Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Restricted cash:
Restricted cash	$ 338	$ 263
Short-term investments:
Total short-term investments	7,173	6,409
Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,513	3,225
Short-term investments:
Total short-term investments	7,173	6,409
Total assets measured at fair value	9,287	10,160
Government and agencies [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	500
Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	749	1,198
Short-term investments:
Total short-term investments	5,276	3,874
Corporate debt [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents		750
Short-term investments:
Total short-term investments	1,897	2,535
Money market funds [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	264	1,277
Restricted cash:
Restricted cash	601	526
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	264	1,277
Short-term investments:
Total assets measured at fair value	865	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	264	1,277
Restricted cash:
Restricted cash	601	526
Direct or Indirect Observable Inputs (Level 2) [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	1,249	1,948
Short-term investments:
Total short-term investments	7,173	6,409
Total assets measured at fair value	8,422	8,357
Direct or Indirect Observable Inputs (Level 2) [Member] | Government and agencies [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	500
Direct or Indirect Observable Inputs (Level 2) [Member] | Corporate commercial paper [Member] | Recurring basis [Member]
Cash equivalents:
Total cash equivalents	749	1,198
Short-term investments:
Total short-term investments	5,276	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	$ 1,897	$ 2,535