Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Short-term investments:
Total short-term investments	$ 13,627	$ 18,888
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	7,198	3,296
Short-term investments:
Total short-term investments	13,627	18,888
Total assets measured at fair value	20,825	22,184
Corporate commercial paper [Member]
Short-term investments:
Total short-term investments	7,174	11,465
Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	2,999	500
Short-term investments:
Total short-term investments	7,174	11,465
Corporate debt securities [Member]
Short-term investments:
Total short-term investments	6,453	7,423
Corporate debt securities [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	3,752
Short-term investments:
Total short-term investments	6,453	7,423
Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	447	2,796
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	447	2,796
Short-term investments:
Total assets measured at fair value	447	2,796
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	447	2,796
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	6,751	500
Short-term investments:
Total short-term investments	13,627	18,888
Total assets measured at fair value	20,378	19,388
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	2,999	500
Short-term investments:
Total short-term investments	7,174	11,465
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate debt securities [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	3,752
Short-term investments:
Total short-term investments	$ 6,453	$ 7,423