Supplemental Gas and Oil Information (Unaudited) Capitalized costs (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Capitalized Costs, Oil and Gas Producing Activities, Net [Abstract]
Proved properties	$ 12,278.7	$ 11,571.4
Unproved properties, net	825.2	665.1
Total proved and unproved properties	13,103.9	12,236.5
Accumulated depreciation, depletion and amortization	(6,153.0)	(4,930.9)
Net capitalized costs	$ 6,950.9	$ 7,305.6

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

Name:	us-gaap_CapitalizedCostsAccumulatedDepreciationDepletionAmortizationAndValuationAllowanceForRelatingToOilAndGasProducingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_CapitalizedCostsOilAndGasProducingActivitiesGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

+ Details

Name:	us-gaap_CapitalizedCostsOilAndGasProducingActivitiesNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_CapitalizedCostsOilAndGasProducingActivitiesNetAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61901-109447

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

-Glossary Properties

-URI http://asc.fasb.org/extlink&oid=21914647

+ Details

Name:	us-gaap_CapitalizedCostsProvedProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61901-109447

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Properties

-URI http://asc.fasb.org/extlink&oid=21914647

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

+ Details

Name:	us-gaap_CapitalizedCostsUnprovedProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant