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Income Taxes - Reconciliation of the Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Reconciliation of the unrecognized tax benefits      
Balance beginning of year $ 29 $ 44 $ 49
Reductions related to expirations of statute of limitations (6) (15) (5)
Balance end of year $ 23 $ 29 $ 44