Supplementary Financial Data (Unaudited) - Supplementary Financial Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	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, 2018		Dec. 31, 2017	Dec. 31, 2016
Net revenue	$ 45,469	$ 40,573	$ 42,553	$ 34,125	$ 47,632	$ 38,173	$ 36,389	$ 29,299
Cost of revenue	26,683	18,688	20,176	16,791	20,299	15,963	15,343	13,778
Gross profit	18,786	21,885	22,377	17,334	27,333	22,210	21,046	15,521	$ 80,382		$ 86,110	$ 68,135
Sales and marketing	15,318	14,479	15,535	13,088	15,362	13,148	12,787	10,773	58,420		52,070	41,563
Research and development	3,464	3,244	4,095	3,556	3,481	3,467	2,981	2,945	14,359		12,874	11,232
General and administrative	5,494	5,160	4,902	5,439	3,947	3,379	3,548	3,216	20,995		14,090	12,943
Lease termination income						(4,000)					(4,000)
Total operating expenses	24,276	22,883	24,532	22,083	22,790	15,994	19,316	16,934	93,774		75,034	65,738
Income (loss) from operations	(5,490)	(998)	(2,155)	(4,749)	4,543	6,216	1,730	(1,413)	(13,392)		11,076	2,397
Interest and other income, net	(44)	(49)	(129)	98	138	197	276	273	(123)	[1]	884	323
Income (loss) before income taxes	(5,534)	(1,047)	(2,284)	(4,651)	4,681	6,413	2,006	(1,140)	(13,515)		11,960	2,720
Income tax provision	20,760	(174)	(712)	(2,619)	(18,199)	225	59	(118)	17,255		(18,033)	143
Net income (loss)	$ (26,293)	$ (873)	$ (1,572)	$ (2,032)	$ 22,880	$ 6,188	$ 1,947	$ (1,022)	$ (30,770)		$ 29,993	$ 2,577
Net income (loss) per share—basic (in dollars per share)	$ (1.89)	$ (0.06)	$ (0.11)	$ (0.15)	$ 1.66	$ 0.44	$ 0.14	$ (0.07)	$ (2.23)		$ 2.16	$ 0.19
Net income (loss) per share—diluted (in dollars per share)	$ (1.89)	$ (0.06)	$ (0.11)	$ (0.15)	$ 1.57	$ 0.42	$ 0.13	$ (0.07)	$ (2.23)		$ 2.04	$ 0.19
Basic (in shares)	13,932	13,851	13,709	13,587	13,744	13,973	13,935	13,840	13,771		13,873	13,225
Diluted (in shares)	13,932	13,851	13,709	13,587	14,569	14,767	14,629	13,840	13,771		14,728	13,753

[1] Included in interest and other income, net, is the estimated interest expense for advance payment related to service contracts under the new revenue standard.