Changes in total proved undeveloped reserves (Details)	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Gas (MMCF)
Proved undeveloped reserves - Gas	2,168	2,168	2,100
Revaluation of undeveloped reserves - Gas	0	(68)	0
Conversion to proved developed reserves - Gas	0	0	0
Oil (MBBLS)
Proved undeveloped reserves - Oil	68	68	0
Revaluation of undeveloped reserves - Oil	0	(68)	0
Conversion to proved developed reserves - Oil	0	0	0