Inventories (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Inventories
Materials and supplies	$ 77,868	$ 77,736
Less: Obsolescence allowance	(1,126)us-gaap_InventoryValuationReserves	(1,102)us-gaap_InventoryValuationReserves
Material and supplies, net	76,742	76,634
Coal inventory	1,232us-gaap_EnergyRelatedInventoryCoal	3,168us-gaap_EnergyRelatedInventoryCoal
Inventories, net	$ 77,975us-gaap_InventoryNet	$ 79,802us-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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Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

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

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Aggregated amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed. This amount is net of valuation reserves and adjustments.

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Amount of valuation reserve for inventory.

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