Derivative instruments: (Details 5) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Natural gas swaps
Assets:
Gross Amounts of Recognized Assets (Liabilities)	$ 1,069,000	$ 80,000
Gross Amounts offset on the Balance Sheet	(58,000)	(1,165,000)
Net Amounts of Assets (Liabilities) Presented on the Balance Sheet	1,011,000
Liabilities:
Net Amounts of Assets (Liabilities) Presented on the Balance Sheet		(1,085,000)
Interest rate options
Assets:
Gross Amounts of Recognized Assets (Liabilities)	63,471,000	25,783,000
Cash Collateral	(34,970,000)	(8,950,000)
Net Amounts of Assets (Liabilities) Presented on the Balance Sheet	$ 28,501,000	$ 16,833,000