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Fair Value Measurements - Schedule of Investment Maturities (Details) - Fair Value - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Less than 1 year $ 615 $ 18
1 - 5 years 17 11
5 - 10 years 25 26
Greater than 10 years 49 40
Total investments at fair value $ 706 $ 95