Derivative instruments (Details)	9 Months Ended
	Sep. 30, 2016 USD ($) bbl MMBTU	Dec. 31, 2015 USD ($) bbl MMBTU	Sep. 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	552,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount | MMBTU	0	0	920,000
Interest rate swap [Member]
Derivative instruments [Line Items]
Derivative, notional amount	$ 0	$ 0	$ 0