Cash and Investments - Cash, Cash Equivalents and Short-Term Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash And Cash Equivalents [Abstract]
Cash	$ 6,780				$ 6,340
Cash equivalents	3,225				6,519
Restricted cash	263
Restricted cash, long term	263
Total cash, cash equivalents and restricted cash as presented in the statement of cash flows	10,531				12,859				$ 14,312
Short-term investments	6,409				11,895
Total cash, cash equivalents, restricted cash and short-term investments	$ 16,940	$ 17,271	$ 17,861	$ 21,435	$ 24,754	$ 26,758	$ 27,296	$ 30,984