Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Restricted cash:
Restricted cash	$ 600	$ 263
Short-term investments:
Total short-term investments	5,627	6,409
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,141	3,225
Short-term investments:
Total short-term investments	5,627	6,409
Total assets measured at fair value	7,368	10,160
Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents		1,198
Short-term investments:
Total short-term investments	4,130	3,874
Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents		750
Short-term investments:
Total short-term investments	1,497	2,535
Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,141	1,277
Restricted cash:
Restricted cash	600	526
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,141	1,277
Short-term investments:
Total assets measured at fair value	1,741	1,803
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,141	1,277
Restricted cash:
Restricted cash	600	526
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents		1,948
Short-term investments:
Total short-term investments	5,627	6,409
Total assets measured at fair value	5,627	8,357
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents		1,198
Short-term investments:
Total short-term investments	4,130	3,874
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents		750
Short-term investments:
Total short-term investments	$ 1,497	$ 2,535