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Derivative instruments (Details) (USD $)
 9 Months Ended
Sep. 30, 2014
Dec. 31, 2013
Sep. 30, 2013
Mar. 31, 2013
Derivative instruments [Line Items]        
Amount of unrealized gain (loss) on hedge at de-designation date to be reclassified into AOCI       $ 1,800,000
Amount of time remaining to transfer accumulated other comprehensive income to earnings 3 months      
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value 44,000 7,500,000 9,900,000  
Assets needed for immediate settlement aggregate fair value for cash flow hedging instruments 44,000 7,500,000 9,900,000  
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings $ (553,000)      
Oil swap and\or collar [Member]
        
Derivative instruments [Line Items]        
Derivative, nonmonetary notional amount 1,400,000 2,900,000 3,900,000  
Natural gas swap and\or collar [Member]
        
Derivative instruments [Line Items]        
Derivative, nonmonetary notional amount 7,300,000 18,300,000 16,500,000