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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, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Unrecognized tax benefits, beginning balance $ 36 $ 33 $ 31
Increases for tax positions taken during the period 1 6 5
Decreases for tax positions taken during a prior period (5) (3) (3)
Unrecognized tax benefits, ending balance $ 32 $ 36 $ 33