Fair Value Measurements - Schedule of Available for Sale Securities at Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 224,287	$ 259,833
Unrealized Gain	111
Unrealized Loss		(108)
Fair Value	224,398	259,725
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	64,578	45,017
Fair Value	64,578	45,017
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	60,241	110,859
Unrealized Gain	27
Unrealized Loss		(91)
Fair Value	60,268	110,768
U.S. Government Treasury and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	99,468	103,957
Unrealized Gain	84
Unrealized Loss		(17)
Fair Value	$ 99,552	$ 103,940