Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Short-term investments:
Total short-term investments	$ 13,263	$ 7,295
Restricted cash—money market fund	250	250
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	10,364	10,989
Short-term investments:
Total short-term investments	13,263	7,295
Restricted cash—money market fund	250	250
Total assets measured at fair value	23,877	18,534
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	9,362	8,989
Short-term investments:
Restricted cash—money market fund	250	250
Total assets measured at fair value	9,612	9,239
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,002	2,000
Short-term investments:
Total short-term investments	13,263	7,295
Total assets measured at fair value	14,265	9,295
Corporate debt [Member]
Short-term investments:
Total short-term investments	12,713	4,794
Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,002	2,000
Short-term investments:
Total short-term investments	12,713	4,794
Corporate debt [Member] | Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,002	2,000
Short-term investments:
Total short-term investments	12,713	4,794
Corporate commercial paper [Member]
Short-term investments:
Total short-term investments	550	1,500
Corporate commercial paper [Member] | Recurring basis [Member]
Short-term investments:
Total short-term investments	550	1,500
Corporate commercial paper [Member] | Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Short-term investments:
Total short-term investments	550	1,500
Foreign government bonds [Member]
Short-term investments:
Total short-term investments		1,001
Foreign government bonds [Member] | Recurring basis [Member]
Short-term investments:
Total short-term investments		1,001
Foreign government bonds [Member] | Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Short-term investments:
Total short-term investments		1,001
Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	9,362	8,989
Money market funds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	$ 9,362	$ 8,989