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Income Taxes - Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Summary of unrecognized income tax benefits      
Unrecognized tax benefits balance at beginning of year $ 48 $ 47 $ 43
Additions for tax positions of current year 7 6 6
Additions for tax positions of prior years 0 1 1
Reductions for tax positions of prior years (6) 0 (1)
Reductions for settlements of tax positions (1) 0 0
Reductions for lapses in statutes of limitations 0 (6) (2)
Unrecognized tax benefits balance at end of year $ 48 $ 48 $ 47