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Coal and Other Mineral Rights (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Coal and other mineral rights |  |  |
Coal and other mineral rights | $ 1,645,451,000 | $ 1,629,286,000 |
Less accumulated depletion and amortization | (387,950,000) | (347,650,000) |
Net book value | 1,257,501,000 | 1,281,636,000 |
Total depletion and amortization expense on coal and other mineral interests | 47,230,000 | 38,501,000 | 48,591,000
Coal and Other Mineral Rights (Textuals) [Abstract] |  |  |
Terminated lease charges from mine closure |  |  | $ 8,200,000

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- Definition Carrying amount at the balance sheet date of mineral properties, gross of adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 15 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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. + Details
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- Definition Mineral properties, net of adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 19 -Paragraph 15 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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. + Details
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