Basis Of Presentation And Recently Issued Accounting Pronouncements (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Impact of ASU 2018-02 [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 605	$ 814	$ 605	$ 814	$ 611
Document Period End Date			Sep. 30, 2018
Revenues	2,778	2,752	$ 7,074	6,829
Operating Costs and Expenses	(1,294)	(1,256)	(3,561)	(3,413)
Costs and Expenses	(2,423)	(2,388)	(6,810)	(6,619)
Income before income taxes	355	364	264	210
Provision for income taxes	142	119	112	69
Net Income (Loss) Attributable to Parent	213	245	152	141
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	207	279	104	251
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	1,065		1,065		931
Total assets exclusive of assets under vehicle programs	6,139		6,139		5,820
Assets	19,741		19,741		17,699
Accounts payable and other current liabilities	1,885		1,885		1,619
Total current liabilities	1,908		1,908		1,645
Other non-current liabilities	767		767		717
Total Liabilities Exclusive Of Liabilities Under Vehicle Programs	6,213		6,213		5,935
Retained Earnings (Accumulated Deficit)	(1,103)		(1,103)		(1,222)
Total stockholders’ equity	521		521		573
Program cash	151	180	151	180	283
Restricted Cash and Cash Equivalents	12	7	12	7
Decrease Increase In Program Cash				0
Payments for (Proceeds from) Other Investing Activities			(44)	6
Net Cash Provided by (Used in) Investing Activities			(3,677)	(2,976)
Effect of changes in exchange rates on cash and cash equivalents, program and restricted cash			(5)	41
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect			(133)	281
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	768	1,001	768	1,001	901	$ 720
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					6
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Impact of ASU 2018-02 [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					12
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Impact of ASU 2018-02 [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					2
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Impact of ASU 2018-02 [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					(1)
Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
Impact of ASU 2018-02 [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					(7)
Accounting Standards Update 2018-02 [Member]
Impact of ASU 2018-02 [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					4
Accounting Standards Update 2018-02 [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Impact of ASU 2018-02 [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					(12)
Accounting Standards Update 2018-02 [Member] | Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Impact of ASU 2018-02 [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					0
Accounting Standards Update 2018-02 [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Impact of ASU 2018-02 [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					1
Accounting Standards Update 2018-02 [Member] | Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
Impact of ASU 2018-02 [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets					$ 7
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Impact of ASU 2018-02 [Line Items]
Revenues			7,083
Operating Costs and Expenses			(3,564)
Costs and Expenses			(6,813)
Income before income taxes			270
Provision for income taxes			114
Net Income (Loss) Attributable to Parent			156
Comprehensive Income (Loss), Net of Tax, Attributable to Parent			108
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	1,053		1,053
Total assets exclusive of assets under vehicle programs	6,127		6,127
Assets	19,729		19,729
Accounts payable and other current liabilities	1,879		1,879
Total current liabilities	1,902		1,902
Other non-current liabilities	717		717
Total Liabilities Exclusive Of Liabilities Under Vehicle Programs	6,163		6,163
Retained Earnings (Accumulated Deficit)	(1,059)		(1,059)
Total stockholders’ equity	565		565
Restatement Adjustment [Member]
Impact of ASU 2018-02 [Line Items]
Revenues			(9)
Operating Costs and Expenses			(3)
Costs and Expenses			(3)
Income before income taxes			(6)
Provision for income taxes			(2)
Net Income (Loss) Attributable to Parent			(4)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent			(4)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	12		12
Total assets exclusive of assets under vehicle programs	12		12
Assets	12		12
Accounts payable and other current liabilities	6		6
Total current liabilities	6		6
Other non-current liabilities	50		50
Total Liabilities Exclusive Of Liabilities Under Vehicle Programs	50		50
Retained Earnings (Accumulated Deficit)	(44)		(44)
Total stockholders’ equity	$ (44)		$ (44)
Scenario, Previously Reported [Member]
Impact of ASU 2018-02 [Line Items]
Decrease Increase In Program Cash				53
Payments for (Proceeds from) Other Investing Activities				(5)
Net Cash Provided by (Used in) Investing Activities				(2,924)
Effect of changes in exchange rates on cash and cash equivalents, program and restricted cash				32
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect				324
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		814		814		490
Restatement Adjustment [Member]
Impact of ASU 2018-02 [Line Items]
Decrease Increase In Program Cash				(53)
Payments for (Proceeds from) Other Investing Activities				(1)
Net Cash Provided by (Used in) Investing Activities				(52)
Effect of changes in exchange rates on cash and cash equivalents, program and restricted cash				9
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect				(43)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		$ 187		$ 187		$ 230