Quarterly Financial Statements (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
Quarterly Financial Information Disclosure [Abstract]
Sales revenues	$ 78,062	$ 67,113	$ 67,677	$ 64,075	$ 64,551	$ 64,663	$ 68,415	$ 69,643	$ 276,927	$ 267,272	$ 182,174
Gross profit	45,380	35,475	36,885	33,557	33,491	32,767	34,913	36,089	151,297	137,260	94,458
Operating income (loss)	(18,118)	5,622	4,448	3,234	4,092	4,244	5,263	776	(4,814)	14,375	25,086
Net income (loss)	$ (21,537)	$ 2,288	$ 1,587	$ 246	$ 2,001	$ 1,599	$ 1,996	$ (1,663)	$ (17,416)	$ 3,933	$ 18,343
Net income (loss) per share
Net income (loss) per share - basic (in dollars per share)	$ (1.57)	$ 0.17	$ 0.11	$ 0.02	$ 0.15	$ 0.12	$ 0.15	$ (0.13)	$ (1.27)	$ 0.29	$ 1.62
Net income (loss) per share - diluted (in dollars per share)	$ (1.57)	$ 0.17	$ 0.11	$ 0.02	$ 0.15	$ 0.12	$ 0.15	$ (0.13)	$ (1.27)	$ 0.29	$ 1.62