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Income Taxes - Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Summary of unrecognized income tax benefits      
Unrecognized tax benefits balance at beginning of year $ 46 $ 42 $ 39
Additions for tax positions of current year 8 6 11
Additions for tax positions of prior years 2 1 3
Reductions for tax positions of prior years (1) (1) (1)
Reductions for lapses in statutes of limitations (6) (2) (10)
Unrecognized tax benefits balance at end of year $ 49 $ 46 $ 42