Administrative Law Judge Recommended Refund

No definition available.

Name:	ale_AdministrativeLawJudgeRecommendedRefund
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Estimated Annual Reduction to Revenue Due to Idled Customers net of associated expense savings such as fuel costs for Keetac and Verso.

+ References

No definition available.

+ Details

Name:	ale_EstimatedAnnualReductionToRevenueDueToIdledCustomers
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Estimated Market Value - Reservoir Land to be Sold

+ References

No definition available.

+ Details

Name:	ale_EstimatedMarketValueReservoirLandToBeSold
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Forced Outage Costs

+ References

No definition available.

+ Details

Name:	ale_ForcedOutageCosts
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Proposed Ongoing Customer Rate Increase

+ References

No definition available.

+ Details

Name:	ale_ProposedOngoingCustomerRateIncrease
Namespace Prefix:	ale_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Public Utilities Reserve for Interim Rates

+ References

No definition available.

+ Details

Name:	ale_PublicUtilitiesReserveForInterimRates
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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

- Definition

Amount of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

+ References

No definition available.

+ Details

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

- Definition

Percentage of public utility's interim rate increase (decrease) allowed while waiting final decision of regulatory agency.

+ References

No definition available.

+ Details

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

- Definition

Percentage of public utility's requested equity to capital, including debt and equity.

+ References

No definition available.

+ Details

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

- Definition

Amount of public utility's requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

+ Details

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

- Definition

Percentage of public utility's requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

+ Details

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

- Definition

Percentage of public utility's requested return on equity.

+ References

No definition available.

+ Details

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

- Definition

The amount of regulated other operating revenues recognized during the period.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_RegulatoryAgencyAxis=ale_MPUCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=ale_CIPConsolidatedFilingMember
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:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=ale_FuelAdjustmentClauseFilingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=ale_COVID19RelatedDeferredAccountingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=ale_MinnesotaPowerLandSalesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ale_RateTypeAxis=ale_ElectricRatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=ale_CurrentCostRecoveryRiderMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ale_CustomerClassAxis=ale_RetailCustomersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=ale_A2020MinnesotaGeneralRateReviewMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=ale_A2022MinnesotaGeneralRateReviewMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Regulatory Matters - Electric Rates (Details) - MPUC [Member] - Minnesota Power [Member] - USD ($) $ in Millions				4 Months Ended	9 Months Ended		18 Months Ended
	Nov. 01, 2021	May 01, 2020	Nov. 01, 2019	Apr. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2020
CIP Consolidated Filing [Member]
Regulatory Matters [Line Items]
Revenue from Cost Recovery Riders and Other Regulatory Programs					$ 2.4	$ 2.4
Fuel Adjustment Clause Filing [Member]
Regulatory Matters [Line Items]
Forced Outage Costs							$ 8.0
Administrative Law Judge Recommended Refund					5.0
COVID-19 Related Deferred Accounting [Member]
Regulatory Matters [Line Items]
Estimated Annual Reduction to Revenue Due to Idled Customers Keetac and Verso					30.0
Minnesota Power Land Sales
Regulatory Matters [Line Items]
Estimated Market Value - Reservoir Land to be Sold					100.0
Electric Rates [Member] \| Minnesota Cost Recovery Riders [Member] \| Retail Customers [Member]
Regulatory Matters [Line Items]
Revenue from Cost Recovery Riders and Other Regulatory Programs					$ 29.2	$ 22.6
Electric Rates [Member] \| 2020 Minnesota General Rate Case [Member] \| Retail Customers [Member]
Regulatory Matters [Line Items]
Annual Additional Revenue Generated from Requested Final Rate Increase			$ 66.0
Requested Rate Increase Percent			10.60%
Interim Rate Increase				$ 36.1
Requested Return on Equity			10.05%
Requested Equity Capital Structure			53.81%
Ongoing Customer Rate Increase		4.10%
Public Utilities, Interim Rate Increase (Decrease), Percentage				5.80%
Public Utilities Reserve for Interim Rates								$ 11.7
Electric Rates [Member] \| 2022 Minnesota General Rate Review \| Retail Customers [Member] \| Subsequent Event [Member]
Regulatory Matters [Line Items]
Annual Additional Revenue Generated from Requested Final Rate Increase	$ 108.0
Requested Rate Increase Percent	18.00%
Interim Rate Increase	$ 87.0
Requested Return on Equity	10.25%
Requested Equity Capital Structure	53.81%
Public Utilities, Interim Rate Increase (Decrease), Percentage	14.00%