Quarterly Financial Data (Unaudited) - Summarized unaudited quarterly financial data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 48,632	$ 50,086	$ 60,479	$ 74,194	$ 51,120	$ 49,609	$ 58,145	$ 68,955	$ 233,391	$ 227,829	$ 238,250
Gross profit	17,463	16,889	19,252	24,013	18,235	17,212	19,370	22,304	77,617	77,121	80,165
Income from operations	7,142	7,047	6,532	7,497	7,266	7,209	6,326	6,204	28,218	27,005	26,022
Net income	$ 4,526	$ 4,541	$ 4,150	$ 4,755	$ 4,602	$ 4,577	$ 4,034	$ 3,952	$ 17,972	$ 17,165	$ 16,659
Diluted net income per common share (in dollars per share)	$ 0.23	$ 0.23	$ 0.21	$ 0.24	$ 0.23	$ 0.23	$ 0.20	$ 0.20	$ 0.90	$ 0.86	$ 0.80