Property, Plant and Equipment (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of Property, plant and equipment
Property, plant and equipment	$ 779,785,000	$ 752,726,000
Less allowances for depreciation, amortization and depletion	(507,243,000)	(489,328,000)
Property, plant and equipment - net	272,542,000	263,398,000
Property, Plant and Equipment (Textual) [Abstract]
Cost of project	104,900,000
Capitalization of fixed assets	14,700,000
Capitalization of capital leases	10,000,000
Reimbursements	63,500,000
Depreciation expense	2,300,000
Cost of sales	2,300,000
Depreciation and amortization expense	37,046,000	43,635,000	35,394,000
Land [Member]
Summary of Property, plant and equipment
Property, plant and equipment	8,797,000	9,123,000
Buildings [Member]
Summary of Property, plant and equipment
Property, plant and equipment	127,691,000	114,090,000
Machinery and equipment [Member]
Summary of Property, plant and equipment
Property, plant and equipment	570,001,000	485,593,000
Computer software [Member]
Summary of Property, plant and equipment
Property, plant and equipment	31,640,000	29,277,000
Construction in progress [Member]
Summary of Property, plant and equipment
Property, plant and equipment	15,389,000	86,149,000
Land Building Machinery and Equipment [Member]
Summary of Property, plant and equipment
Less allowances for depreciation, amortization and depletion	(503,560,000)	(476,443,000)
Property, plant and equipment - net	249,956,000	247,789,000
Capital leases [Member]
Summary of Property, plant and equipment
Property, plant and equipment	10,912,000	10,898,000
Less allowances for depreciation, amortization and depletion	(1,440,000)	(837,000)
Property, plant and equipment - net	9,472,000	10,061,000
Property, Plant and Equipment (Textual) [Abstract]
Depreciation and amortization expense	31,000,000	37,500,000	29,000,000
Mineral resources [Member]
Summary of Property, plant and equipment
Property, plant and equipment	4,979,000	5,029,000
Mine development [Member]
Summary of Property, plant and equipment
Property, plant and equipment	10,377,000	12,567,000
Productive land [Member]
Summary of Property, plant and equipment
Less allowances for depreciation, amortization and depletion	(2,243,000)	(12,048,000)
Property, plant and equipment - net	$ 13,113,000	$ 5,548,000