Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 71,284	$ 69,588	$ 68,975	$ 66,306	$ 67,418	$ 65,605	$ 63,557	$ 59,241	$ 276,153	$ 255,821	$ 234,496
Gross profit	52,507	50,284	49,643	47,753	49,237	48,523	46,474	42,239	200,187	186,473	169,611
Loss from operations	(4,699)	(4,723)	(7,375)	(6,401)	(4,655)	(3,923)	(6,963)	(6,193)	(23,198)	(21,734)	(15,894)
Net loss	$ (5,980)	$ (6,561)	$ (8,491)	$ (9,352)	$ (11,093)	$ (4,351)	$ (5,673)	$ (5,379)	$ (30,384)	$ (26,496)	$ (15,278)
Net loss per common share
Basic (in dollars per share)	$ (0.10)	$ (0.11)	$ (0.15)	$ (0.17)	$ (0.20)	$ (0.08)	$ (0.10)	$ (0.10)	$ (0.53)	$ (0.47)	$ (0.28)
Diluted (in dollars per share)	$ (0.10)	$ (0.11)	$ (0.15)	$ (0.17)	$ (0.20)	$ (0.08)	$ (0.10)	$ (0.10)	$ (0.53)	$ (0.47)	$ (0.28)