Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Restricted cash:
Restricted cash	$ 263
Short-term investments:
Total short-term investments	6,409	$ 11,895
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	3,225	6,519
Short-term investments:
Total short-term investments	6,409	11,895
Total assets measured at fair value	10,160	18,414
Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,198	3,245
Short-term investments:
Total short-term investments	3,874	5,480
Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	750	1,000
Short-term investments:
Total short-term investments	2,535	6,415
Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,277	2,274
Restricted cash:
Restricted cash	526
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,277	2,274
Short-term investments:
Total assets measured at fair value	1,803	2,274
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,277	2,274
Restricted cash:
Restricted cash	526
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,948	4,245
Short-term investments:
Total short-term investments	6,409	11,895
Total assets measured at fair value	8,357	16,140
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,198	3,245
Short-term investments:
Total short-term investments	3,874	5,480
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	750	1,000
Short-term investments:
Total short-term investments	$ 2,535	$ 6,415