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Fair Value Measurements - Investment Maturities (Details) - Fair Value - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Less than 1 year $ 2 $ 2
1 - 5 years 23 22
5 - 10 years 20 23
Greater than 10 years 50 54
Total investments at fair value $ 95 $ 101