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Commitments, Guarantees and Contingencies - Transmission (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
CapX2020 [Member]  
Transmission [Line Items]  
Number of CapX2020 Projects 3ale_NumberOfProjects
/ ale_TransmissionAxis
= ale_Capx2020Member
Estimated Capital Expenditures $ 100ale_EstimatedCapitalExpendituresIncludingPastExpenditures
/ ale_TransmissionAxis
= ale_Capx2020Member
Great Northern Transmission Line [Member]  
Transmission [Line Items]  
Transmission Line Length (Miles) 220ale_TransmissionLineLengthMiles
/ ale_TransmissionAxis
= ale_GreatNorthernTransmissionLineMember
Transmission Line Capacity (kV) 500ale_TransmissionLineCapacityKv
/ ale_TransmissionAxis
= ale_GreatNorthernTransmissionLineMember
Great Northern Transmission Line [Member] | Minimum [Member]  
Transmission [Line Items]  
Estimated Capital Expenditures 560ale_EstimatedCapitalExpendituresIncludingPastExpenditures
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ ale_TransmissionAxis
= ale_GreatNorthernTransmissionLineMember
Great Northern Transmission Line [Member] | Maximum [Member]  
Transmission [Line Items]  
Estimated Capital Expenditures $ 710ale_EstimatedCapitalExpendituresIncludingPastExpenditures
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ ale_TransmissionAxis
= ale_GreatNorthernTransmissionLineMember
Completed Projects - Current Year [Member] | CapX2020 [Member]  
Transmission [Line Items]  
Number of CapX2020 Projects 1ale_NumberOfProjects
/ ale_ProjectStatusAxis
= ale_CompletedProjectCurrentYearMember
/ ale_TransmissionAxis
= ale_Capx2020Member
Completed Projects - Prior Years [Member] | CapX2020 [Member]  
Transmission [Line Items]  
Number of CapX2020 Projects 2ale_NumberOfProjects
/ ale_ProjectStatusAxis
= ale_CompletedProjectPriorYearsMember
/ ale_TransmissionAxis
= ale_Capx2020Member