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Derivative instruments (Details) (USD $)
 3 Months Ended 9 Months Ended
Sep. 30, 2012
MMBTU
Sep. 30, 2011
Sep. 30, 2012
MMBTU
Sep. 30, 2011
Derivative instruments [Line Items]        
Natural gas swap agreement total forward notional volumes in MMBtu 31,000   31,000  
Change in fair value of derivative instruments not designated as hedging instruments recorded as increase (decrease) to regulatory assets $ (175,000) $ 414,000 $ (384,000) $ (8,100,000)
Derivative instruments with credit-risk-related contingent features, net liability position, aggregate fair value 53,000   53,000  
Aggregate fair value of assets needed for immediate settlement if credit-risk-related contingent features were triggered 53,000   53,000  
Notional amount of interest rate derivatives 60,000,000   60,000,000  
Amount of gain (loss) of hedge ineffectiveness on oil and natural gas derivatives (500,000)   (900,000)  
Gain (loss) on derivatives that did not qualify for hedge accounting (600,000) 200,000 400,000 300,000
Maximum length of time hedged in swap, collar and/or put cash flow hedge agreements (in months)     15 months  
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings     9,800,000  
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value 9,900,000   9,900,000  
Assets needed for immediate settlement, of credit-risk-related contingent features, aggregate fair value $ 9,900,000   $ 9,900,000  
Oil swap and\or collar [Member]
        
Derivative instruments [Line Items]        
Nonmonetary notional amount of derivative instruments 3,300,000   3,300,000  
Natural gas swap and\or collar [Member]
        
Derivative instruments [Line Items]        
Nonmonetary notional amount of derivative instruments 8,200,000   8,200,000  
Natural gas basis swap [Member]
        
Derivative instruments [Line Items]        
Nonmonetary notional amount of derivative instruments 874,000   874,000