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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
Unrecognized Tax Benefits      
Gross unrecognized tax benefit balances $ 18 $ 16 $ 34
Reductions based on settlements with taxing authority 0 $ (21) $ (73)
Unrecognized Tax Benefits, Other Disclosures      
Unrecognized tax benefits, if recognized, which would affect effective tax rate 18    
Estimate of decrease in unrecognized tax benefits in next year 1    
Unrecognized tax benefits balance reasonably possible next year $ 17