Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected quarterly financial data [Abstract]
Net sales	$ 148,650	$ 156,341	$ 99,439	$ 82,646	$ 91,016	$ 101,164	$ 93,981	$ 63,028	$ 487,076	$ 349,189	$ 286,594
Cost of sales	120,781	128,250	81,493	68,576	73,108	81,132	78,717	53,932	399,100	286,952	240,115
Net earnings	2,020	1,507	3,065	2,503	7,397	7,380	1,689	(558)	9,095	15,908	2,373
Net earnings (Loss) per share [Abstract]
Restructuring charges									1,832	1,387	5,245
Class A [Member]
Selected quarterly financial data [Abstract]
Net earnings									1,580	2,868	372
Net earnings (Loss) per share [Abstract]
Common share - basic and diluted (in dollars per share)	$ 0.16	$ 0.12	$ 0.25	$ 0.20	$ 0.61	$ 0.62	$ 0.14	$ (0.05)	$ 0.73	$ 1.32	$ 0.17
Class B [Member]
Selected quarterly financial data [Abstract]
Net earnings									$ 7,515	$ 13,040	$ 2,001
Net earnings (Loss) per share [Abstract]
Common share - basic and diluted (in dollars per share)	$ 0.17	$ 0.13	$ 0.27	$ 0.22	$ 0.65	$ 0.65	$ 0.15	$ (0.05)	$ 0.79	$ 1.41	$ 0.21