Fair Value Measurements - Schedule of Available for Sale Securities at Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 290,872	$ 175,745
Unrealized Loss	(97)	(42)
Fair Value	290,775	175,703
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	182,149	66,478
Fair Value	182,149	66,478
U.S. government agency obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	46,283	57,183
Unrealized Loss	(43)	(30)
Fair Value	46,240	57,153
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	62,440	52,084
Unrealized Loss	(54)	(12)
Fair Value	$ 62,386	$ 52,072