Quarterly Results Quarterly Results (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Results [Abstract]
Total revenue	$ 134,872,000	$ 127,854,000	$ 125,468,000	$ 115,623,000	$ 120,051,000	$ 122,472,000	$ 110,190,000	$ 94,706,000	$ 503,817,000	$ 447,419,000	$ 370,868,000
Gross profit	68,514,000	71,740,000	68,855,000	62,045,000	64,299,000	67,344,000	59,685,000	53,631,000	271,154,000	244,959,000	213,674,000
Income from operations	14,622,000	18,008,000	14,318,000	4,594,000	9,875,000	6,185,000	(1,877,000)	5,252,000	51,542,000	19,435,000	50,928,000
Income before provision for income taxes	13,287,000	16,490,000	12,532,000	3,020,000	7,342,000	4,629,000	(3,446,000)	4,800,000	45,329,000	13,325,000	51,257,000
Net income	11,790,000	9,393,000	6,623,000	2,666,000	3,270,000	2,825,000	(2,271,000)	2,759,000	30,472,000	6,583,000	33,220,000
Earnings per share, Basic	$ 0.26	$ 0.21	$ 0.15	$ 0.06	$ 0.07	$ 0.06	$ (0.05)	$ 0.06	$ 0.68	$ 0.15	$ 0.76
Earnings per share, Diluted	$ 0.26	$ 0.21	$ 0.15	$ 0.06	$ 0.07	$ 0.06	$ (0.05)	$ 0.06	$ 0.67	$ 0.15	$ 0.75
Subscription revenue attributable to presentation change from net to gross	$ 8,500,000