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Commitments, Guarantees and Contingencies - North Dakota Wind Development (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
kV
Miles
DC Transmission Line from Square Butte [Member]  
North Dakota Wind Development [Line Items]  
Transmission Line Length (Miles) 465ale_TransmissionLineLengthMiles
/ ale_NorthDakotaWindDevelopmentAxis
= ale_DCTransmissionLinefromSquareButteMember
Transmission Line Capacity (kV) 250ale_TransmissionLineCapacityKv
/ ale_NorthDakotaWindDevelopmentAxis
= ale_DCTransmissionLinefromSquareButteMember
Bison Wind Energy Center [Member]  
North Dakota Wind Development [Line Items]  
Generating Unit Capacity (MW) 497.0ale_GeneratingCapacityMw
/ ale_NorthDakotaWindDevelopmentAxis
= ale_BisonWindEnergyCenterMember
Bison 4 Wind Project [Member]  
North Dakota Wind Development [Line Items]  
Generating Unit Capacity (MW) 205.0ale_GeneratingCapacityMw
/ ale_NorthDakotaWindDevelopmentAxis
= ale_Bison4Member
Capital Cost Spent to Date 333ale_CapitalCostSpentToDate
/ ale_NorthDakotaWindDevelopmentAxis
= ale_Bison4Member