Proved reserves (Details 6)	Dec. 31, 2015 MBoe MMcf MBbls	Dec. 31, 2014 MBoe MMcf MBbls	Dec. 31, 2013 MBoe MMcf MBbls	Dec. 31, 2012 MBoe MMcf MBbls
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, energy | MBoe	11,865	65,086	66,140
Proved undeveloped reserves, energy | MBoe	1,456	26,854	14,555
Balance at end of year, energy | MBoe	13,321	91,940	80,695	80,486
Changes in proved and undeveloped reserves [Abstract]
Change in PUD reserve | MBoe	(25,400)
Oil (MBbls)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	11,380	30,130	31,394
Proved undeveloped reserves, volume	1,307	13,788	9,625
Balance at end of year, volume	12,687	43,918	41,019	33,453
NGL (MBbls)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	144	4,217	5,322
Proved undeveloped reserves, volume	67	2,970	1,280
Balance at end of year, volume	211	7,187	6,602	7,153
Natural Gas (MMcf)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume | MMcf	2,033	184,437	176,546
Proved undeveloped reserves, volume | MMcf	498	60,574	21,899
Balance at end of year, volume | MMcf	2,531	245,011	198,445	239,278