Fair Value Measurements - Schedule of Available for Sale Securities at Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 196,927	$ 259,833
Unrealized Gain	54
Unrealized Loss		(108)
Fair Value	196,981	259,725
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	64,611	45,017
Fair Value	64,611	45,017
Corporate Securities and Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	51,618	110,859
Unrealized Gain	12
Unrealized Loss		(91)
Fair Value	51,630	110,768
U.S. Government Treasury and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	80,698	103,957
Unrealized Gain	42
Unrealized Loss		(17)
Fair Value	$ 80,740	$ 103,940