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, 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
Net revenue	$ 47,632	$ 38,173	$ 36,389	$ 29,299	$ 37,875	$ 30,281	$ 27,477	$ 22,423	$ 151,493	$ 118,056	$ 94,761
Cost of revenue	20,299	15,963	15,343	13,778	15,962	12,538	11,472	9,949	65,383	49,921	40,478
Gross profit	27,333	22,210	21,046	15,521	21,913	17,743	16,005	12,474	86,110	68,135	54,283
Sales and marketing	15,362	13,148	12,787	10,773	11,561	10,574	10,712	8,716	52,070	41,563	35,942
Research and development	3,481	3,467	2,981	2,945	2,897	2,914	2,712	2,709	12,874	11,232	10,733
General and administrative	3,947	3,379	3,548	3,216	3,010	2,716	3,997	3,220	14,090	12,943	12,129
Lease termination income		(4,000)							(4,000)
Total operating expenses	22,790	15,994	19,316	16,934	17,468	16,204	17,421	14,645	75,034	65,738	58,804
Income (loss) from operations	4,543	6,216	1,730	(1,413)	4,445	1,539	(1,416)	(2,171)	11,076	2,397	(4,521)
Interest and other income, net	138	197	276	273	(204)	166	217	144	884	323	293
Income (loss) before income taxes	4,681	6,413	2,006	(1,140)	4,241	1,705	(1,199)	(2,027)	11,960	2,720	(4,228)
Income tax provision (benefit)	(18,199)	225	59	(118)	28	61	30	24	(18,033)	143	212
Net income (loss)	$ 22,880	$ 6,188	$ 1,947	$ (1,022)	$ 4,213	$ 1,644	$ (1,229)	$ (2,051)	$ 29,993	$ 2,577	$ (4,440)
Net income (loss) per share—basic (in dollars per share)	$ 1.66	$ 0.44	$ 0.14	$ (0.07)	$ 0.31	$ 0.12	$ (0.09)	$ (0.16)	$ 2.16	$ 0.19	$ (0.32)
Net income (loss) per share—diluted (in dollars per share)	$ 1.57	$ 0.42	$ 0.13	$ (0.07)	$ 0.30	$ 0.12	$ (0.09)	$ (0.16)	$ 2.04	$ 0.19	$ (0.32)
Basic (in shares)	13,744	13,973	13,935	13,840	13,591	13,163	13,131	13,010	13,873	13,225	13,960
Diluted (in shares)	14,569	14,767	14,629	13,840	14,201	13,544	13,131	13,010	14,728	13,753	13,960