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Income Taxes - Liability Related to Uncertain Tax Positions (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]    
Balance at January 1 $ 30 $ 38
Gross decreases for prior period tax positions (1) (7)
Gross increases for current period tax positions 3 3
Decreases due to the lapse of applicable statute of limitations (4) (4)
Balance at December 31 $ 28 $ 30