Derivative instruments (Details) MMBTU in Millions	6 Months Ended
	Jun. 30, 2015 USD ($) bbl MMBTU	Dec. 31, 2014 USD ($) bbl MMBTU	Jun. 30, 2014 USD ($) bbl MMBTU
Derivative instruments [Line Items]
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	$ 3,500,000	$ 0	$ 17,400,000
Assets needed for immediate settlement aggregate fair value for cash flow hedging instruments	3,500,000		$ 17,400,000
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	1,100,000	270,000	2,500,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount | MMBTU	1.8	5.0	11.0