Oil and natural gas properties (Details 6) (USD $)	3 Months Ended	12 Months Ended
Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Natural gas and oil properties [Abstract]
Discount rate used in calculating present value of future net cash flows from proved reserves (in hundredths)	10.00%		10.00%
NYMEX oil price	94.71	94.97	94.71
Henry Hub natural gas price	2.76	2.83	2.76
Write-downs of oil and natural gas properties	$ 231,700,000	$ 160,100,000	$ 391,800,000	$ 0	$ 0
Noncash write-downs of oil and natural gas properties, after tax	145,900,000	100,900,000	246,800,000
Additional write-downs of its oil and natural gas properties that would have been recognized without effects of cash flow hedges	20,800,000	19,500,000	20,800,000
Additional write-downs of its oil and natural gas properties that would have been recognized without effects of cash flow hedges, after tax	13,100,000	12,300,000	13,100,000
Oil and natural gas properties not subject to amortization [Abstract]
Acquisition	144,521,000		144,521,000
Development	7,415,000		7,415,000
Exploration	36,246,000		36,246,000
Capitalized interest	3,612,000		3,612,000
Total costs not subject to amortization	191,794,000		191,794,000	232,462,000	182,402,000
Acquisition costs, period cost			26,318,000	38,186,000	22,142,000	57,875,000
Development costs, period cost			6,858,000	399,000	77,000	81,000
Exploration costs, period cost			34,407,000	856,000	643,000	340,000
Capitalized interest costs, period costs			1,297,000	757,000	439,000	1,119,000
Capitalized costs of unproved properties excluded from amortization, period cost			$ 68,880,000	$ 40,198,000	$ 23,301,000	$ 59,415,000
Time period for evaluating the majority of costs not subject to amortization (in years)			five years

Capitalized interest costs, period costs.

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Cumulative interest costs incurred that are capitalized as of the balance-sheet date.

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Discount rate used in calculating the present value of future net cash flows from proved reserves as mandated under the rules of the SEC.

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The dollar amount that would have been charged to income after tax had the effects of the cash flow hedges not been considered in calculating the ceiling limitation at an oil and gas producing company subject to the full cost rules.

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Henry Hub natural gas price used to calculate the ceiling test.

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The after tax expense recorded to reduce the value of oil and gas assets consisting of proved properties and unproved properties as the estimate of future successful production from these properties is reduced.

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NYMEX oil price used to calculate the ceiling test

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Time period for evaluating the majority of costs not subject to amortization and including the costs in the amortization base as the properties are evaluated and/or developed.

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The capitalized costs incurred during the period (excluded from amortization) to purchase, lease or otherwise acquire an unproved property, including costs of lease bonuses and options to purchase or lease properties, the portion of costs applicable to minerals when land including mineral rights is purchased in fee, brokers' fees, recording fees, legal costs, and other costs incurred in acquiring properties.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-10.(c)(7)(ii))

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 10

-Paragraph c

-Subparagraph 3(ii)(A)

-Article 4

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The capitalized costs incurred (excluded from amortization), as of the date of the balance sheet, to purchase, lease or otherwise acquire an unproved property, including costs of lease bonuses and options to purchase or lease properties, the portion of costs applicable to minerals when land including mineral rights is purchased in fee, brokers' fees, recording fees, legal costs, and other costs incurred in acquiring properties.

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The sum of the capitalized costs incurred during the period of unproved properties excluded from amortization including acquisition costs, exploration costs, development costs, and production costs.

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The sum of the capitalized costs incurred, as of the balance sheet date, of unproved properties excluded from amortization including acquisition costs, exploration costs, development costs, and production costs.

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The capitalized costs incurred during the period (excluded from amortization) to obtain access to proved reserves and to provide facilities for extracting, treating, gathering and storing the oil and gas.

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The capitalized costs incurred (excluded from amortization), as of the date of the balance sheet, to obtain access to proved reserves and to provide facilities for extracting, treating, gathering and storing the oil and gas.

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Amount that would have been charged to income had the effects of the cash flow hedges not been considered in calculating the ceiling limitation at an oil and gas producing company subject to the full cost rules.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 360

-Section S99

-Paragraph 2

-Subparagraph (SAB TOPIC 12.D.3(b))

-URI http://asc.fasb.org/extlink&oid=6601679&loc=d3e516453-122869

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-Name Staff Accounting Bulletin (SAB)

-Number Topic 12

-Section D

-Paragraph b

-Subsection 3

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The capitalized costs incurred during the period (excluded from amortization) in identifying areas that may warrant examination and in examining specific areas that are considered to have prospects of containing oil and gas reserves, including costs of drilling exploratory wells and exploratory-type stratigraphic test wells. Exploration costs may be incurred both before acquiring the related property (sometimes referred to in part as prospecting costs) and after acquiring the property. Principal types of exploration costs, which include depreciation and applicable operating costs of support equipment and facilities and other costs of exploration activities, are: costs of topographical, geographical and geophysical studies, rights of access to properties to conduct those studies, and salaries and other expenses of geologists, geophysical crews, and others conducting those studies. Collectively, these are sometimes referred to as geological and geophysical or "G&G" costs. Exploration costs also include costs of carrying and retaining undeveloped properties, such as delay rentals, ad valorem taxes on properties, legal costs for title defense, the maintenance of land and lease records, dry hole contributions and bottom hole contributions, costs of drilling and equipping exploratory wells and costs of drilling exploratory-type stratigraphic test wells.

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The capitalized costs incurred (excluded from amortization), as of the date of the balance sheet, in identifying areas that may warrant examination and in examining specific areas that are considered to have prospects of containing oil and gas reserves, including costs of drilling exploratory wells and exploratory-type stratigraphic test wells. Exploration costs may be incurred both before acquiring the related property (sometimes referred to in part as prospecting costs) and after acquiring the property. Principal types of exploration costs, which include depreciation and applicable operating costs of support equipment and facilities and other costs of exploration activities, are: costs of topographical, geographical and geophysical studies, rights of access to properties to conduct those studies, and salaries and other expenses of geologists, geophysical crews, and others conducting those studies. Collectively, these are sometimes referred to as geological and geophysical or "G&G" costs. Exploration costs also include costs of carrying and retaining undeveloped properties, such as delay rentals, ad valorem taxes on properties, legal costs for title defense, the maintenance of land and lease records, dry hole contributions and bottom hole contributions, costs of drilling and equipping exploratory wells and costs of drilling exploratory-type stratigraphic test wells.

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The expense recorded to reduce the value of oil and gas assets consisting of proved properties and unproved properties as the estimate of future successful production from these properties is reduced.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 27, 28, 29

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

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