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Regulatory Matters - Utility Rates (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended |
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Jun. 01, 2011 | Jan. 01, 2013 | Dec. 31, 2014
Hydro Project [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Estimated Capital Expenditures |  |  | $ 90 ale_EstimatedCapitalExpendituresIncludingPastExpenditures / ale_HydroOperationsAxis = ale_HydroProjectMember
Bison Wind Energy Center [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Generating Capacity (MW) |  |  | 497.0 ale_GeneratingCapacityMw / ale_NorthDakotaWindDevelopmentAxis = ale_BisonWindEnergyCenterMember
Bison 4 Wind Project [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Generating Capacity (MW) |  |  | 205.0 ale_GeneratingCapacityMw / ale_NorthDakotaWindDevelopmentAxis = ale_Bison4Member
Capital Cost Spent to Date |  |  | $ 333 ale_CapitalCostSpentToDate / ale_NorthDakotaWindDevelopmentAxis = ale_Bison4Member
Municipal Customers [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Number of Customers |  |  | 16 ale_NumberOfCustomers / ale_CustomerClassAxis = ale_MunicipalCustomersMember
MPUC 2010 MN Rate Case [Member] | Electric Rates [Member] | Retail Customers [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Approved Rate of Return on Common Equity | 10.38%us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage / ale_CustomerClassAxis = ale_RetailCustomersMember / ale_RateTypeAxis = ale_ElectricRatesMember / us-gaap_RegulatoryAgencyAxis = ale_A2010RateCaseMember |  |
Approved Percentage of Capital Structure Related to Equity | 54.29%us-gaap_PublicUtilitiesApprovedEquityCapitalStructurePercentage / ale_CustomerClassAxis = ale_RetailCustomersMember / ale_RateTypeAxis = ale_ElectricRatesMember / us-gaap_RegulatoryAgencyAxis = ale_A2010RateCaseMember |  |
FERC-Approved Wholesale Rates [Member] | Electric Rates [Member] | Municipal Customers [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Number of Customers |  |  | 16 ale_NumberOfCustomers / ale_CustomerClassAxis = ale_MunicipalCustomersMember / ale_RateTypeAxis = ale_ElectricRatesMember / us-gaap_RegulatoryAgencyAxis = ale_FercApprovedWholesaleRatesMember
Length of Notice Required to Terminate (Years) |  |  | 3 ale_LengthOfNoticeRequiredToTerminateYears / ale_CustomerClassAxis = ale_MunicipalCustomersMember / ale_RateTypeAxis = ale_ElectricRatesMember / us-gaap_RegulatoryAgencyAxis = ale_FercApprovedWholesaleRatesMember
PSCW 2012 Wisconsin Rate Case [Member] | Retail Customers [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Approved Rate of Return on Common Equity |  | 10.90%us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage / ale_CustomerClassAxis = ale_RetailCustomersMember / us-gaap_RegulatoryAgencyAxis = ale_A2012WisconsinRateCaseMember |

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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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