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Property, Plant and Equipment (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment – Net		$ 5,100.2	$ 4,840.8
Asset Retirement Obligation [Roll Forward]
Beginning Obligation		166.6	160.3
Accretion		8.8	8.3
Liabilities Recognized		4.5	1.4
Liabilities Settled		(5.2)	(3.6)
Revisions in Estimated Cash Flows		9.8	0.2
Ending Obligation		184.5	166.6
Regulated Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment in Service		5,028.7	4,972.3
Construction Work in Progress		108.7	79.4
Accumulated Depreciation		(1,834.6)	(1,758.0)
Property, Plant and Equipment – Net		$ 3,302.8	3,293.7
Regulated Operations [Member] | Generation [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		2 years
Regulated Operations [Member] | Generation [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		50 years
Regulated Operations [Member] | Transmission [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		50 years
Regulated Operations [Member] | Transmission [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		75 years
Regulated Operations [Member] | Distribution [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		18 years
Regulated Operations [Member] | Distribution [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		70 years
ALLETE Clean Energy [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment in Service		$ 1,716.9	1,275.4
Construction Work in Progress		79.9	261.0
Accumulated Depreciation		(149.0)	(112.9)
Property, Plant and Equipment – Net		$ 1,647.8	1,423.5
ALLETE Clean Energy [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		5 years
ALLETE Clean Energy [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		35 years
Corporate and Other [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment in Service	[1]	$ 267.1	240.5
Construction Work in Progress	[1]	19.8	9.6
Accumulated Depreciation	[1]	(137.3)	(126.5)
Property, Plant and Equipment – Net	[1]	$ 149.6	$ 123.6
Corporate and Other [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		3 years
Corporate and Other [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		50 years
[1] Primarily includes BNI Energy and a small amount of non-rate base generation.

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Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount of structure or a modification to a structure under construction. Includes recently completed structures or modifications to structures that have not been placed into service.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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