Derivative instruments (Details) MMBTU in Millions	6 Months Ended
	Jun. 30, 2016 USD ($) bbl MMBTU	Dec. 31, 2015 USD ($) bbl MMBTU	Jun. 30, 2015 USD ($) bbl MMBTU
Derivative instruments [Line Items]
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	0	0	1,100,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount | MMBTU	0.0	0.0	1.8
Interest Rate Swap [Member]
Derivative instruments [Line Items]
Derivative, Notional Amount	$ 0	$ 0	$ 0