Schedule II - Valuation and Qualifying Accounts and Reserves (Detail) - Allowance for Uncollectible Accounts - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		$ 1	$ 1	$ 1
Charged to Income		7	5	3
Other Charges		0	0	0
Payments & Deductions	[1]	6	5	3
Balance at End of Period		$ 2	$ 1	$ 1

[1] Write-off of individual bad debt accounts