Proved reserves (Details 4)	Dec. 31, 2016 MBoe MMcf MBbls	Dec. 31, 2015 MBoe MMcf MBbls	Dec. 31, 2014 MBoe MMcf MBbls	Dec. 31, 2013 MBoe MMcf MBbls
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, energy | MBoe	0	11,865	65,086
Proved undeveloped reserves, energy | MBoe	0	1,456	26,854
Balance at end of year, energy | MBoe	0	13,321	91,940	80,695
Changes in proved and undeveloped reserves [Abstract]
Change in PUD reserve | MBoe	(1,500)
Oil (MBbls)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	0	11,380	30,130
Proved undeveloped reserves, volume	0	1,307	13,788
Balance at end of year, volume	0	12,687	43,918	41,019
NGL (MBbls)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	0	144	4,217
Proved undeveloped reserves, volume	0	67	2,970
Balance at end of year, volume	0	211	7,187	6,602
Natural Gas (MMcf)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume | MMcf	0	2,033	184,437
Proved undeveloped reserves, volume | MMcf	0	498	60,574
Balance at end of year, volume | MMcf	0	2,531	245,011	198,445