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Fair Value Measurements - Investment Maturities (Details) - Fair Value - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Less than 1 year $ 822 $ 253
1 - 5 years 8 14
5 - 10 years 24 26
Greater than 10 years 52 48
Total investments at fair value $ 906 $ 341