Inventories (Inventories) (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Inventory Disclosure [Abstract]
Coal	$ 182,998,000	$ 180,917,000
Repair parts and supplies	170,040,000	172,139,000
Work-in-process	15,202,000	12,368,000
Inventories	368,240,000	365,424,000
Allowance for slow-moving and obsolete inventories	$ 14,000,000	$ 13,600,000

The amount of the valuation account as of the balance sheet date which reduces the carrying amount of inventory to net realizable value; takes into consideration such factors as market value, excessive quantities based on expected sales, technological obsolescence, and shrinkage. May also provide for estimated product returns or price concessions pertaining to product cost.

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

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

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Reference 7: http://www.xbrl.org/2003/role/presentationRef

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+ Details