Estimated quantities of proved reserves (Details 5)	12 Months Ended
Dec. 31, 2015 MBoe MMcf MBbls	Dec. 31, 2014 MBoe MMcf MBbls	Dec. 31, 2013 MBoe MMcf MBbls
Proved developed and undeveloped reserves:
Balance at beginning of year, BOE \| MBoe	91,940	80,695	80,486
Production, BOE \| MBoe	(6,471)	(8,998)	(10,264)
Extensions and discoveries, BOE \| MBoe	888	24,025	19,050
Improved recovery, BOE \| MBoe	0	0	0
Purchases of proved reserves, BOE \| MBoe	0	6,748	0
Sales revisions, BOE \| MBoe	(57,097)	(14,796)	(7,987)
Revisions of previous estimate, BOE \| MBoe	(15,939)	4,266	(590)
Balance at end of year, BOE \| MBoe	13,321	91,940	80,695
Oil (MBbls)
Proved developed and undeveloped reserves:
Balance at beginning of year	43,918	41,019	33,453
Production	(3,286)	(4,919)	(4,815)
Extensions and discoveries	744	9,654	13,313
Improved recovery	0	0	0
Purchases of proved reserves	0	5,463	0
Sales of proved reserves	(16,474)	(4,945)	(1,286)
Revisions of previous estimates	(12,215)	(2,354)	354
Balance at end of year	12,687	43,918	41,019
NGL (MBbls)
Proved developed and undeveloped reserves:
Balance at beginning of year	7,187	6,602	7,153
Production	(393)	(609)	(781)
Extensions and discoveries	29	3,634	1,333
Improved recovery	0	0	0
Purchases of proved reserves	0	0	0
Sales of proved reserves	(6,864)	(3,109)	(25)
Revisions of previous estimates	252	669	(1,078)
Balance at end of year	211	7,187	6,602
Natural Gas (MMcf)
Proved developed and undeveloped reserves:
Balance at beginning of year \| MMcf	245,011	198,445	239,278
Production \| MMcf	(16,747)	(20,822)	(28,008)
Extensions and discoveries \| MMcf	681	64,420	26,428
Improved recovery \| MMcf	0	0	0
Purchases of proved reserves \| MMcf	0	7,711	0
Sales of proved reserves \| MMcf	(202,560)	(40,451)	(40,055)
Revisions of previous estimates \| MMcf	(23,854)	35,708	802
Balance at end of year \| MMcf	2,531	245,011	198,445

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Energy measure of increase in quantity of proved reserve of oil and gas resulting from the extension of the proved acreage of previously discovered reservoir or discovery of new field with proved reserve or of new reservoir of proved reserve in old field, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Energy measure of increase in quantity of proved reserve of oil and gas resulting from application of improved recovery techniques, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Energy measure of proved reserve of oil and gas, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Energy measure of decrease in quantity of proved reserve of oil and gas resulting from production, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Energy measure of increase in quantity of proved reserve of oil and gas resulting from purchase of mineral in place, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Energy measure of increase (decrease) in quantity of proved reserve of oil and gas related to revision of a previous estimate resulting from new information, except for an increase in proved acreage, or change in economic factors, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Additions to proved reserves that result from (1) extension of the proved acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved reserves or of new reservoirs of proved reserves in old fields.

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Changes in proved reserve estimates resulting from application of improved recovery techniques.

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The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

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Production of proved reserves.

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Energy measure of decrease in quantity of proved reserve of oil and gas resulting from sale of mineral in place, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

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Sales of minerals in place.

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