Quarterly Financial Information (Unaudited) - Summary of Condensed Consolidated Balance Sheets (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
Statement Of Financial Position [Abstract]
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
Total current assets	29,996	32,102	35,514	39,913	44,253	45,021	47,077	49,092
Total assets	31,994	37,648	41,000	45,110	49,434	50,247	52,395	54,403
Total current liabilities	12,466	12,744	14,189	15,289	17,427	14,351	14,445	14,310
Total shareholders' equity	$ 18,341	$ 23,858	$ 25,635	$ 29,340	$ 31,430	$ 35,238	$ 37,099	$ 39,206	$ 38,112