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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
Reconciliation of unrecognized tax benefits      
Balance at January 1, $ 30 $ 24 $ 17
Additions based on tax positions related to the current year 0 0 2
Additions for tax positions of prior years 0 7 6
Reductions for tax positions of prior years (1) (1) 0
Reductions related to settlements with taxing authorities (9) 0 (1)
Balance at December 31, $ 20 $ 30 $ 24