Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								9 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	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Revenues		$ 71,658	$ 68,153	$ 64,878		$ 59,490	$ 55,302	$ 53,121	$ 278,889	$ 230,828	$ 278,889	$ 230,828	$ 197,925	$ 197,925
Cost of revenues											66,185	51,580	43,128
Gross profit											212,704	179,248	154,797
Operating expenses:
Research and development											53,255	42,816	36,591
Sales and marketing											70,039	63,855	58,985
General and administrative											39,049	35,334	29,114
Total operating expenses											162,343	142,005	124,690
Income from operations	$ 12,943	18,117	10,895	8,406	$ 9,729	10,849	9,009	7,656			50,361	37,243	30,107
Other income (expense), net	1,862	1,116	884	1,245	652	671	360	453			5,107	2,135	322
Income before income taxes	14,805	19,233	11,779	9,651	10,381	11,520	9,369	8,109			55,468	39,378	30,429
Provision for (benefit from) income taxes											(1,836)	(1,062)	11,205
Net income	$ 14,400	$ 23,469	$ 10,293	$ 9,142	$ 2,857	$ 8,452	$ 7,202	$ 21,930	$ 57,304	$ 40,440	$ 57,304	$ 40,440	$ 19,224	$ 19,224
Earnings Per Share [Abstract]
Basic (in usd per share)	$ 0.37	$ 0.60	$ 0.26	$ 0.24	$ 0.07	$ 0.22	$ 0.19	$ 0.60		$ 1.08	$ 1.47	$ 1.08	$ 0.55	$ 0.55
Diluted (usd per share)	$ 0.35	$ 0.56	$ 0.24	$ 0.22	$ 0.07	$ 0.21	$ 0.18	$ 0.56		$ 1.01	$ 1.37	$ 1.01	$ 0.50	$ 0.50