Business Segments - Summary of Operating Results by Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue:
Total revenue	$ 822	$ 697	$ 2,363	$ 1,979
Operating profit (loss):
Total operating loss	$ 26	$ (6)	$ 51	$ (41)
Operating profit (loss) % of revenue:
Total operating loss %	3.20%	(0.90%)	2.20%	(2.10%)
United States [Member]
Revenue:
Total revenue	$ 630	$ 506	$ 1,792	$ 1,426
Operating profit (loss):
Total operating loss	$ 21	$ (10)	$ 40	$ (52)
Operating profit (loss) % of revenue:
Total operating loss %	3.30%	(2.00%)	2.20%	(3.60%)
Canada [Member]
Revenue:
Total revenue	$ 93	$ 96	$ 270	$ 271
Operating profit (loss):
Total operating loss	$ 5	$ 4	$ 10	$ 9
Operating profit (loss) % of revenue:
Total operating loss %	5.40%	4.20%	3.70%	3.30%
International [Member]
Revenue:
Total revenue	$ 99	$ 95	$ 301	$ 282
Operating profit (loss):
Total operating loss			$ 1	$ 2
Operating profit (loss) % of revenue:
Total operating loss %	0.00%	0.00%	0.30%	0.70%