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Investment in ATC - FERC Complaint (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Loss Contingencies [Line Items] |
Equity Method Investment, Proposed Return on Common Equity | 9.15%ale_EquityMethodInvestmentProposedReturnonCommonEquity
Equity Method Investment, Approved Return on Common Equity | 12.20%ale_EquityMethodInvestmentApprovedReturnOnCommonEquity
Equity Method Investment, Ownership Percentage | 8.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
Sensitivity Analysis [Member] |
Sensitivity Analysis [Line Items] |
Basis Point Reduction on Approved Rate of Return on Common Equity | 50 ale_BasisPointReductiononApprovedRateofReturnonCommonEquity / ale_SensitivityAnalysisAxis = ale_SensitivityAnalysisMember
Effect on Future Earnings, Amount | $ 0.5 us-gaap_EffectOnFutureEarningsAmount / ale_SensitivityAnalysisAxis = ale_SensitivityAnalysisMember
ATC [Member] |
Loss Contingencies [Line Items] |
Loss Contingency, Estimate of Possible Loss | 18 us-gaap_LossContingencyEstimateOfPossibleLoss / dei_LegalEntityAxis = ale_ATCMember
Rainy River Energy [Member] |
Loss Contingencies [Line Items] |
Loss Contingency, Estimate of Possible Loss | $ 1 us-gaap_LossContingencyEstimateOfPossibleLoss / dei_LegalEntityAxis = ale_RainyRiverEnergyMember

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- Definition The approved percentage return on common equity dedicated to the utility plant in the investee accounted for under the equity method of accounting. + References No definition available. + Details
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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 + Details
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- Definition Reflects the estimated amount of loss from the specified contingency as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349 + Details
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