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Commitments, Guarantees and Contingencies - Environmental Matters (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Jun. 30, 2014 | Dec. 31, 2014 MW
Clean Water Act - Aquatic Organisms [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Environmental Compliance Costs |  | $ 15 ale_EstimatedEnvironmentalComplianceCosts / us-gaap_EnvironmentalRemediationContingencyAxis = ale_CleanWaterActAquaticOrganismsMember
Coal Combustion Residuals [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Environmental Compliance Costs |  | 130 ale_EstimatedEnvironmentalComplianceCosts / us-gaap_EnvironmentalRemediationContingencyAxis = ale_CoalCombustionResidualsMember
Boswell Unit 4 [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Capital Expenditures |  | 250 ale_EstimatedCapitalExpendituresIncludingPastExpenditures / ale_GeneratingFacilityAxisAxis = ale_BoswellUnit4Member
Capital Cost Spent to Date |  | 145 ale_CapitalCostSpentToDate / ale_GeneratingFacilityAxisAxis = ale_BoswellUnit4Member
NOV Consent Decree Obligation [Member] |  |
Environmental Matters [Line Items] |  |
Generating Capacity (MW) |  | 200.0 ale_GeneratingCapacityMw / ale_EnvironmentalConsentDecreeAxis = ale_NOVConsentDecreeObligationMember
Environmental Remediation Expense |  | 4.2 us-gaap_EnvironmentalRemediationExpense / ale_EnvironmentalConsentDecreeAxis = ale_NOVConsentDecreeObligationMember
Civil Penalty | 1.4 ale_CivilPenalty / ale_EnvironmentalConsentDecreeAxis = ale_NOVConsentDecreeObligationMember |
NOV Consent Decree [Member] |  |
Environmental Matters [Line Items] |  |
Generating Capacity (MW) |  | 500.0 ale_GeneratingCapacityMw / ale_EnvironmentalConsentDecreeAxis = ale_NOVConsentDecreeMember
Estimated Capital Expenditures |  | 600 ale_EstimatedCapitalExpendituresIncludingPastExpenditures / ale_EnvironmentalConsentDecreeAxis = ale_NOVConsentDecreeMember
NOV Consent Decree [Member] | Minimum [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Capital Expenditures |  | 20 ale_EstimatedCapitalExpenditures / ale_EnvironmentalConsentDecreeAxis = ale_NOVConsentDecreeMember / us-gaap_RangeAxis = us-gaap_MinimumMember
NOV Consent Decree [Member] | Maximum [Member] |  |
Environmental Matters [Line Items] |  |
Estimated Capital Expenditures |  | $ 40 ale_EstimatedCapitalExpenditures / ale_EnvironmentalConsentDecreeAxis = ale_NOVConsentDecreeMember / us-gaap_RangeAxis = us-gaap_MaximumMember

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- Definition Capital cost spent to date for project. + References No definition available. + Details
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- Definition Civil Penalty + References No definition available. + Details
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- Details
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- Definition The estimated amount of certain future capital expenditures related to environmental compliance, regulatory matters, or other committments and/or contingencies. + References No definition available. + Details
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- Definition The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period. + References No definition available. + Details
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- Definition Estimated Environmental Compliance Costs + References No definition available. + Details
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- Definition The charge against earnings in the period for known or estimated future costs arising from requirements to perform environmental remediation activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13296-110859 + Details
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- Details
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