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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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