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Income Taxes - Reconciliation of Uncertain Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Uncertain tax benefits    
Liability for uncertainty in income taxes, noncurrent $ 33 $ 37
Unrecognized tax benefits, income tax penalties expense 1  
Accrued interest and penalties related to unrecognized tax benefits 3 4
Uncertain tax benefits reconciliation    
Balance as of January 1 34 32
Increase due to current year positions 4 8
Decrease due to prior year positions (8) 0
Decrease due to settlements and payments 0 (6)
Uncertain tax benefits as of December 31 $ 30 $ 34