QUARTERLY FINANCIAL DATA (UNAUDITED) - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Revenue	$ 67,801	$ 62,653	$ 65,692	$ 61,885	$ 61,791	$ 59,310	$ 59,227	$ 56,306	$ 258,031	$ 236,634	$ 216,007
Gross profit	43,578	42,228	43,170	40,406	38,360	39,798	39,596	36,702	169,382	154,456	144,216
Loss from operations	(8,881)	(7,348)	(8,156)	(9,072)	(9,363)	(6,097)	(9,342)	(9,929)	(33,457)	(34,731)	(36,270)
Net loss	$ (8,748)	$ (8,465)	$ (9,059)	$ (10,873)	$ (12,467)	$ (7,910)	$ (11,098)	$ (10,185)	$ (37,145)	$ (41,660)	$ (39,216)
Net loss per common share (in dollars per share)	$ (0.20)	$ (0.20)	$ (0.21)	$ (0.26)	$ (0.30)	$ (0.19)	$ (0.27)	$ (0.25)	$ (0.87)	$ (1.00)	$ (0.97)
Income tax benefit due to change in Federal tax rate	$ 1,400