Cash, Cash Equivalents and Investments - Cash, Cash Equivalents and Short-Term Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash And Cash Equivalents [Abstract]
Cash	$ 7,901	$ 6,340
Cash equivalents	2,791	6,519
Total cash and cash equivalents	10,692	12,859	$ 9,704	$ 14,312
Short-term investments	10,743	11,895
Total cash, cash equivalents and short-term investments	$ 21,435	$ 24,754