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

No definition available.

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- Definition

Purchase option to acquire an asset.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

Unsecured Term Loan, Amount

+ References

No definition available.

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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

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- 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

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- Definition

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

+ References

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- 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

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- 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=6877327&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

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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

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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=114775744&loc=d3e28555-108399

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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- References

No definition available.

+ Details

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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ 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
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-Topic 470
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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Balance Type:	credit
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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

+ Details

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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- Definition

Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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- 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

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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=118932676&loc=d3e3255-108585

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Data Type:	xbrli:monetaryItemType
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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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

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- Details

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- Details

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Balance Type:
Period Type:

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Short-Term and Long-Term Debt - Long-Term Debt (Details) $ in Millions				12 Months Ended
	Jan. 10, 2020 USD ($)	Aug. 14, 2019 USD ($)	Mar. 01, 2019 USD ($)	Dec. 31, 2019 USD ($) Years	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Long-Term Debt				$ 1,400.9	$ 1,428.5
Long-Term Debt - Unamortized Debt Issuance Costs				8.4
Debt Instrument [Abstract]
Long-Term Debt Maturing in 2020				213.3
Long-Term Debt Maturing in 2021				98.6
Long-Term Debt Maturing in 2022				88.8
Long-Term Debt Maturing in 2023				88.8
Long-Term Debt Maturing in 2024				73.5
Long-Term Debt Maturing Thereafter				$ 1,059.6
First Mortgage Bonds - 4.08% Due March 2029 [Member]
Debt Instrument [Abstract]
Proceeds from Issuance of First Mortgage Bond			$ 70.0
Interest Rate			4.08%	4.08%
Camp Ripley Financing [Member]
Debt Instrument [Abstract]
Annual Financing Payment				$ 1.4
Financing Renewal Term (Years) \| Years				2
Purchase Option				$ 4.0
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member]
Debt Instrument [Abstract]
Interest Rate				3.11%
ALLETE 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 Bonds 4.07% Due April 2048 [Member]
Debt Instrument [Abstract]
Interest Rate				4.07%
Unsecured Term Loan Due in 2020 [Member]
Debt Instrument [Abstract]
Proceeds from Issuance of Unsecured Debt		$ 110.0
Debt Instrument, Description of Variable Rate Basis		LIBOR
Debt Instrument, Basis Spread on Variable Rate		1.025%
ALLETE Bonds 4.47% Due March 2049 [Member]
Debt Instrument [Abstract]
Proceeds from Issuance of First Mortgage Bond			$ 30.0
Interest Rate			4.47%	4.47%
Subsequent Event [Member] \| Unsecured Term Loan Due in 2021 [Member]
Debt Instrument [Line Items]
Unsecured Term Loan, Amount	$ 200.0
Debt Instrument [Abstract]
Proceeds from Issuance of Unsecured Debt	60.0
Debt Instrument, Unused Borrowing Capacity, Amount	$ 140.0
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	0.55%