General - Audited Consolidated Statement of Operations (Detail) - USD ($) $ / shares in Units, $ 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
Revenues	$ 65,189	$ 62,868	$ 61,091	$ 59,092	$ 58,057	$ 56,057	$ 54,092	$ 51,879	$ 248,240	$ 220,085	$ 193,153
Operating expenses
Sales and marketing									71,719	71,261	66,876
Income from operations	8,209	8,257	5,965	4,300	292	3,354	2,956	3,379	26,731	9,981	6,385
Income tax expense									4,468	10,342	2,755
Net income (loss)	$ 7,141	$ 8,061	$ 5,416	$ 3,254	$ (6,778)	$ 2,176	$ 1,968	$ 2,985	$ 23,872	$ 351	$ 4,963
Net income per share
Basic									$ 1.39	$ 0.02	$ 0.29
Diluted	$ 0.40	$ 0.45	$ 0.31	$ 0.19	$ (0.39)	$ 0.13	$ 0.11	$ 0.17	$ 1.36	$ 0.02	$ 0.29
ASU 2014-09 [Member] | As Previously Reported [Member]
Revenues										$ 220,566	$ 193,295
Operating expenses
Sales and marketing										73,295	65,886
Income from operations										8,428	7,517
Income tax expense										11,580	3,140
Net income (loss)										$ (2,440)	$ 5,710
Net income per share
Basic										$ (0.14)	$ 0.34
Diluted										$ (0.14)	$ 0.33
ASU 2014-09 [Member] | Adjustments [Member]
Revenues										$ (481)	$ (142)
Operating expenses
Sales and marketing										(2,034)	990
Income from operations										1,553	(1,132)
Income tax expense										(1,238)	(385)
Net income (loss)										$ 2,791	$ (747)
Net income per share
Basic										$ 0.16	$ (0.05)
Diluted										$ 0.16	$ (0.04)