Fair Value Measurements - Schedule of Fair Value of Financial Assets Measured on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial Assets
Total financial assets	$ 115,319	$ 99,979
Money Market Funds [Member]
Financial Assets
Total financial assets	115,019	99,792
Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	300	187
Level 1 [Member]
Financial Assets
Total financial assets	115,319	99,979
Level 1 [Member] | Money Market Funds [Member]
Financial Assets
Total financial assets	115,019	99,792
Level 1 [Member] | Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	$ 300	$ 187