Fair Value Measurements - Schedule of Fair Value Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Assets
Total financial assets	$ 99,980	$ 107,243
Money Market Funds [Member]
Financial Assets
Total financial assets	99,792	107,043
Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	188	200
Level 1 [Member]
Financial Assets
Total financial assets	99,980	107,243
Level 1 [Member] | Money Market Funds [Member]
Financial Assets
Total financial assets	99,792	107,043
Level 1 [Member] | Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	$ 188	$ 200