Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financial Assets
Financial assets, fair value	$ 83,852	$ 71,458
Money market funds
Financial Assets
Financial assets, fair value	13,447	7,180
Reverse repurchase agreements
Financial Assets
Financial assets, fair value	6,250	6,250
U.S. government securities
Financial Assets
Financial assets, fair value	750	4,112
Commercial paper
Financial Assets
Financial assets, fair value	23,118	53,916
Corporate bonds
Financial Assets
Financial assets, fair value	40,287
Level 1
Financial Assets
Financial assets, fair value	13,447	7,180
Level 1 | Money market funds
Financial Assets
Financial assets, fair value	13,447	7,180
Level 2
Financial Assets
Financial assets, fair value	70,405	64,278
Level 2 | Reverse repurchase agreements
Financial Assets
Financial assets, fair value	6,250	6,250
Level 2 | U.S. government securities
Financial Assets
Financial assets, fair value	750	4,112
Level 2 | Commercial paper
Financial Assets
Financial assets, fair value	23,118	$ 53,916
Level 2 | Corporate bonds
Financial Assets
Financial assets, fair value	$ 40,287