Property, Plant, Equipment and Mine Development (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Property, Plant, Equipment and Mine Development, Net [Line Items]
Land and coal interests	$ 11,029.8btu_LandAndCoalInterests	$ 11,021.1btu_LandAndCoalInterests
Buildings and improvements	1,599.3us-gaap_BuildingsAndImprovementsGross	1,569.1us-gaap_BuildingsAndImprovementsGross
Machinery and equipment	2,662.2us-gaap_MachineryAndEquipmentGross	2,685.7us-gaap_MachineryAndEquipmentGross
Less: Accumulated depreciation, depletion and amortization	(4,839.5)	(4,698.6)
Total, net	$ 10,451.8us-gaap_PropertyPlantAndEquipmentNet	$ 10,577.3us-gaap_PropertyPlantAndEquipmentNet

Land and coal interests.

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- Definition

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Amount before accumulated depreciation of building structures held for productive use including addition, improvement, or renovation to the structure, including, but not limited to, interior masonry, interior flooring, electrical, and plumbing.

+ References

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

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- Definition

Amount before accumulated depreciation of tangible personal property used to produce goods and services, including, but is not limited to, tools, dies and molds, computer and office equipment.

+ References

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

-Publisher FASB

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- Definition

Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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-Topic 210

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