Guarantees (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011
Magnum [Member]
Guarantees and Commitments
Guarantee obligations, maximum exposure, undiscounted	$ 15.3
Excess of the current market cost of purchasing coal over the selling price	199.4
Volume of purchased coal in tons	9,800,000
Performance Guarantee [Member] \| Magnum [Member]
Guarantees and Commitments
Volume of purchased coal in tons	700,000
Bonding Requirements [Member] \| Magnum [Member]
Guarantees and Commitments
Surety bonds posted in the company's favor	48.9
Letters of credit posted in the Company's favor	16.1
Guarantee obligations, maximum exposure, undiscounted	38.5
Indemnification Agreement [Member]
Guarantees and Commitments
Guarantee obligations, maximum exposure, undiscounted	19.3
Coal, materials and supplies and capital commitments, other than reserve acquisitions, and transportation capacity agreements [Member]
Guarantees and Commitments
Unrecorded Unconditional Purchase Obligation Due Current	487.5
Unrecorded Unconditional Purchase Obligation Due in Two Years	139.2
Unrecorded Unconditional Purchase Obligation Due in Three Years	175.0
Unrecorded Unconditional Purchase Obligation Due in Four Years	163.9
Unrecorded Unconditional Purchase Obligation Due in Five Years	128.3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	$ 154.8

Letters of credit posted in the Company's favor

No definition available.

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- Definition

Purchased Tons Needed To Fulfill Commitment

+ References

No definition available.

+ Details

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- Definition

Surety Bonds posted in the Company's favor

+ References

No definition available.

+ Details

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- Details

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- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

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

-Publisher FASB

-Name FASB Interpretation (FIN)

-Number 45

-Paragraph 13

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 460

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

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- Definition

The floor amount as of the balance sheet date that the entity must expend to satisfy the terms of disclosed arrangements (excluding long-term commitments) in which the entity must commit resources to supply goods or services to one or more customers.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.25)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.17)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 25

-Article 5

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.(a)(19))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 17

-Article 9

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 19

+ Details

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- Definition

The remaining amount of the fixed and determinable portion of an unrecorded unconditional purchase obligation to be paid in more than four and within five years from the balance sheet date.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 47

-Paragraph 7

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 440

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6394976&loc=d3e25336-109308

+ Details

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- Definition

The remaining amount of the fixed and determinable portion of an unrecorded unconditional purchase obligation to be paid within one year of the balance sheet date.

+ References

+ Details

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- Definition

The remaining amount of the fixed and determinable portion of an unrecorded unconditional purchase obligation to be paid in more than three and within four years from the balance sheet date.

+ References

+ Details

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- Definition

The remaining amount of the fixed and determinable portion of an unrecorded unconditional purchase obligation to be paid in more than one and within two years from the balance sheet date.

+ References

+ Details

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- Definition

The remaining amount of the fixed and determinable portion of an unrecorded unconditional purchase obligation to be paid in more than two and within three years from the balance sheet date.

+ References

+ Details

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- Definition

The remaining amount of the fixed and determinable portion of an unrecorded unconditional purchase obligation to be paid after five years from the balance sheet date.

+ References

No definition available.

+ Details

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