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Income Taxes - Reconciliation of Gross Amounts of Tax Contingencies (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Income Tax Disclosure [Abstract]    
Balance, beginning of year $ 255 $ 268
Additions based on tax positions related to the current year 19 26
Additions for tax positions of prior years 39 32
Reductions for tax positions of prior years (5) (32)
Reductions for tax positions as a result of lapse of statute (37) (14)
Settlements (1) (25)
Balance, end of year $ 270 $ 255