Acquisitions & Divestitures Acquisitions (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Business Acquisition [Line Items]
Bargain purchase gain from acquisition	$ 0.0	$ 0.4
Payments to acquire oil and gas property	36.2	68.2
Goodwill		$ 5.3
2017 Permian Basin Acquisition [Member]
Business Acquisition [Line Items]
Payments to acquire oil and gas property	$ 35.7

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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- Definition

In a business combination in which the amount of net identifiable assets acquired and liabilities assumed exceeds the aggregate consideration transferred or to be transferred (as defined), this element represents the amount of gain recognized by the entity.

+ References

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- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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- Definition

The cash outflow to purchase of mineral interests in oil and gas properties for use in the normal oil and gas operations and not intended for resale.

+ References

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