Capital cost spent to date for project.

No definition available.

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

- Definition

The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period.

+ References

No definition available.

+ Details

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

- Definition

Generating Capacity Counterparty Owned (MW)

+ References

No definition available.

+ Details

Name:	ale_GeneratingCapacityCounterpartyOwnedMW
Namespace Prefix:	ale_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	instant

- Definition

Generating Capacity Jointly Owned (MW)

+ References

No definition available.

+ Details

Name:	ale_GeneratingCapacityJointlyOwnedMW
Namespace Prefix:	ale_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	instant

- Definition

Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased

+ References

No definition available.

+ Details

Name:	ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputPurchasedMw
Namespace Prefix:	ale_
Data Type:	num:powerItemType
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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.D)
-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

+ Details

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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

No definition available.

+ Details

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

- Details

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

- Details

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

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	ale_GenerationTypeAxis=us-gaap_NaturalGasProcessingPlantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=us-gaap_JointlyOwnedElectricityGenerationPlantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ale_GenerationTypeAxis=ale_WindTurbineGeneratorsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=ale_Nobles2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongTermContractForPurchaseOfElectricPowerAxis=ale_Nobles2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Regulatory Matters - Integrated Resource Plan (Details) $ in Millions	Jul. 28, 2017 MW	Dec. 31, 2019 USD ($)	Aug. 22, 2018 MW
Minnesota Power [Member] \| Nemadji Trail Energy Center [Member]
Regulatory Matters [Line Items]
Estimated Capital Expenditures, Including Past Expenditures \| $		$ 350
Maximum [Member] \| Minnesota Power [Member] \| Nemadji Trail Energy Center [Member]
Regulatory Matters [Line Items]
Estimated Capital Expenditures, Including Past Expenditures \| $		700
Capital Cost Spent to Date \| $		$ 12
MPUC [Member] \| Minnesota Power [Member] \| Resource Package [Member] \| Natural Gas PPA [Member]
Regulatory Matters [Line Items]
Output Being Purchased (MW)	250
MPUC [Member] \| Natural Gas-Fired [Member] \| Minnesota Power [Member] \| Resource Package [Member] \| Natural Gas PPA [Member]
Regulatory Matters [Line Items]
Expected Output Entitlement (Percent)	50.00%
MPUC [Member] \| Natural Gas-Fired [Member] \| Minimum [Member] \| Resource Package [Member] \| Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] \| Natural Gas PPA [Member] \| Combined-Cycle Natural Gas-Fired Generating Facility [Member] \| Jointly-Owned Electricity Generation Plant [Member]
Regulatory Matters [Line Items]
Generating Capacity Jointly Owned (MW)	525
MPUC [Member] \| Natural Gas-Fired [Member] \| Maximum [Member] \| Resource Package [Member] \| Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] \| Natural Gas PPA [Member] \| Combined-Cycle Natural Gas-Fired Generating Facility [Member] \| Jointly-Owned Electricity Generation Plant [Member]
Regulatory Matters [Line Items]
Generating Capacity Jointly Owned (MW)	625
MPUC [Member] \| Wind Turbine Generators [Member] \| Minnesota Power [Member] \| Resource Package [Member] \| Nobles 2 [Member] \| Nobles 2 [Member]
Regulatory Matters [Line Items]
Generating Capacity Counterparty Owned (MW)	250		250