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Derivative instruments (Details) (USD $)
 3 Months Ended
Mar. 31, 2014
Dec. 31, 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 9 months    
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings (449,000)    
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value 12,200,000 7,500,000 12,400,000
Assets needed for immediate settlement aggregate fair value for cash flow hedging instruments 12,200,000 7,500,000 12,400,000
Oil swap and\or collar [Member]
      
Derivative instruments [Line Items]      
Derivative, nonmonetary notional amount 2,700,000 2,900,000 2,800,000
Natural gas swap and\or collar [Member]
      
Derivative instruments [Line Items]      
Derivative, nonmonetary notional amount 14,700,000 18,300,000 25,900,000
Interest rate swap [Member]
      
Derivative instruments [Line Items]      
Derivative, notional amount     $ 40,000,000