Derivative Instruments (Details 3) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Natural gas swaps
Liabilities:
Fair Value of liabilities	$ 197,000	$ 1,085,000
Interest rate options
Assets:
Fair value of assets	43,531,000	25,783,000
Not designated as hedges | Natural gas swaps
Liabilities:
Fair Value of liabilities	197,000	1,085,000
Not designated as hedges | Interest rate options
Assets:
Fair value of assets	43,531,000	25,783,000
Liabilities:
Liability associated with cash collateral, which is recorded as an offset to the fair value of derivatives	$ 31,060,000	$ 8,950,000