Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 339	$ 3,713
Fair value, derivative liability	420	3,688
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	298	3,606
Fair value, derivative liability	420	3,688
Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	41	107
Interest Rate Contracts | Designated as Hedging Instrument | Derivative Asset
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	298	3,606
Natural Gas Contracts | Designated as Hedging Instrument | Derivative Liability - Current
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	420	3,390
Natural Gas Contracts | Designated as Hedging Instrument | Other Long-Term Liabilities
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	0	298
Currency Contracts | Not Designated as Hedging Instrument | Prepaid and Other Current Assets
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 41	$ 107