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Investments - Available-for-Sale Investments (Details)
$ in Millions
Sep. 30, 2020
USD ($)
Security
Dec. 31, 2019
USD ($)
Security
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value $ 139 $ 365
Unrealized Losses $ 2 $ 2
Total Number of Positions | Security 25 239
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value $ 35 $ 222
Unrealized Losses $ 1 $ 1
Total Number of Positions | Security 18 167
Mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Estimated Fair Value $ 104 $ 143
Unrealized Losses $ 1 $ 1
Total Number of Positions | Security 7 72