Estimated quantities of proved reserves (Details 4)	12 Months Ended
	Dec. 31, 2014 MBoe	Dec. 31, 2013 MBoe	Dec. 31, 2012 MBoe
Proved developed and undeveloped reserves:
Balance at beginning of year, BOE	80,695	80,486	97,651
Production, BOE	(8,998)	(10,264)	(10,058)
Extensions and discoveries, BOE	24,025	19,050	14,756
Improved recovery, BOE	0	0	0
Purchases of proved reserves, BOE	6,748	0	0
Sales revisions, BOE	(14,796)	(7,987)	(423)
Revisions of previous estimate, BOE	4,266	(590)	(21,440)
Balance at end of year, BOE	91,940	80,695	80,486
Oil (MBbls)
Proved developed and undeveloped reserves:
Balance at beginning of year	41,019	33,453	27,005
Production	(4,919)	(4,815)	(3,694)
Extensions and discoveries	9,654	13,313	9,874
Improved recovery	0	0	0
Purchases of proved reserves	5,463	0	0
Sales of proved reserves	(4,945)	(1,286)	(39)
Revisions of previous estimates	(2,354)	354	307
Balance at end of year	43,918	41,019	33,453
NGL (MBbls)
Proved developed and undeveloped reserves:
Balance at beginning of year	6,602	7,153	7,342
Production	(609)	(781)	(828)
Extensions and discoveries	3,634	1,333	1,817
Improved recovery	0	0	0
Purchases of proved reserves	0	0	0
Sales of proved reserves	(3,109)	(25)	0
Revisions of previous estimates	669	(1,078)	(1,178)
Balance at end of year	7,187	6,602	7,153
Natural Gas (MMcf)
Proved developed and undeveloped reserves:
Balance at beginning of year	198,445	239,278	379,827
Production	(20,822)	(28,008)	(33,214)
Extensions and discoveries	64,420	26,428	18,386
Improved recovery	0	0	0
Purchases of proved reserves	7,711	0	0
Sales of proved reserves	(40,451)	(40,055)	(2,307)
Revisions of previous estimates	35,708	802	(123,414)
Balance at end of year	245,011	198,445	239,278