Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Short-term investments:
Total short-term investments	$ 10,743	$ 11,895
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	2,791	6,519
Short-term investments:
Total short-term investments	10,743	11,895
Total assets measured at fair value	13,534	18,414
Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	2,494	3,245
Short-term investments:
Total short-term investments	4,970	5,480
Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents		1,000
Short-term investments:
Total short-term investments	5,773	6,415
Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	297	2,274
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	297	2,274
Short-term investments:
Total assets measured at fair value	297	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	297	2,274
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	2,494	4,245
Short-term investments:
Total short-term investments	10,743	11,895
Total assets measured at fair value	13,237	16,140
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	2,494	3,245
Short-term investments:
Total short-term investments	4,970	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	$ 5,773	$ 6,415