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Investments - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Investments, Debt and Equity Securities [Abstract]        
Gross realized investment gains $ 0 $ 1 $ 0 $ 6
Gross realized investment losses 0 $ 0 0 $ 0
Debt securities held to maturity 145   145  
Amortized Cost, Due in one year or less 137   137  
Amortized Cost, Due one year through five years $ 8   $ 8