Reconciliation of Segment Information, Total Assets (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets:
Assets	$ 11,759,530,000us-gaap_Assets	$ 11,393,667,000us-gaap_Assets	$ 12,997,594,000us-gaap_Assets
Items excluded from segment assets:
Income Taxes Receivable, Current	20,401,000us-gaap_IncomeTaxesReceivable	10,705,000us-gaap_IncomeTaxesReceivable
Deferred Tax Assets	924,618,000us-gaap_DeferredTaxAssetsNet	1,025,808,000us-gaap_DeferredTaxAssetsNet
Total Property, Plant and Equipment-Net	10,162,472,000us-gaap_PropertyPlantAndEquipmentNet	9,442,262,000us-gaap_PropertyPlantAndEquipmentNet
Total Reportable Business Segments
Assets:
Assets	11,471,946,000	10,682,752,000
Coal Segment [Member]
Assets:
Assets	4,107,761,000	4,348,284,000	4,333,486,000
Gas
Assets:
Assets	7,364,185,000	6,334,468,000	5,768,882,000
Other Segments [Member]
Assets:
Assets	15,089,000	132,487,000	135,170,000
Excludes Gas Segment [Member]
Items excluded from segment assets:
Cash and Other Investments	147,210,000	321,992,000
Intersegment Eliminations [Member]
Assets:
Assets	272,495,000	578,428,000	2,760,056,000
Items excluded from segment assets:
Income Taxes Receivable, Current	20,401,000	10,705,000
Deferred Tax Assets	66,569,000	211,303,000
Bond Issuance Costs	38,315,000	34,428,000
UNITED STATES
Segment Reporting, Asset Reconciling Item [Line Items]
Disclosure of Long-Live Assets by Geographic Region	10,151,448,000	9,431,238,000
CANADA
Segment Reporting, Asset Reconciling Item [Line Items]
Disclosure of Long-Live Assets by Geographic Region	$ 11,024,000	$ 11,024,000

Disclosure of Long-Live Assets by Geographic Region

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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