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Fair Value Measurements - Investment Maturities (Details) - Fair Value - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Less than 1 year $ 1 $ 996
1 - 5 years 18 10
5 - 10 years 30 25
Greater than 10 years 53 53
Total investments at fair value $ 102 $ 1,084