Selected Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure (Unaudited) [Abstract]
Revenue	$ 67,747	$ 65,407	$ 64,293	$ 61,888	$ 60,275	$ 58,554	$ 57,481	$ 61,194	$ 259,335	$ 237,504	$ 197,688
Cost of revenue	33,529	30,444	31,542	28,920	32,641	29,766	24,694	23,670	124,435	110,771	82,323
Selling, general and administrative expenses	18,059	17,786	18,579	17,255	16,732	15,584	15,736	14,473	71,679	62,525	53,590
(Gain) loss on sale of patent assets, net	0	(8)	(699)	0	0	0	126	0	(707)	126	(177)
Operating income	16,159	17,185	14,871	15,713	10,902	13,204	16,925	23,051	63,928	64,082	61,952
Other income (expense), net	88	73	94	99	43	56	63	51	354	213	117
Income before provision for income taxes	16,247	17,258	14,965	15,812	10,945	13,260	16,988	23,102	64,282	64,295	62,069
Provision for income taxes	6,998	6,425	5,566	5,952	3,951	4,863	6,291	8,407	24,941	23,512	23,112
Net income	9,249	10,833	9,399	9,860	6,994	8,397	10,697	14,695	39,341	40,783	38,957
Net income available to common stockholders:
Basic	9,249	10,833	9,399	9,860	6,994	8,395	10,691	14,678	39,341	40,763	38,455
Diluted	9,249	10,833	9,399	9,860	6,994	8,395	10,691	14,678	39,341	40,763	38,474
Net income available to common stockholders per common share(1):
Basic	$ 0.17	$ 0.20	$ 0.18	$ 0.19	$ 0.13	$ 0.16	$ 0.21	$ 0.29	$ 0.74	$ 0.78	$ 0.77
Diluted	$ 0.17	$ 0.20	$ 0.17	$ 0.18	$ 0.13	$ 0.16	$ 0.20	$ 0.28	$ 0.72	$ 0.76	$ 0.74
Other Data:
Deferred revenue, including current portion	136,209	112,820	131,207	140,534	137,743	105,230	117,573	118,177	136,209	137,743
Stock-based compensation expense	$ 4,302	$ 4,486	$ 5,065	$ 3,803	$ 4,033	$ 3,895	$ 4,478	$ 3,708