Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fair value of derivative instruments
Total assets derivatives designated as hedging instruments	$ 0	$ 22
Total liabilities derivatives designated as hedging instruments	0	12,475
Interest rate swaps - cash flow hedges [Member]
Fair value of derivative instruments
Total assets derivatives designated as hedging instruments	0	0
Total liabilities derivatives designated as hedging instruments	0	12,227
Natural gas swaps [Member]
Fair value of derivative instruments
Total assets derivatives designated as hedging instruments	0	22
Total liabilities derivatives designated as hedging instruments	$ 0	$ 248