Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Advertising Expense										$ 121	$ 116	$ 111
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	$ 56				$ 48					56	48
Other related Revenue										144
Equity Securities without Readily Determinable Fair Value, Amount	8									8
Revenues	2,162	$ 2,753	$ 2,337	$ 1,920	2,050	$ 2,778	$ 2,328	$ 1,968		9,172	9,124	8,848
Currency translation adjustment	(9)				3					(9)	3	(71)	$ 39
Net carrying value of software developed	261				188					261	188
Vehicle interest income										15	15	8
Retained risks of liability to third parties	441				421					441	421
Workers compensation liabilities	56				60					56	60
Operating Costs and Expenses										4,698	4,639	4,472
Costs and Expenses										8,885	8,857	8,637
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest										287	267	211
Income Tax Expense (Benefit)										(15)	102	(150)
Net income	142	$ 189	$ 62	$ (91)	13	$ 213	$ 26	(87)		302	165	361
Comprehensive Income (Loss), Net of Tax, Attributable to Parent										277	62	491
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	1,662				1,301					1,662	1,301
Total Assets Exclusive Of Assets Under Vehicle Programs	9,311				6,370					9,311	6,370
Assets	23,126				19,149					23,126	19,149
Accounts Payable and Accrued Liabilities, Current	2,206				1,693					2,206	1,693
Liabilities, Current	2,225				1,716					2,225	1,716
Other Liabilities, Noncurrent	757				767					757	767
Total Liabilities Exclusive Of Liabilities Under Vehicle Programs	8,538				6,011					8,538	6,011
Income (Loss) from Equity Method Investments										10	5
Proceeds from Sale of Equity Method Investments										64
Gain (Loss) on Sale of Equity Investments										44
Gain on Sale of Investments										4
Operating Lease, Right-of-Use Asset	2,596				0					2,596	0
Cumulative Effect of New Accounting Principle in Period of Adoption					5						5	(40)	56
Equity Securities without Readily Determinable Fair Value, Upward Price Adjustment, Annual Amount										12
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 900				735					$ 900	735	901	$ 720
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, 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					0						0	2
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)	$ (1)
Greece [Member]
Summary Of Significant Accounting Policies [Line Items]
Payments to Acquire Equity Method Investments								$ 20	$ 19
Equity Method Investment, Ownership Percentage	40.00%							20.00%		40.00%
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 5									$ 5
Property leases [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating Lease, Right-of-Use Asset	2,596									2,596
Property leases [Member] | Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating Lease, Right-of-Use Asset	2,811									2,811
Vehicle leases [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating Lease, Right-of-Use Asset	195									195
Vehicle leases [Member] | Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating Lease, Right-of-Use Asset	$ 183									$ 183