Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.D)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480114/980-10-S99-1

+ Details

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerAmountOfLongTermDebtOrLeaseObligationOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The allocable portion of interest included in charges under a long-term contract to purchase electric power.

+ References

+ Details

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerInterestIncludedInContractCharges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.D)
-SubTopic 10
-Topic 980
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480114/980-10-S99-1

+ Details

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Share of plant output being purchased under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization.

+ References

+ Details

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of operating expense for purchased power procured under arrangements extending beyond one year or the normal operating cycle, if longer, by regulated operation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 980
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 10.D)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480114/980-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.2(b))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2

+ Details

Name:	us-gaap_UtilitiesOperatingExpensePurchasedPowerUnderLongTermContracts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	srt_CounterpartyNameAxis=ale_SquareButteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerAxis=ale_SquareButtePpaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ale_MinnesotaPowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ale_GeneratingFacilityAxisAxis=ale_SquareButteCoalFiredUnitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=ale_MinnkotaPowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ale_LongtermContractsforSaleofElectricPowerAxis=ale_MinnkotaSalesAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) - Square Butte PPA - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Square Butte
Power Purchase Agreements [Line Items]
PPA counterparty total debt outstanding	$ 162.6
Square Butte \| Minnesota Power
Power Purchase Agreements [Line Items]
Cost of power purchased	22.9	$ 22.1
Pro rata share of PPA counterparty interest expense	$ 1.3	$ 1.3
Square Butte \| Minnesota Power \| Square Butte Coal-Fired Unit
Power Purchase Agreements [Line Items]
Expected output entitlement	50.00%
Minnkota Power \| Minnesota Power \| Square Butte Coal-Fired Unit \| Minnkota Power PSA
Power Purchase Agreements [Line Items]
Expected output entitlement	41.00%		37.00%