Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Short-term investments:
Total short-term investments	$ 15,926	$ 13,263
Restricted cash—money market fund	250	250
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	11,513	10,364
Short-term investments:
Total short-term investments	15,926	13,263
Restricted cash—money market fund	250	250
Total assets measured at fair value	27,689	23,877
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	10,192	9,362
Short-term investments:
Restricted cash—money market fund	250	250
Total assets measured at fair value	10,442	9,612
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,321	1,002
Short-term investments:
Total short-term investments	15,926	13,263
Total assets measured at fair value	17,247	14,265
Corporate commercial paper [Member]
Short-term investments:
Total short-term investments	2,496	550
Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,000
Short-term investments:
Total short-term investments	2,496	550
Corporate commercial paper [Member] | Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,000
Short-term investments:
Total short-term investments	2,496	550
Corporate debt securities [Member]
Short-term investments:
Total short-term investments	13,430	12,713
Corporate debt securities [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	321	1,002
Short-term investments:
Total short-term investments	13,430	12,713
Corporate debt securities [Member] | Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	321	1,002
Short-term investments:
Total short-term investments	13,430	12,713
Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	10,192	9,362
Money market funds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	$ 10,192	$ 9,362