Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-term investments:
Total short-term investments	$ 11,895	$ 18,888
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	6,519	3,296
Short-term investments:
Total short-term investments	11,895	18,888
Total assets measured at fair value	18,414	22,184
Recurring basis [Member] | Corporate commercial paper [Member]
Cash equivalents:
Total cash equivalents	3,245	500
Short-term investments:
Total short-term investments	5,480	11,465
Recurring basis [Member] | Corporate debt [Member]
Cash equivalents:
Total cash equivalents	1,000
Short-term investments:
Total short-term investments	6,415	7,423
Recurring basis [Member] | Money market funds [Member]
Cash equivalents:
Total cash equivalents	2,274	2,796
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	2,274	2,796
Short-term investments:
Total assets measured at fair value	2,274	2,796
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member] | Money market funds [Member]
Cash equivalents:
Total cash equivalents	2,274	2,796
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	4,245	500
Short-term investments:
Total short-term investments	11,895	18,888
Total assets measured at fair value	16,140	19,388
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member] | Corporate commercial paper [Member]
Cash equivalents:
Total cash equivalents	3,245	500
Short-term investments:
Total short-term investments	5,480	11,465
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member] | Corporate debt [Member]
Cash equivalents:
Total cash equivalents	1,000
Short-term investments:
Total short-term investments	$ 6,415	$ 7,423