Commitments and Contingencies (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Contingencies And Commitments [Line Items]
Maximum self insurance exposures per incident under general liability insurance program	$ 5
Maximum self insurance exposures per incident under workers' compensation insurance program	5
Per incident base deductible under auto liability insurance program	10
Insurance claims receivable	153	$ 171
Beginning Balance	588	643
Self-insurance expense (benefit)	142	71
Cash (paid) received	(148)	(126)
Ending Balance	582	588
Current portion	107	133
Long-term portion	$ 475	455
Expected time period in years for cash settlement of recorded obligations associated with insurance liabilities	6 years
Revolving Credit Facility [Member]
Contingencies And Commitments [Line Items]
Credit Facility, aggregate capacity	$ 2,250	$ 2,250