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Income Taxes - Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Unrecognized tax benefits roll forward      
Gross unrecognized tax benefits at beginning of period $ (11) $ (9) $ (3)
Increases in tax positions for current year (1) (1) (1)
Increases in tax positions for prior years (4) (1) (5)
Decreases in tax positions for prior years 3 0 0
Gross unrecognized tax benefits at end of period $ (13) $ (11) $ (9)