Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation account for accounts receivable:
Valuation Account
Balance at Beginning of Period	$ 3	$ 3	$ 6
Charged to Costs and Expenses	1	1	0
Deductions	1	1	3
Balance at End of Period	3	3	3
Valuation account for deferred tax assets:
Valuation Account
Balance at Beginning of Period	134	47	48
Charged to Costs and Expenses	0	91	18
Deductions	78	4	19
Balance at End of Period	$ 56	$ 134	$ 47