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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
Reconciliation of unrecognized tax benefits      
Balance at January 1, $ 16 $ 26 $ 14
Additions based on tax positions related to the current year 2 2 4
Additions for tax positions of prior years 0 0 8
Reductions for tax positions of prior years (1) (8) 0
Reductions related to settlements with taxing authorities 0 (4) 0
Balance at December 31, $ 17 $ 16 $ 26