Estimated quanities of proved reserves (Details 5)	12 Months Ended
	Dec. 31, 2012 MBoe	Dec. 31, 2011 MBoe	Dec. 31, 2010 MBoe
Proved developed and undeveloped reserves:
Balance at beginning of year, BOE	97,651	107,599	108,954
Production, BOE	(10,058)	(11,099)	(11,661)
Extensions and discoveries, BOE	14,756	10,842	9,421
Improved recovery, BOE	0	0	0
Purchases of proved reserves, BOE	0	247	10,165
Sales of proved reserves, BOE	(423)	0	(34)
Revisions of previous estimates, BOE	(21,440)	(9,938)	(9,246)
Balance at end of year, BOE	80,486	97,651	107,599
Reduction in proved undeveolped reserves specified properties		8,900
Oil (MBbls)
Proved developed and undeveloped reserves:
Balance at beginning of year	27,005	25,666	25,930
Production	(3,694)	(2,724)	(2,767)
Extensions and discoveries	9,874	4,717	2,793
Improved recovery	0	0	0
Purchases of proved reserves	0	223	911
Sales of proved reserves	(39)	0	(18)
Revisions of previous estimates	307	(877)	(1,183)
Balance at end of year	33,453	27,005	25,666
NGL (MBbls)
Proved developed and undeveloped reserves:
Balance at beginning of year	7,342	7,201	8,286
Production	(828)	(776)	(495)
Extensions and discoveries	1,817	1,421	596
Improved recovery	0	0	0
Purchases of proved reserves	0	16	68
Sales of proved reserves	0	0	0
Revisions of previous estimates	(1,178)	(520)	(1,254)
Balance at end of year	7,153	7,342	7,201
Natural Gas (MMcf)
Proved developed and undeveloped reserves:
Balance at beginning of year	379,827	448,397	448,425
Production	(33,214)	(45,598)	(50,391)
Extensions and discoveries	18,386	28,221	36,191
Improved recovery	0	0	0
Purchases of proved reserves	0	54	55,119
Sales of proved reserves	(2,307)	0	(92)
Revisions of previous estimates	(123,414)	(51,247)	(40,855)
Balance at end of year	239,278	379,827	448,397