Financial Instruments and Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2013 lb		Dec. 31, 2012 lb
Forward Contracts [Member]
Derivatives designated as hedging instruments, Cash Flow Hedges:
Derivatives Designated As Hedging Instruments Cash Flow Hedges Notional Amount	$ 69,250	[1]	$ 36,500	[1]
Derivatives not designated as hedging instruments:
Notional amount of derivatives not designated as hedging instruments	12,625	[1]	12,643	[1]
Commodity Contract [Member]
Derivatives designated as hedging instruments, Cash Flow Hedges:
Fixed Price Commodity Contracts (in pounds)	2,827,000	[1]	2,436,000	[1]
Interest Rate Swap [Member]
Derivatives designated as hedging instruments, Fair Value Hedge:
Derivatives Designated As Hedging Instruments Fair Value Hedges Notional Amount	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	573		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	818		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	16		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	1,111		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.