Fair Value Measurements - Schedule of Fair Value of Investment Assets (Details) - Fair Value - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
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