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

X
- Definition Capital cost spent to date for project. + References No definition available. + Details
Name: | ale_CapitalCostSpentToDate
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Capacity of a generating facility, unit, or project in megawatts. + References No definition available. + Details
Name: | ale_GeneratingCapacityMw
Namespace Prefix: | ale_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | ale_NorthDakotaWindDevelopmentLineItems
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Capacity of a transmission line in kilovolts. + References No definition available. + Details
Name: | ale_TransmissionLineCapacityKv
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Length of a transmission line in miles. + References No definition available. + Details
Name: | ale_TransmissionLineLengthMiles
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | ale_NorthDakotaWindDevelopmentAxis=ale_Bison4Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ale_NorthDakotaWindDevelopmentAxis=ale_BisonWindEnergyCenterMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | ale_NorthDakotaWindDevelopmentAxis=ale_DCTransmissionLinefromSquareButteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |