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Investments - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2021
Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]      
Gross realized investment gains $ 6 $ 7 $ 6
Debt securities held to maturity 156 156  
Amortized Cost, Due in one year or less 148 148  
Amortized Cost, Due one year through five years $ 8 $ 8