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Regulatory Matters - Great Northern Transmission Line (GNTL) (Details) (Great Northern Transmission Line [Member])
Dec. 31, 2014
kV
Miles
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