Acquisitions and Divestitures (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Impairment of Oil and Gas Properties	$ 1,143.2us-gaap_ImpairmentOfOilAndGasProperties	$ 33.5us-gaap_ImpairmentOfOilAndGasProperties
Permian Basin Acquisition [Member]
Business Acquisition [Line Items]
Aggregate purchase price	941.8
Net acres of producing and undeveloped oil and gas properties	26,500
Vertical producing oil and gas wells	270
Escrow Deposit	50.0
Borrowing from Company's expanded term loan	300.0
Revenue of acquired properties since acquisition date	159.5
Net income of acquired properties since acquisition date	(438.3)
Impairment of Oil and Gas Properties	467.7
Transaction costs	0.6
Debt issuance cost	1.1
Williston Basin Acquisition [Member]
Business Acquisition [Line Items]
Aggregate purchase price		1,392.7
Revenue of acquired properties since acquisition date	767.3	300.0	63.7
Net income of acquired properties since acquisition date	402.1	67.0	14.9
Transaction costs			$ 1.1

Business Acquisition Transaction Costs

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- Definition

Vertical Producing Oil And Gas Wells

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- Definition

Amount of liabilities incurred by the acquirer as part of consideration transferred in a business combination.

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-Name Accounting Standards Codification

-Topic 805

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This element represents the amount of earnings or loss of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.

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This element represents the amount of revenue of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.

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- Definition

Amount of debt issuance costs (for example, but not limited to, legal, accounting, broker, and regulatory fees).

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The designation of funds furnished by a borrower to a lender to assure future payments of the borrower's real estate taxes and insurance obligations with respect to a mortgaged property. Escrow deposits may be made for a variety of other purposes such as earnest money and contingent payments. This element excludes replacement reserves which are an escrow separately provided for within the US GAAP taxonomy.

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Fractional interest owned based on working interests or other economic arrangements of area spaced or assigned to productive 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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The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

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