Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Coal and other plant and equipment	$ 6,086,734	$ 6,030,620
Drilling Cost	1,721,384	1,550,297
Proven gas properties	1,597,626	1,596,838
Coal properties and surface lands	1,455,541	1,346,151
Unproven gas properties	1,371,532	1,266,017
Gas gathering equipment	1,034,927	1,006,882
Airshafts	727,674	706,388
Mine development	583,494	537,939
Leased coal lands	529,700	529,758
Gas wells and related equipment	542,284	492,367
Coal advance mining royalties	396,034	391,501
Other gas assets	125,194	82,217
Gas advance royalties	22,127	8,229
Total property plant and equipment	16,194,251	15,545,204
Less Accumulated depreciation, depletion and amortization	5,770,506	5,354,237
Total Property, Plant and Equipment-Net	$ 10,423,745	$ 10,190,967

Coal And Other Plant And Equipment

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Coal Properties And Surface Lands

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Drilling Cost

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Gas Gathering Systems, Net

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Gas Wells and Related Equipment

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Mine Development

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Other Gas Assets

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The cumulative amount of depreciation, depletion and amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement.

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Discloses the period end balance sheet amount of advances for gas exploration, development and production.

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Carrying amount as of the balance sheet date of amounts paid for royalties in advance of when the expense is to be recognized, which will occur within one year or in the normal operating cycle, if longer.

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Gross amount of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Amount, net of accumulated depreciation, depletion and amortization, of long-lived physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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This element represents capitalized assets classified as property, plant and equipment not otherwise defined in the taxonomy.

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The gross amount of lessor property subject to or available for lease as of the balance sheet date.

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Oil and Gas properties with proved reserves under the successful effort method.

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Oil and Gas properties with unproved reserves under the successful effort method.

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