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Financial Instruments and Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Pound
Dec. 31, 2012
Pound
Forward Contracts [Member]
    
Derivatives designated as hedging instruments, Cash Flow Hedges:    
Notional Amount of Cash Flow Hedge Instruments 27,375 [1] 36,500 [1]
Derivatives not designated as hedging instruments:    
Notional amount of derivatives not designated as hedging instruments 12,585 [1] 12,643 [1]
Commodity Contract [Member]
    
Derivatives designated as hedging instruments, Cash Flow Hedges:    
Fixed Price Commodity Contracts (in pounds) 2,339,000 [1] 2,436,000 [1]
Interest Rate Swap [Member]
    
Derivatives designated as hedging instruments, Cash Flow Hedges:    
Notional Amount of Cash Flow Hedge Instruments 45,000 [1] 45,000 [1]
Other Assets [Member] | Forward Contracts [Member]
    
Derivatives not designated as hedging instruments:    
Other Derivatives Not Designated as Hedging Instruments at Fair Value, Net 0 0
Derivatives designated as hedging instruments, Cash Flow Hedges:    
Cash Flow Hedges Derivative Instruments at Fair Value, Net 2,866 1,800
Other Assets [Member] | Commodity Contract [Member]
    
Derivatives designated as hedging instruments, Cash Flow Hedges:    
Cash Flow Hedges Derivative Instruments at Fair Value, Net 0 340
Other Assets [Member] | Interest Rate Swap [Member]
    
Derivatives designated as hedging instruments, Fair Value Hedge:    
Interest Rate Fair Value Hedge Derivative at Fair Value, Net 2,109 2,212
Other Liabilities [Member] | Forward Contracts [Member]
    
Derivatives not designated as hedging instruments:    
Other Derivatives Not Designated as Hedging Instruments at Fair Value, Net 15 191
Derivatives designated as hedging instruments, Cash Flow Hedges:    
Cash Flow Hedges Derivative Instruments at Fair Value, Net 0 0
Other Liabilities [Member] | Commodity Contract [Member]
    
Derivatives designated as hedging instruments, Cash Flow Hedges:    
Cash Flow Hedges Derivative Instruments at Fair Value, Net 467 0
Other Liabilities [Member] | Interest Rate Swap [Member]
    
Derivatives designated as hedging instruments, Fair Value Hedge:    
Interest Rate Fair Value Hedge Derivative at Fair Value, Net 0 0
[1] Notional amounts represent the gross contract / notional amount of the derivatives outstanding. The fixed price commodity notional amounts are in pounds.