Derivative instruments (Details 5) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Natural gas swaps
Assets:
Gross Amounts of Recognized Assets (Liabilities)	$ 261,000	$ 1,069,000
Gross Amounts offset on the Balance Sheet	(2,025,000)	(58,000)
Net Amounts of Assets Presented on the Balance Sheet	(1,764,000)
Net Amounts of Assets Presented on the Balance Sheet		1,011,000
Interest rate options
Assets:
Gross Amounts of Recognized Assets (Liabilities)	13,110,000	63,471,000
Cash Collateral		(34,970,000)
Net Amounts of Assets Presented on the Balance Sheet	$ 13,110,000	$ 28,501,000