Derivative instruments: (Details 3) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Liabilities:
Fair Value of liabilities	$ 1,085,000	$ 7,220,000
Natural gas swaps
Liabilities:
Fair Value of liabilities	1,085,000	7,220,000
Interest rate options
Assets:
Fair value of assets	25,783,000	69,446,000
Designated as hedges | Natural gas swaps
Liabilities:
Fair Value of liabilities		7,220,000
Not designated as hedges | Natural gas swaps
Liabilities:
Fair Value of liabilities	1,085,000
Not designated as hedges | Nuclear decommissioning trust
Assets:
Fair value of assets		(982,000)
Not designated as hedges | Interest rate options
Assets:
Fair value of assets	$ 25,783,000	$ 69,446,000