Investments - Schedule of Available-for-sale Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	$ 374,161	$ 110,095
Unrealized Gain	383
Unrealized Loss	(1)	(107)
Estimated Fair Value	374,543	109,988
Commercial Paper
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	82,165	16,887
Estimated Fair Value	82,165	16,887
U.S. Government Debt and Agency Securities
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	269,366	85,722
Unrealized Gain	351
Unrealized Loss	(1)	(99)
Estimated Fair Value	269,716	85,623
Corporate Bonds
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	22,630	7,486
Unrealized Gain	32
Unrealized Loss		(8)
Estimated Fair Value	$ 22,662	$ 7,478