Supplemental Gas and Oil Information (Unaudited) Capitalized costs (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Capitalized Costs, Oil and Gas Producing Activities, Net [Abstract]
Proved properties	$ 14,232.5	$ 13,314.9
Unproved properties, net	871.5	691.0
Total proved and unproved properties	15,104.0	14,005.9
Accumulated depreciation, depletion and amortization	(8,797.7)	(6,870.2)
Net capitalized costs	$ 6,306.3	$ 7,135.7

Amount of accumulated depreciation, depletion, amortization and valuation allowance relating to oil and gas producing activities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61901-109447

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The sum of capitalized costs relating to oil and gas producing activities before accounting for accumulated depreciation.

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Net amount of capitalized costs relating to oil and gas producing activities.

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No definition available.

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Capitalized costs of proved properties incurred for any combination mineral interests acquisitions; wells and related equipment; support equipment and facilities; and uncompleted wells and equipment and other costs not previously disclosed within this table.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Properties
-URI http://asc.fasb.org/extlink&oid=21914647

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 360
-Glossary Proved Properties
-URI http://asc.fasb.org/extlink&oid=6522428

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61901-109447

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Capitalized costs of unproved properties incurred for any combination mineral interests acquisitions and other costs not previously disclosed within this table.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Properties
-URI http://asc.fasb.org/extlink&oid=21914647

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e61901-109447

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-Glossary Unproved Properties
-URI http://asc.fasb.org/extlink&oid=6527831

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