Property, Plant and Equipment (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment – Net		$ 4,840.8	$ 4,377.0
Asset Retirement Obligation [Roll Forward]
Beginning Obligation		160.3	138.6
Accretion		8.3	7.2
Liabilities Recognized		1.4	1.4
Liabilities Settled		(3.6)	(4.6)
Revisions in Estimated Cash Flows		0.2	17.7
Ending Obligation		166.6	160.3
Regulated Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment in Service		4,972.3	4,555.8
Construction Work in Progress		79.4	383.6
Accumulated Depreciation		(1,758.0)	(1,635.3)
Property, Plant and Equipment – Net		$ 3,293.7	3,304.1
Regulated Operations [Member] | Generation [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		3 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		52 years
Regulated Operations [Member] | Transmission [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		71 years
Regulated Operations [Member] | Distribution [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		19 years
Regulated Operations [Member] | Distribution [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Lives		68 years
ALLETE Clean Energy [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment in Service		$ 1,275.4	686.0
Construction Work in Progress		261.0	351.3
Accumulated Depreciation		(112.9)	(86.8)
Property, Plant and Equipment – Net		$ 1,423.5	950.5
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]	$ 240.5	231.9
Construction Work in Progress	[1]	9.6	3.8
Accumulated Depreciation	[1]	(126.5)	(113.3)
Property, Plant and Equipment – Net	[1]	$ 123.6	$ 122.4
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.