Summary of Changes in Contract Assets (Detail) - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2018	Jun. 30, 2018
Contract assets
Beginning balance	[1]	$ 8
Increases due to cumulative catch up adjustment	[1]		$ 7
Increases due to revenue recognized	[1]	7	15
Decreases due to transfer to receivables	[1]	(8)	(15)
Ending balance	[1],[2]	$ 7	$ 7

[1] The contract asset is determined at each period end, this table reflects the rollforward of the period end balance.

[2] Included in prepaid expenses and contract assets