Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenues	$ 85,573,000	$ 88,837,000	$ 168,203,000	$ 170,196,000
Cost of goods sold	41,727,000	42,890,000	82,048,000	84,487,000
Gross profit	43,846,000	45,947,000	86,155,000	85,709,000
Operating expenses
Sales and marketing	12,130,000	11,603,000	22,770,000	21,817,000
General and administrative	11,575,000	11,203,000	24,118,000	23,473,000
Research and development	4,215,000	5,244,000	8,204,000	10,595,000
Total operating expenses	27,920,000	28,050,000	55,092,000	55,885,000
Operating income	15,926,000	17,897,000	31,063,000	29,824,000
Interest expense	4,298,000	4,285,000	8,348,000	9,705,000
Loss on extinguishment of debt	0	0	0	2,161,000
Other income	(336,000)	(552,000)	(1,256,000)	(1,129,000)
Income before income taxes	11,964,000	14,164,000	23,971,000	19,087,000
Income tax expense	2,219,000	569,000	6,015,000	1,354,000
Net income	$ 9,745,000	$ 13,595,000	$ 17,956,000	$ 17,733,000
Net income per common share:
Basic (in dollars per share)	$ 0.25	$ 0.37	$ 0.47	$ 0.48
Diluted (in dollars per share)	$ 0.25	$ 0.35	$ 0.45	$ 0.46
Weighted-average common shares outstanding:
Basic (in shares)	38,233	37,235	38,060	37,063
Diluted (in shares)	39,398	38,900	39,468	38,726