Derivative Instruments (Details 3) - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Interest rate options
Assets:
Fair value of assets	$ 4,715,000	$ 4,371,000
Natural gas swaps
Liabilities:
Fair Value of liabilities	15,942,000	18,914,000
Not designated as hedges | Interest rate options | Other Deferred Charges
Assets:
Fair value of assets	4,715,000	4,371,000
Not designated as hedges | Natural gas swaps | Other Current Liabilities
Liabilities:
Fair Value of liabilities	10,198,000	13,418,000
Not designated as hedges | Natural gas swaps | Other Deferred Credits
Liabilities:
Fair Value of liabilities	$ 5,744,000	$ 5,496,000