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Investments - Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]    
Gross realized investment gains $ 6 $ 13
Amortized Cost, Due in one year or less 134  
Amortized Cost, Due one year through five years $ 2