Derivative instruments (Details) (USD $)	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 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	6 months
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings	(535,000)
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	17,400,000	7,500,000	1,400,000
Assets needed for immediate settlement aggregate fair value for cash flow hedging instruments	$ 17,400,000	$ 7,500,000	$ 1,400,000
Oil swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	2,500,000	2,900,000	3,100,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	11,000,000	18,300,000	21,400,000