Fair Value Measurements - Schedule of Fair Values of Financial Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments	$ 62,894	$ 94,806
Long-term investments		3,246
Carrying Value [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	26,705	35,168
Short-term investments	62,894	94,806
Long-term investments		3,246
Total assets	89,599	133,220
Quoted Prices in Active Markets Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	25,706	24,972
Total assets	25,706	24,972
Significant Other Observable Inputs Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	999	10,196
Short-term investments	62,894	94,806
Long-term investments		3,246
Total assets	$ 63,893	$ 108,248