Fair Value Measurements - Schedule of Available for Sale Securities at Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 243,107	$ 259,833
Unrealized Gain	32
Unrealized Loss	(3)	(108)
Fair Value	243,136	259,725
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	39,971	45,017
Fair Value	39,971	45,017
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	91,913	110,859
Unrealized Loss	(3)	(91)
Fair Value	91,910	110,768
U.S. Government Treasury and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	111,223
Unrealized Gain	32
Fair Value	$ 111,255
U.S. Government Agency Obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		103,957
Unrealized Loss		(17)
Fair Value		$ 103,940