Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	$ 752.5us-gaap_AssetRetirementObligation	$ 712.8us-gaap_AssetRetirementObligation	$ 687.5us-gaap_AssetRetirementObligation
Asset Retirement Obligation, Current	30.2	21.0
Asset Retirement Obligation, Liabilities Incurred	22.7	0
Liabilities settled or disposed	(19.7)	(15.4)
Accretion expense	39.3us-gaap_AccretionExpense	39.3us-gaap_AccretionExpense
Revision to estimates	(2.6)	1.4
Asset retirement obligations	722.3	691.8
Balance at end of year — active locations	676.2	660.8
Balance at end of year — closed or inactive locations	76.3	52.0
Credit adjusted, risk-free interest rates	6.82%btu_CreditAdjustedRiskFreeInterestRates	6.44%btu_CreditAdjustedRiskFreeInterestRates	6.28%btu_CreditAdjustedRiskFreeInterestRates
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities	645.0	568.4
Amount of reclamation self-bonding in certain states in which the company qualifies	1,361.4	1,365.1
Letters of credit in support of reclamation obligations or activities	17.6	17.6
Eastern US Coal Reserves [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Liabilities Incurred	$ 22.2

Asset Retirement Obligation Active Locations.

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- Definition

Asset Retirement Obligation Closed or Inactive Locations.

+ References

No definition available.

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Credit-adjusted, risk-free interest rates.

+ References

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- Definition

Letters of credit in support of reclamation obligations or activities

+ References

No definition available.

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- Definition

Reclassification of self bonding in certain states in which the company qualifies.

+ References

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- Definition

Surety Bonds Outstanding For Reclamation.

+ References

No definition available.

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- Definition

Amount recognized for the passage of time, typically for liabilities, that have been discounted to their net present values. Excludes accretion associated with asset retirement obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 420

-SubTopic 10

-Section 35

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6394232&loc=d3e17558-110866

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- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Asset Retirement Obligation

-URI http://asc.fasb.org/extlink&oid=6505190

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Current portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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- Definition

Amount of asset retirement obligations incurred during the period.

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Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

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- Definition

Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

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Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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