Selected Quarterly Financial Data (Unaudited) - Selected Unaudited Quarterly Consolidated Statements of Operations Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Operations Data
Revenues	$ 140,535	$ 129,764	$ 119,436	$ 111,791	$ 104,503	$ 96,839	$ 91,844	$ 86,538	$ 501,526	$ 379,724	$ 296,228
Gross profit	106,915	98,980	89,875	84,485	79,851	73,384	69,480	64,798	380,255	287,513	208,957
Operating loss	(6,185)	(6,246)	(7,543)	(7,301)	(6,606)	(7,061)	(6,304)	(5,981)	(27,275)	(25,952)	(30,932)
Net loss	$ (6,089)	$ (5,712)	$ (7,031)	$ (7,309)	$ (6,946)	$ (7,979)	$ (7,771)	$ (6,613)	$ (26,141)	$ (29,309)	$ (32,099)
Basic and diluted net loss per share	$ (0.08)	$ (0.07)	$ (0.09)	$ (0.10)	$ (0.09)	$ (0.11)	$ (0.11)	$ (0.09)	$ (0.34)	$ (0.40)	$ (0.46)