Derivative instruments (Details)	9 Months Ended
	Sep. 30, 2015 USD ($) bbl MMBTU	Dec. 31, 2014 USD ($) bbl MMBTU	Sep. 30, 2014 USD ($) bbl MMBTU
Derivative instruments [Line Items]
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	$ 0	$ 0	$ 44,000
Assets needed for immediate settlement aggregate fair value for cash flow hedging instruments			$ 44,000
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings	$ (400,000)
Oil swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount | bbl	552,000	270,000	1,400,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount | MMBTU	920,000	5,000,000	7,300,000