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Income Taxes - Unrecognized tax benefit (expense) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Balance, beginning of period $ 12 $ 12 $ 48
Gross increases - tax positions in prior period 3 0 12
Gross decreases - tax positions in prior period 0 0 (48)
Gross decreases - tax reform (6) 0 0
Balance, end of period $ 9 $ 12 $ 12