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Regulatory Matters - Great Northern Transmission Line (GNTL) (Details) - Great Northern Transmission Line [Member]	Sep. 30, 2015 Miles kV
Transmission [Line Items]
Transmission Line Length (Miles)	220
Transmission Line Capacity (kV) | kV	500

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

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	ale_TransmissionLineItems
Namespace Prefix:	ale_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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_TransmissionAxis=ale_GreatNorthernTransmissionLineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: