Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial Assets
Financial assets, fair value	$ 63,003	$ 46,406
U.S. treasury securities
Financial Assets
Financial assets, fair value		1,501
Commercial paper
Financial Assets
Financial assets, fair value	32,230
Money market funds
Financial Assets
Financial assets, fair value	6,550	1,674
Reverse repurchase agreements
Financial Assets
Financial assets, fair value	6,250	7,250
U.S. government securities
Financial Assets
Financial assets, fair value	17,973	35,981
Level 1
Financial Assets
Financial assets, fair value	6,550	3,175
Level 1 | U.S. treasury securities
Financial Assets
Financial assets, fair value		1,501
Level 1 | Money market funds
Financial Assets
Financial assets, fair value	6,550	1,674
Level 2
Financial Assets
Financial assets, fair value	56,453	43,231
Level 2 | Commercial paper
Financial Assets
Financial assets, fair value	32,230
Level 2 | Reverse repurchase agreements
Financial Assets
Financial assets, fair value	6,250	7,250
Level 2 | U.S. government securities
Financial Assets
Financial assets, fair value	$ 17,973	$ 35,981