Quarterly Financial Information (Unaudited) - Summary of Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Cash, cash equivalents and short-term investments	$ 24,754	$ 26,758	$ 27,296	$ 30,984	$ 33,200	$ 31,577	$ 26,898	$ 27,626
Total current assets	44,253	45,021	47,077	49,092	55,657	51,844	47,059	49,683
Total assets	49,434	50,247	52,395	54,403	61,008	57,355	52,680	55,389
Total current liabilities	17,427	14,351	14,445	14,310	20,221	15,502	13,407	16,322
Total shareholders' equity	$ 31,430	$ 35,238	$ 37,099	$ 39,206	$ 38,112	$ 38,790	$ 38,166	$ 37,873	$ 36,907