Fair Value Measurements - Schedule of Available for Sale Securities at Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 277,574	$ 175,745
Unrealized Loss	(84)	(42)
Fair Value	277,490	175,703
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	104,286	66,478
Fair Value	104,286	66,478
Overnight Repurchase Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	40,000
Fair Value	40,000
U.S. government agency obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	46,779	57,183
Unrealized Loss	(24)	(30)
Fair Value	46,755	57,153
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	86,509	52,084
Unrealized Loss	(60)	(12)
Fair Value	$ 86,449	$ 52,072