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Income Taxes - Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Summary of unrecognized income tax benefits      
Unrecognized tax benefits balance at beginning of year $ 49 $ 49 $ 46
Additions for tax positions of current year 8 8 8
Additions for tax positions of prior years 3 0 2
Reductions for tax positions of prior years (1) (7) (1)
Reductions for settlements of tax positions 0 (1) 0
Reductions for lapses in statutes of limitations (5) 0 (6)
Unrecognized tax benefits balance at end of year $ 54 $ 49 $ 49