policies - Additional Information (Detail) - 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	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Equity Securities without Readily Determinable Fair Value, Amount	$ 8								$ 8
Deferred Revenue	187				$ 194				187	$ 194
Revenues	2,050	$ 2,778	$ 2,328	$ 1,968	2,019	$ 2,752	$ 2,238	$ 1,839	9,124	8,848	$ 8,659
Cash and cash equivalents	615				611				615	611
Program cash	115				283				115	283
Restricted Cash	5				7				5	7
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	735				901				735	901	720	$ 717
Currency translation adjustment	3				(71)				3	(71)	39	80
Net carrying value of software developed	188				196				188	196
Vehicle interest income									15	8	18
Advertising expense									116	111	127
Investments in joint ventures	48				32				48	32
Retained risks of liability to third parties	421				422				421	422
Workers compensation liabilities	60				66				60	66
Gain or loss of currency transactions									(3)	(3)	6
Increase (Decrease) in Customer Loyalty Program Liability									50
Increase (Decrease) in Customer Loyalty Program Liability (Net of Tax)									40
Deferred Revenue, Additions									1,982
Deferred Revenue, Revenue Recognized									1,989
Operating Costs and Expenses									4,639	4,472	4,382
Costs and Expenses									8,857	8,637	8,380
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									267	211	279
Income Tax Expense (Benefit)									102	(150)	116
Net income									165	361	163
Net Income (Loss) Attributable to Parent	13	$ 213	$ 26	$ (87)	220	$ 245	$ 3	$ (107)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent									62	491	156
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	1,301				931				1,301	931
Total Assets Exclusive Of Assets Under Vehicle Programs	6,370				5,820				6,370	5,820
Assets	19,149				17,699				19,149	17,699
Accounts Payable and Accrued Liabilities, Current	1,693				1,619				1,693	1,619
Liabilities, Current	1,716				1,645				1,716	1,645
Other Liabilities, Noncurrent	767				717				767	717
Total Liabilities Exclusive Of Liabilities Under Vehicle Programs	6,011				5,935				6,011	5,935
Retained Earnings (Accumulated Deficit)	(1,091)				(1,222)				(1,091)	(1,222)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	414				573				414	573	$ 221	$ 439
Income (Loss) from Equity Method Investments									5
Assets Held-for-sale, Not Part of Disposal Group, Current, Other	25								25
Gain on Sale of Investments									4
Operating Lease, Right-of-Use Asset	3,000								3,000
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					(4)					(4)
Accumulated Foreign Currency Adjustment Attributable to Noncontrolling Interest [Member] | Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					(7)					(7)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					12					12
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Summary Of Significant Accounting Policies [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					2					2
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					0					0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Summary Of Significant Accounting Policies [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					(1)					(1)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					$ (1)					$ (1)
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Summary Of Significant Accounting Policies [Line Items]
Revenues									9,124
Operating Costs and Expenses									4,642
Costs and Expenses									8,860
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent									264
Income Tax Expense (Benefit)									101
Net Income (Loss) Attributable to Parent									163
Comprehensive Income (Loss), Net of Tax, Attributable to Parent									60
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	1,292								1,292
Total Assets Exclusive Of Assets Under Vehicle Programs	6,361								6,361
Assets	19,140								19,140
Accounts Payable and Accrued Liabilities, Current	1,688								1,688
Liabilities, Current	1,711								1,711
Other Liabilities, Noncurrent	725								725
Total Liabilities Exclusive Of Liabilities Under Vehicle Programs	5,964								5,964
Retained Earnings (Accumulated Deficit)	(1,053)								(1,053)
Stockholders' Equity Attributable to Parent	452								452
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Summary Of Significant Accounting Policies [Line Items]
Revenues									0
Other Operating Income (Expense), Net									(3)
Costs and Expenses									(3)
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent									3
Income Tax Expense (Benefit)									1
Net Income (Loss) Attributable to Parent									2
Comprehensive Income (Loss), Net of Tax, Attributable to Parent									2
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	9								9
Total Assets Exclusive Of Assets Under Vehicle Programs	9								9
Assets	9								9
Accounts Payable and Accrued Liabilities, Current	5								5
Liabilities, Current	5								5
Other Liabilities, Noncurrent	42								42
Total Liabilities Exclusive Of Liabilities Under Vehicle Programs	47								47
Retained Earnings (Accumulated Deficit)	(38)								(38)
Stockholders' Equity Attributable to Parent	$ (38)								$ (38)