Property, Plant and Equipment Assets Amortized by Units of Production (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Unproved Oil and Gas Property, Successful Effort Method	$ 1,258,027	$ 2,206,399
Coal Properties And Surface Lands	1,340,757	1,292,701
Leased coal lands	540,817	536,603
Mine development	457,179	587,518
Coal advance mining royalties	393,340	389,379
Airshafts	659,736	662,315
Gas advance royalties	4,065	3,078
Property, Plant and Equipment, Gross	14,087,319	14,951,358
Assets amortized by units of production [Member]
Property, Plant and Equipment [Line Items]
Unproved Oil and Gas Property, Successful Effort Method	1,258,027	2,206,399
Coal Properties And Surface Lands	386,402	394,635
Leased coal lands	178,988	171,056
Mine development	78,990	34,907
Coal advance mining royalties	54,533	67,674
Airshafts	47,437	73,703
Gas advance royalties	3,884	2,800
Property, Plant and Equipment, Gross	$ 2,008,261	$ 2,951,174

Carrying amount as of the balance sheet date of amounts paid for royalties in advance of when the expense is to be recognized, which will occur within one year or in the normal operating cycle, if longer.

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Gross amount, at the balance sheet date, of long-lived physical assets used in the normal conduct of business and not intended for resale. This can include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software.

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- Definition

The gross amount of lessor property subject to or available for lease as of the balance sheet date.

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- Definition

Oil and Gas properties with unproved reserves under the successful effort method.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Unproved Properties

-URI http://asc.fasb.org/extlink&oid=6527831

-Topic 932

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 59N

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 11

-Subparagraph a

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 19

-Paragraph 147, 148

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 14

-URI http://asc.fasb.org/extlink&oid=6821537&loc=d3e61926-109447

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 360

-Section 25

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6474861&loc=d3e64746-109465

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