Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Short-term investments:
Total short-term investments	$ 17,464	$ 13,263
Restricted cash—money market fund	250	250
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	9,924	10,364
Short-term investments:
Total short-term investments	17,464	13,263
Restricted cash—money market fund	250	250
Total assets measured at fair value	27,638	23,877
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	9,674	9,362
Short-term investments:
Restricted cash—money market fund	250	250
Total assets measured at fair value	9,924	9,612
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	250	1,002
Short-term investments:
Total short-term investments	17,464	13,263
Total assets measured at fair value	17,714	14,265
Corporate debt securities [Member]
Short-term investments:
Total short-term investments	16,065	12,713
Corporate debt securities [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	250	1,002
Short-term investments:
Total short-term investments	16,065	12,713
Corporate debt securities [Member] | Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	250	1,002
Short-term investments:
Total short-term investments	16,065	12,713
Corporate commercial paper [Member]
Short-term investments:
Total short-term investments	1,399	550
Corporate commercial paper [Member] | Recurring basis [Member]
Short-term investments:
Total short-term investments	1,399	550
Corporate commercial paper [Member] | Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Short-term investments:
Total short-term investments	1,399	550
Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	9,674	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	$ 9,674	$ 9,362