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Fair Value Measurements - Schedule of Fair Value of Investment Assets (Details) - Fair Value - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total investments at fair value | $ 341 | $ 95
Level 2 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total investments at fair value | 341 | 95
Level 2 | Certificates of Deposit and Commercial Paper |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total investments at fair value | 250 | 0
Level 2 | Corporate Bonds |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total investments at fair value | 56 | 61
Level 2 | Government Securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total investments at fair value | $ 35 | $ 34

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