Fair Value Measurements - Schedule of Available for Sale Securities at Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 259,833	$ 175,745
Unrealized Loss	(108)	(42)
Fair Value	259,725	175,703
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	45,017	66,478
Fair Value	45,017	66,478
U.S. government agency obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	103,957	57,183
Unrealized Loss	(17)	(30)
Fair Value	103,940	57,153
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	110,859	52,084
Unrealized Loss	(91)	(12)
Fair Value	$ 110,768	$ 52,072