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Fair Value Measurements - Investment Maturities (Details) - Fair Value - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Less than 1 year $ 1,037 $ 253
1 - 5 years 9 14
5 - 10 years 26 26
Greater than 10 years 54 48
Total investments at fair value $ 1,126 $ 341