Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Short-term investments:
Total short-term investments	$ 5,879	$ 11,895
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	3,727	6,519
Short-term investments:
Total short-term investments	5,879	11,895
Total assets measured at fair value	9,606	18,414
Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,746	3,245
Short-term investments:
Total short-term investments	3,231	5,480
Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents		1,000
Short-term investments:
Total short-term investments	2,648	6,415
Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,981	2,274
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,981	2,274
Short-term investments:
Total assets measured at fair value	1,981	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	1,981	2,274
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,746	4,245
Short-term investments:
Total short-term investments	5,879	11,895
Total assets measured at fair value	7,625	16,140
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,746	3,245
Short-term investments:
Total short-term investments	3,231	5,480
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents		1,000
Short-term investments:
Total short-term investments	$ 2,648	$ 6,415