Financial Instruments and Fair Value Measurements (Details Textual) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended									6 Months Ended		12 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Reclassification out of Accumulated Other Comprehensive Income [Member]	Jun. 30, 2013 Interest Rate Swap [Member]	Jun. 30, 2012 Interest Rate Swap [Member]	Jun. 30, 2013 Interest Rate Swap [Member]	Jun. 30, 2012 Interest Rate Swap [Member]	Jun. 30, 2013 Forward Contracts [Member]		Jun. 30, 2012 Forward Contracts [Member]	Dec. 31, 2012 Forward Contracts [Member]		Jun. 30, 2013 Forward Contracts [Member] Period From July 2013 Through December 2013 [Member]	Jun. 30, 2013 Forward Contracts [Member] Period From January 2014 Through December 2014 [Member]	Jun. 30, 2013 Commodity Contract [Member] lb		Dec. 31, 2012 Commodity Contract [Member] lb		Jun. 30, 2013 Commodity Contract [Member] Period From July 2013 Through December 2013 [Member] lb	Jun. 30, 2013 Commodity Contract [Member] Period From January 2014 Through December 2014 [Member] lb	Jun. 30, 2013 Senior Notes [Member]	Dec. 31, 2012 Senior Notes [Member]
Secured Long-term Debt, Noncurrent																							$ 175,000	$ 175,000
Long-Term Debt, Fair Value																							191,853	188,895
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net		1,435		558						(278)		85
Derivatives Designated As Hedging Instruments Cash Flow Hedges Notional Amount										69,250	[1]		36,500	[1]	24,250	45,000
Derivatives Not Designated As Hedging Instruments Notional Amount										12,625	[1]		12,643	[1]
Fixed Price Commodity Contracts (in pounds)																	2,827,000	[1]	2,436,000	[1]	1,653,000	1,174,000
Derivative Instruments in Hedges, at Fair Value, Net, Total						45,000		45,000
Derivative, Variable Interest Rate						7.20%		7.20%
Derivative, Fixed Interest Rate						9.50%		9.50%
Increase Decrease In Derivative Interest Expenses						191	168	422	393
Debt Instrument, Interest Rate, Stated Percentage						9.50%		9.50%
Cash Flow Hedge Gain Reclassified To Earnings	$ 6,391	$ (5,298)	$ 10,674	$ 448	$ 538

[1] Notional amounts represent the gross contract / notional amount of the derivatives outstanding. The fixed price commodity notional amounts are in pounds.