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Fair Value Measurements - Fair Value of Investment Assets (Details) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total investments at amortized cost $ 91 $ 1,076
Fair Value | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total investments at fair value 102 1,084
Fair Value | Level 2 | Commercial Paper and Certificates of Deposit    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total investments at fair value 0 989
Fair Value | Level 2 | Corporate Bonds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total investments at fair value 68 59
Fair Value | Level 2 | Government Securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total investments at fair value $ 34 $ 36