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Investments - Available-for-Sale Investments (Details)
$ in Millions
Sep. 30, 2021
USD ($)
position
Dec. 31, 2020
USD ($)
position
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value $ 779 $ 77
Unrealized Losses $ 3 $ 1
Total Number of Positions | position 268 21
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value $ 509 $ 0
Unrealized Losses $ 2 $ 0
Total Number of Positions | position 174 0
Mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value $ 270 $ 77
Unrealized Losses $ 1 $ 1
Total Number of Positions | position 94 21