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Property, Plant and Equipment (Tables)
 12 Months Ended
Dec. 31, 2024
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment and Estimated Useful Lives of Property, Plant and Equipment
Property, Plant and Equipment
As of December 3120242023
Millions
Regulated Operations
Property, Plant and Equipment in Service$5,387.8 $5,167.2 
Construction Work in Progress197.5 146.7 
Accumulated Depreciation(2,077.8)(1,969.4)
Regulated Operations – Net3,507.5 3,344.5 
ALLETE Clean Energy
Property, Plant and Equipment in Service1,605.3 1,612.8 
Construction Work in Progress51.9 48.9 
Accumulated Depreciation(272.9)(229.1)
ALLETE Clean Energy – Net1,384.3 1,432.6 
Corporate and Other (a)
Property, Plant and Equipment in Service377.4 355.8 
Construction Work in Progress77.5 27.5 
Accumulated Depreciation(165.2)(147.0)
Corporate and Other – Net289.7 236.3 
Property, Plant and Equipment – Net$5,181.5 $5,013.4 
(a)Primarily includes BNI Energy and a small amount of non-rate base generation.
Estimated Useful Lives of Property, Plant and Equipment (Years)
Regulated Operations
   Generation
3 to 50
ALLETE Clean Energy
5 to 35
   Transmission
50 to 75
Corporate and Other
3 to 50
   Distribution
18 to 70
Asset Retirement Obligations
(a)     The increase in liabilities recognized in 2024 reflects the impact of estimated compliance costs related to the EPA’s CCR Legacy Impoundment Rule finalized in May 2024. (See Note 9. Commitments, Guarantees and Contingencies.)