Estimated quantities of proved reserves (Details 3)	12 Months Ended
	Dec. 31, 2016 MBoe MMcf MBbls	Dec. 31, 2015 MBoe MMcf MBbls	Dec. 31, 2014 MBoe MMcf MBbls
Proved developed and undeveloped reserves:
Balance at beginning of year, BOE | MBoe	13,321	91,940	80,695
Production, BOE | MBoe	0	(6,471)	(8,998)
Extensions and discoveries, BOE | MBoe	0	888	24,025
Improved recovery, BOE | MBoe	0	0	0
Purchases of proved reserves, BOE | MBoe	0	0	6,748
Sales revisions, BOE | MBoe	(13,321)	(57,097)	(14,796)
Revisions of previous estimate, BOE | MBoe	0	(15,939)	4,266
Balance at end of year, BOE | MBoe	0	13,321	91,940
Oil (MBbls)
Proved developed and undeveloped reserves:
Balance at beginning of year	12,687	43,918	41,019
Production	0	(3,286)	(4,919)
Extensions and discoveries	0	744	9,654
Improved recovery	0	0	0
Purchases of proved reserves	0	0	5,463
Sales of proved reserves	(12,687)	(16,474)	(4,945)
Revisions of previous estimates	0	(12,215)	(2,354)
Balance at end of year	0	12,687	43,918
NGL (MBbls)
Proved developed and undeveloped reserves:
Balance at beginning of year	211	7,187	6,602
Production	0	(393)	(609)
Extensions and discoveries	0	29	3,634
Improved recovery	0	0	0
Purchases of proved reserves	0	0	0
Sales of proved reserves	(211)	(6,864)	(3,109)
Revisions of previous estimates	0	252	669
Balance at end of year	0	211	7,187
Natural Gas (MMcf)
Proved developed and undeveloped reserves:
Balance at beginning of year | MMcf	2,531	245,011	198,445
Production | MMcf	0	(16,747)	(20,822)
Extensions and discoveries | MMcf	0	681	64,420
Improved recovery | MMcf	0	0	0
Purchases of proved reserves | MMcf	0	0	7,711
Sales of proved reserves | MMcf	(2,531)	(202,560)	(40,451)
Revisions of previous estimates | MMcf	0	(23,854)	35,708
Balance at end of year | MMcf	0	2,531	245,011