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Income Taxes Changes in Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Changes in Unrecognized Tax Benefits [Abstract]      
Balance at beginning of year $ 94 $ 624 $ 609
Additions attributable to tax positions taken in the current year 0 0 8
Additions attributable to tax positions taken in prior years 0 11 7
Reduction attributable to tax positions taken in prior years 0 (541) 0
Lapse of statute 4 0 0
Balance at end of year $ 90 $ 94 $ 624