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Income Taxes - Progression of the liability for uncertain tax positions, excluding interest and penalties (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Progression of the liability for uncertain tax positions, excluding interest and penalties      
Balance at January 1 $ 1 $ 1 $ 0
Additions for tax positions of prior years 0 0 1
Reductions for tax positions of prior years 0 0 0
Additions for tax positions of current year 0 0 0
Settlements (1) 0 0
Balance at December 31 $ 0 $ 1 $ 1