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Derivative instruments (Details 3) (USD $)
Sep. 30, 2014
Dec. 31, 2013
Interest rate options
   
Assets:    
Fair value of assets $ 13,110,000 $ 63,471,000
Natural gas swaps
   
Assets:    
Fair value of assets   1,011,000
Liabilities:    
Fair Value of liabilities 1,764,000  
Liability associated with cash collateral, which is recorded as an offset to the fair value of derivatives 1,764,000  
Not designated as hedges | Interest rate options
   
Assets:    
Fair value of assets 13,110,000 63,471,000
Liabilities:    
Liability associated with cash collateral, which is recorded as an offset to the fair value of derivatives 0 34,970,000
Not designated as hedges | Natural gas swaps
   
Assets:    
Fair value of assets 258,000 1,011,000
Liabilities:    
Fair Value of liabilities $ 2,022,000