Supplemental Gas and Oil Information (Unaudited) Costs incurred (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Abstract]
Accrued capital costs included in development costs	$ 10.2
Costs Incurred, Asset Retirement Obligation Incurred	51.1
Costs Incurred To Advance The Development Of Proved Undeveloped Reserves	796.7	645.9	513.0
Unproved	496.3	9.3	692.6
Proved	465.4	31.6	714.4
Total property acquisitions	961.7	40.9	1,407.0
Exploration	23.6us-gaap_CostsIncurredExplorationCosts	14.6us-gaap_CostsIncurredExplorationCosts	14.3us-gaap_CostsIncurredExplorationCosts
Development	1,695.1us-gaap_CostsIncurredDevelopmentCosts	1,440.8us-gaap_CostsIncurredDevelopmentCosts	1,310.0us-gaap_CostsIncurredDevelopmentCosts
Total costs incurred	$ 2,680.4	$ 1,496.3	$ 2,731.3

Represents capital costs accrued for the development of proved reserves.

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- Definition

The costs incurred during the period to advance the development of reserves that were classified as proved undeveloped.

+ References

No definition available.

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- Definition

Cost incurred, including capitalized costs and costs charged to expense, in acquisition of oil and gas properties.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

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- Definition

Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have proved reserves.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have unproved reserves.

+ References

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Amount of asset retirement obligations (ARO) costs incurred (whether capitalized or charged to expense) during the period.

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

-Publisher FASB

-Name Accounting Standards Codification

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Development costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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Exploration costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Exploration

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Amount of costs incurred related to oil and gas property acquisition, exploration and development activities, including capitalized costs and costs charged to expense.

+ References

No definition available.

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