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
Property Acquisition [Abstract]
Unproved	$ 11.9	$ 0.3	$ 0.6
Proved	54.1	106.4	0
Exploration (capitalized and expensed)	1.6us-gaap_CostsIncurredExplorationCosts	0us-gaap_CostsIncurredExplorationCosts	0us-gaap_CostsIncurredExplorationCosts
Development	49.1us-gaap_CostsIncurredDevelopmentCosts	133.1us-gaap_CostsIncurredDevelopmentCosts	146.9us-gaap_CostsIncurredDevelopmentCosts
Total costs incurred	116.7	239.8	147.5
Cost incurred to develop proved undeveloped reserves	$ 28.9	$ 106.3	$ 50.4

Development costs incurred in oil and gas activities of proved undeveloped reserves.

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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

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 19

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62038-109447

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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 unproved reserves.

+ References

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- Definition

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

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 18

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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

-URI http://asc.fasb.org/extlink&oid=6512329

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

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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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