INVENTORIES (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Inventory Disclosure [Abstract]
Coal stockpiles	$ 42,266	$ 41,795
Coal fuel inventories	9,524us-gaap_EnergyRelatedInventoryCoal	6,531us-gaap_EnergyRelatedInventoryCoal
Materials and supplies	84,713	88,584
Reserve for obsolete inventory	(2,684)us-gaap_InventoryValuationReserves	(3,055)us-gaap_InventoryValuationReserves
Total	$ 133,819us-gaap_InventoryNet	$ 133,855us-gaap_InventoryNet

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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- Definition

Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

+ References

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- Definition

Amount before last-in first-out (LIFO) and valuation reserves of ore stockpile material contained on a leach pad, for example, but not limited to, copper or gold ore.

+ References

No definition available.

+ Details

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- Definition

Gross amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed.

+ References

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

-Publisher SEC

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- Definition

Amount of valuation reserve for inventory.

+ References

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

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