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Business acquisition purchase price allocation asset retirement obligations.

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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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Percentage of working interest acquired.

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- Definition

Amount of cash paid to acquire the entity.

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

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-Name Statement of Financial Accounting Standard (FAS)

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

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-Name Accounting Standards Codification

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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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Acquisitions (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011	Jun. 18, 2012	Jun. 18, 2012 Mississippi Canyon Block 72 [Member]	Dec. 28, 2011 BP Exploration and Production Inc [Member]	Dec. 28, 2011 Adjacent Mississippi Canyon Block 29 [Member]	Dec. 28, 2011 Mica Field [Member]	Dec. 28, 2011 Pompano Field [Member]	Jun. 18, 2012 Block Deep Water Pompano Field Mississippi Canyon [Member]	Jun. 18, 2012 Mississippi Canyon Block 29 [Member]
Allocation of the recorded value of net assets acquired in the transaction
Proved oil and gas properties			$ 39.2
Unevaluated oil and gas properties			1.6
Asset retirement obligations			(14.4)
Total fair value of net assets			26.4
Pro forma of financial information
Revenues	279.4	515.0
Income from operations	117.2	204.9
Net income	72.1	125.5
Basic earnings per share	$ 1.47	$ 2.56
Diluted earnings per share	$ 1.47	$ 2.56
Acquisitions and Divestitures (Textual) [Abstract]
Percentage of working interest acquired				10.00%					25.00%	14.00%
Percentage of operated working capital acquired					75.00%	51.00%
Percentage of non-operated working capital acquired							50.00%	75.00%
Cash consideration transferred			$ 26.4