Cash, Cash Equivalents and Investments - Cash, Cash Equivalents and Short-Term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Cash	$ 5,933	$ 11,016
Cash equivalents:
Total cash equivalents	7,198	3,296
Total cash and cash equivalents	13,131	14,312	$ 12,346	$ 16,443
Short-term investments:
Total short-term investments	13,627	18,888
Total cash, cash equivalents and short-term investments	26,758	33,200
Corporate commercial paper [Member]
Cash equivalents:
Total cash equivalents	2,999	500
Short-term investments:
Total short-term investments	7,174	11,465
Corporate debt securities [Member]
Cash equivalents:
Total cash equivalents	3,752
Short-term investments:
Total short-term investments	6,453	7,423
Money market funds [Member]
Cash equivalents:
Total cash equivalents	$ 447	$ 2,796