Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 477	$ 403
Fair value, derivative liability	3,489	2,868
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	1,618	1,325
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	477	403
Fair value, derivative liability	1,871	1,543
Currency contracts [Member] | Not Designated as Hedging Instrument [Member] | Prepaid and other current assets [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	477	403
Natural Gas Contracts | Designated as Hedging Instrument [Member] | Derivative Liability, Current [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	1,453	1,222
Natural Gas Contracts | Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	165	103
Natural Gas Contracts | Not Designated as Hedging Instrument [Member] | Derivative Liability, Current [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	1,698	1,431
Natural Gas Contracts | Not Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	$ 173	$ 112