Acquisitions (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2011	Dec. 28, 2011
Allocation of the recorded value of net assets acquired in the transaction
Proved oil and gas properties		$ 208.7
Unevaluated oil and gas properties		17.3
Asset retirement obligations		(58.4)
Total fair value of net assets		167.6
Pro forma of financial information
Revenues	241.1
Income from operations	91.4
Net income	55.9
Basic earnings per share	$ 1.14
Diluted earnings per share	$ 1.14
Acquisitions and Divestitures (Textual) [Abstract]
Cash consideration transferred		$ 167.6
BP Exploration and Production Inc [Member]
Acquisitions and Divestitures (Textual) [Abstract]
Percentage of operated working capital acquired		75.00%
Adjacent Mississippi Canyon Block 29 [Member]
Acquisitions and Divestitures (Textual) [Abstract]
Percentage of operated working capital acquired		51.00%
Mica Field [Member]
Acquisitions and Divestitures (Textual) [Abstract]
Percentage of non-operated working capital acquired		50.00%
Pompano Field [Member]
Acquisitions and Divestitures (Textual) [Abstract]
Percentage of non-operated working capital acquired		75.00%

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Business acquisition purchase price allocation proved oil and gas properties.

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Business acquisition purchase price allocation unevaluated oil and gas properties.

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Percentage of non operated working capital acquired.

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Percentage of operated working capital acquired.

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Amount of cash paid to acquire the entity.

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The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

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The total purchase price of the acquired entity. This includes cash paid to equity interest holders of the acquired entity, fair value of debt and equity securities issued to equity holders of the acquired entity, and transaction costs paid to third parties to consummate the acquisition.

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The pro forma income from continuing operations before changes in accounting and extraordinary items, net of tax effect, for the period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

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