Summary of Geographic Segment 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
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues	$ 2,050	$ 2,778	$ 2,328	$ 1,968	$ 2,019	$ 2,752	$ 2,238	$ 1,839	$ 9,124	$ 8,848	$ 8,659
Assets exclusive of assets under vehicle programs	6,370				5,820				6,370	5,820	6,065
Assets under vehicle programs	12,779				11,879				12,779	11,879	11,578
Long-Lived Assets	2,653				2,627				2,653	2,627	2,562
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									5,708	5,629	5,674
Assets exclusive of assets under vehicle programs	3,494				3,069				3,494	3,069	3,699
Assets under vehicle programs	9,021				8,192				9,021	8,192	8,552
Long-Lived Assets	1,476				1,451				1,476	1,451	1,489
All Other Countries
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues									3,416	3,219	2,985
Assets exclusive of assets under vehicle programs	2,876				2,751				2,876	2,751	2,366
Assets under vehicle programs	3,758				3,687				3,758	3,687	3,026
Long-Lived Assets	$ 1,177				$ 1,176				$ 1,177	$ 1,176	$ 1,073