Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Short-term investments:
Total short-term investments	$ 7,623	$ 11,895
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	3,000	6,519
Short-term investments:
Total short-term investments	7,623	11,895
Total assets measured at fair value	10,623	18,414
Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	500	3,245
Short-term investments:
Total short-term investments	3,729	5,480
Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	783	1,000
Short-term investments:
Total short-term investments	3,894	6,415
Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,717	2,274
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,717	2,274
Short-term investments:
Total assets measured at fair value	1,717	2,274
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,717	2,274
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,283	4,245
Short-term investments:
Total short-term investments	7,623	11,895
Total assets measured at fair value	8,906	16,140
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	500	3,245
Short-term investments:
Total short-term investments	3,729	5,480
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	783	1,000
Short-term investments:
Total short-term investments	$ 3,894	$ 6,415