Acquisitions & Divestitures Purchase Accounting Entries (Details) - 2016 Permian Basin Acquisition [Member] $ in Millions	6 Months Ended
Jun. 30, 2017 USD ($)
Business Combination, Separately Recognized Transactions [Line Items]
Payments to Acquire Businesses, Gross	$ 591.0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	591.0
Proved Property [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	406.2
Unproved Property [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	214.2
Asset Retirement Obligation Costs [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	(11.6)
Bargain Purchase Gain [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	$ (17.8)

Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 55
-Paragraph 37
-URI http://asc.fasb.org/extlink&oid=73531372&loc=d3e2207-128464

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73530397&loc=d3e4845-128472

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- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Investing Activities
-URI http://asc.fasb.org/extlink&oid=6516133

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3213-108585

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