Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial Assets
Financial assets, fair value	$ 64,913	$ 71,458
Commercial paper
Financial Assets
Financial assets, fair value	42,931	53,916
Money market funds
Financial Assets
Financial assets, fair value	14,613	7,180
Reverse repurchase agreements
Financial Assets
Financial assets, fair value	6,250	6,250
Level 1
Financial Assets
Financial assets, fair value	14,613	7,180
Level 1 | Money market funds
Financial Assets
Financial assets, fair value	14,613	7,180
Level 2
Financial Assets
Financial assets, fair value	50,300	64,278
Level 2 | Commercial paper
Financial Assets
Financial assets, fair value	42,931	53,916
Level 2 | Reverse repurchase agreements
Financial Assets
Financial assets, fair value	6,250	6,250
U.S. government securities
Financial Assets
Financial assets, fair value	1,119	4,112
U.S. government securities | Level 2
Financial Assets
Financial assets, fair value	$ 1,119	$ 4,112