Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation account for accounts receivable:
Valuation Account
Balance at Beginning of Period	$ 3	$ 6	$ 1
Charged to Costs and Expenses	1	0	5
Deductions	1	3	0
Balance at End of Period	3	3	6
Valuation account for deferred tax assets:
Valuation Account
Balance at Beginning of Period	47	48	57
Charged to Costs and Expenses	91	18	5
Deductions	4	19	14
Balance at End of Period	$ 134	$ 47	$ 48