Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 116	$ 403
Fair value, derivative liability	2,329	2,868
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	1,092	1,325
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	116	403
Fair value, derivative liability	1,237	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	116	403
Natural Gas Contracts | Designated as Hedging Instrument [Member] | Derivative Liability, Current [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	1,077	1,222
Natural Gas Contracts | Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	4	103
Natural Gas Contracts | Not Designated as Hedging Instrument [Member] | Derivative Liability, Current [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	1,219	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	18	112
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	$ 11	$ 0