Cash and Investments - Cash, Cash Equivalents and Short-Term Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash And Cash Equivalents [Abstract]
Cash	$ 6,340	$ 11,016
Cash equivalents	6,519	3,296
Total cash and cash equivalents	12,859	14,312	$ 16,443
Short-term investments	11,895	18,888
Total cash, cash equivalents, and short-term investments	$ 24,754	$ 33,200