Carrying amount as of the balance sheet date of coal, which is a combustible rock or minerals which are extracted from the ground and used as fuel.

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Carrying amount, net of valuation reserves and adjustments, as of the balance sheet date for elements of machinery or equipment held for the purpose of replacing similar parts in the course of repair or maintenance.

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Amount before valuation and LIFO reserves of merchandise or goods in the production process expected to be completed within one year or operating cycle, if longer.

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Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs.

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Inventories (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Inventory Disclosure [Abstract]
Coal	$ 71,901us-gaap_EnergyRelatedInventoryCoal	$ 117,531us-gaap_EnergyRelatedInventoryCoal
Repair parts and supplies	118,352	137,497
Work-in-process	0us-gaap_InventoryWorkInProcess	9,133us-gaap_InventoryWorkInProcess
Inventories	190,253us-gaap_InventoryNet	264,161us-gaap_InventoryNet
Inventories
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for slow-moving and obsolete inventories	6,625
Valuation allowances and reserves		$ 8,446	$ 12,589	$ 13,107