Commitments (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Future minimum rental payments required under operating leases
2014	$ 6,900,000
2015	4,500,000
2016	3,000,000
2017	2,300,000
2018	1,300,000
Thereafter	1,600,000
Total rental expense on operating leases	7,600,000	7,900,000	9,100,000
Long-term supply contracts
Total payments	52,710,000	45,038,000	46,581,000
Future minimum purchase requirements
2014	41,772,000
2015	35,651,000
2016	19,598,000
2017	19,580,000
2018	20,017,000
Thereafter	2,164,000
Raw and other materials
Long-term supply contracts
Total payments	40,170,000	30,802,000	33,953,000
Future minimum purchase requirements
2014	37,800,000
2015	31,679,000
2016	19,162,000
2017	19,580,000
2018	20,017,000
Thereafter	2,164,000
Transportation
Long-term supply contracts
Total payments	9,794,000	9,731,000	8,782,000
Future minimum purchase requirements
2014	1,575,000
2015	1,575,000
2016	436,000
Information systems and services
Long-term supply contracts
Total payments	2,746,000	4,505,000	3,846,000
Future minimum purchase requirements
2014	2,397,000
2015	$ 2,397,000