Debt Instrument, Interest Rate Floor

No definition available.

Name:	ale_DebtInstrumentInterestRateFloor
Namespace Prefix:	ale_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Renewal term for a debt instrument disclosing for what period of time after maturity the instrument may be extended.

+ References

No definition available.

+ Details

Name:	ale_DebtInstrumentRenewalTerm
Namespace Prefix:	ale_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Purchase option to acquire an asset.

+ References

No definition available.

+ Details

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

- Definition

Unsecured Senior Notes, Amount

+ References

No definition available.

+ Details

Name:	ale_UnsecuredSeniorNotesAmount
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Unsecured Term Loan, Amount

+ References

No definition available.

+ Details

Name:	ale_UnsecuredTermLoanAmount
Namespace Prefix:	ale_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the required periodic payments including both interest and principal payments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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

- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

+ References

+ Details

Name:	us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28555-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing after fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04.(a))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e24072-122690

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

+ Details

Name:	us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in fifth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

Name:	us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

+ Details

Name:	us-gaap_LongTermDebtNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from amounts received from a long-term debt instrument secured by a first mortgage deed of trust, containing a pledge of real property. The lender has the highest claim on the property in case of default.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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

- Definition

The cash inflow from the issuance of long-term debt that is not secured by collateral. Excludes proceeds from tax exempt unsecured debt.

+ References

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_CampRipleyFinancingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_UnsecuredTermLoanDuein2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_UnsecuredTermLoanDueApril2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_ALLETEBonds4.08DueMarch2029Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_ALLETEBonds4.47DueMarch2049Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_UnsecuredSeniorNotesDueSeptember2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_UnsecuredTermLoanDueDecember2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=ale_ALLETECleanEnergyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_ALLETEBonds4.07DueApril2048Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_ALLETETermLoanVariableRateDue2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_ALLETESeniorUnsecuredNotes3.11Due2027Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_ALLETEBonds250DueAugust2030Member
Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ale_ALLETEBonds330DueAugust2050Member
Namespace Prefix:
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- Details

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Short-Term and Long-Term Debt - Long-Term Debt (Details)				12 Months Ended
	Dec. 28, 2020 USD ($)	Jan. 10, 2020 USD ($)	Mar. 01, 2019 USD ($)	Dec. 31, 2020 USD ($) Years	Jan. 31, 2021 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Long-Term Debt				$ 1,593,200,000		$ 1,400,900,000
Long-Term Debt - Unamortized Debt Issuance Costs				9,200,000
Debt Instrument [Abstract]
Long-Term Debt Maturing in 2020				204,000,000.0
Long-Term Debt Maturing in 2021				89,200,000
Long-Term Debt Maturing in 2022				89,100,000
Long-Term Debt Maturing in 2023				73,900,000
Long-Term Debt Maturing in 2024				241,100,000
Long-Term Debt Maturing Thereafter				1,109,100,000
Camp Ripley Financing [Member]
Debt Instrument [Abstract]
Annual Financing Payment				$ 1,400,000
Financing Renewal Term (Years) \| Years				2
Purchase Option				$ 4,000,000
Unsecured Term Loan Due in 2021 [Member]
Debt Instrument [Abstract]
Unsecured Term Loan, Amount		$ 200,000,000
Debt Instrument, Description of Variable Rate Basis		LIBOR
Debt Instrument, Basis Spread on Variable Rate		0.55%
Unsecured Term Loan Due April 2021 [Member]
Debt Instrument [Abstract]
Unsecured Term Loan, Amount				115,000,000
Debt Instrument, Unused Borrowing Capacity, Amount				$ 40,000,000
Debt Instrument, Description of Variable Rate Basis				LIBOR
Debt Instrument, Basis Spread on Variable Rate				1.70%
Debt Instrument, Interest Rate Floor				0.75%
First Mortgage Bonds - 2.50% Due August 2030 [Member]
Debt Instrument [Abstract]
Interest Rate				4.08%
First Mortgage Bonds - 4.47% Series Due 2049 [Member]
Debt Instrument [Abstract]
Interest Rate				4.47%
Unsecured Senior Notes Due September 2025 [Member]
Debt Instrument [Abstract]
Interest Rate				2.65%
Unsecured Senior Notes, Amount				$ 150,000,000
Unsecured Term Loan Due December 2021 [Member] \| ALLETE Clean Energy [Member]
Debt Instrument [Abstract]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	1.125%
Proceeds from Issuance of Unsecured Debt	$ 100,000,000.0			$ 65,000,000
First Mortgage Bonds - 4.07% Series Due 2048 [Member]
Debt Instrument [Abstract]
Interest Rate				4.07%
Unsecured Term Loan Variable Rate Due 2020 [Member]
Debt Instrument [Abstract]
Debt Instrument, Description of Variable Rate Basis				LIBOR
Debt Instrument, Basis Spread on Variable Rate				1.025%
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member]
Debt Instrument [Abstract]
Interest Rate				3.11%
First Mortgage Bonds - 2.50% Series Due 2030
Debt Instrument [Abstract]
Proceeds from Issuance of First Mortgage Bond			$ 46,000,000
Interest Rate			2.50%	2.50%
First Mortgage Bonds - 3.30% Series Due 2050
Debt Instrument [Abstract]
Proceeds from Issuance of First Mortgage Bond			$ 94,000,000
Interest Rate			3.30%	3.30%
Subsequent Event [Member] \| Unsecured Term Loan Due December 2021 [Member] \| ALLETE Clean Energy [Member]
Debt Instrument [Abstract]
Debt Instrument, Unused Borrowing Capacity, Amount					$ 35,000,000