Fair Value Measurements - Schedule of Available for Sale Securities at Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 265,693	$ 175,745
Unrealized Loss	(110)	(42)
Fair Value	265,583	175,703
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	58,404	66,478
Fair Value	58,404	66,478
U.S. government agency obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	89,505	57,183
Unrealized Loss	(43)	(30)
Fair Value	89,462	57,153
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	117,784	52,084
Unrealized Loss	(67)	(12)
Fair Value	$ 117,717	$ 52,072