Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Financial Assets
Financial assets, fair value	$ 100,298	$ 71,458
Money market funds
Financial Assets
Financial assets, fair value	5,947	7,180
Reverse repurchase agreements
Financial Assets
Financial assets, fair value	6,250	6,250
Commercial paper
Financial Assets
Financial assets, fair value	49,435	53,916
Level 1
Financial Assets
Financial assets, fair value	5,947	7,180
Level 1 | Money market funds
Financial Assets
Financial assets, fair value	5,947	7,180
Level 2
Financial Assets
Financial assets, fair value	94,351	64,278
Level 2 | Reverse repurchase agreements
Financial Assets
Financial assets, fair value	6,250	6,250
Level 2 | Commercial paper
Financial Assets
Financial assets, fair value	49,435	53,916
U.S. government securities
Financial Assets
Financial assets, fair value	38,666	4,112
U.S. government securities | Level 2
Financial Assets
Financial assets, fair value	$ 38,666	$ 4,112