Cash, Cash Equivalents and Investments - Cash, Cash Equivalents and Short-Term Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Cash	$ 7,238	$ 6,340
Cash equivalents	3,000	6,519
Total cash and cash equivalents	10,238	12,859
Short-term investments	7,623	11,895
Total cash, cash equivalents and short-term investments	$ 17,861	$ 24,754