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Regulatory Matters - Utility Rates (Details) | 0 Months Ended |
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Jun. 01, 2011 | Jan. 01, 2013 | Mar. 31, 2015 MW
Bison Wind Energy Center [Member] |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Generating Capacity (MW) |  |  | 497 ale_GeneratingCapacityMw / ale_NorthDakotaWindDevelopmentAxis = ale_BisonWindEnergyCenterMember
MPUC 2010 Minnesota 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 Capacity of a generating facility, unit, or project in megawatts. + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The length of notice required to terminate a contract (in years). + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of Customers + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage of public utility's approved equity to capital, including debt and equity. + References No definition available. + Details
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Data Type: | num:percentItemType
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- Definition Percentage of public utility's approved return on equity. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
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- Details
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- Details
Name: | ale_NorthDakotaWindDevelopmentAxis=ale_BisonWindEnergyCenterMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=ale_A2010RateCaseMember
Namespace Prefix: |
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Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=ale_A2012WisconsinRateCaseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAgencyAxis=ale_FercApprovedWholesaleRatesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |