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Commitments, Guarantees and Contingencies - Transmission (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
CapX2020 [Member]  
Transmission [Line Items]  
Number of Projects 3ale_NumberOfProjects
/ ale_TransmissionAxis
= ale_Capx2020Member
Estimated Capital Expenditures $ 105ale_EstimatedCapitalExpendituresIncludingPastExpenditures
/ ale_TransmissionAxis
= ale_Capx2020Member
Capital Cost Spent to Date 99ale_CapitalCostSpentToDate
/ 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
Project in Progress [Member] | CapX2020 [Member]  
Transmission [Line Items]  
Number of Projects 1ale_NumberOfProjects
/ ale_ProjectStatusAxis
= ale_ProjectinProgressMember
/ ale_TransmissionAxis
= ale_Capx2020Member
Completed Project [Member] [Member] | CapX2020 [Member]  
Transmission [Line Items]  
Number of Projects 2ale_NumberOfProjects
/ ale_ProjectStatusAxis
= ale_CompletedProjectMemberMember
/ ale_TransmissionAxis
= ale_Capx2020Member