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Property, Plant and Equipment
 12 Months Ended
Dec. 31, 2020
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment [Text Block] PROPERTY, PLANT AND EQUIPMENT
Property, Plant and Equipment
As of December 3120202019
Millions
Regulated Operations
Property, Plant and Equipment in Service$4,972.3 $4,555.8 
Construction Work in Progress79.4 383.6 
Accumulated Depreciation(1,758.0)(1,635.3)
Regulated Operations – Net3,293.7 3,304.1 
ALLETE Clean Energy
Property, Plant and Equipment in Service1,275.4 686.0 
Construction Work in Progress261.0 351.3 
Accumulated Depreciation(112.9)(86.8)
ALLETE Clean Energy – Net1,423.5 950.5 
Corporate and Other (a)
Property, Plant and Equipment in Service240.5 231.9 
Construction Work in Progress9.6 3.8 
Accumulated Depreciation(126.5)(113.3)
Corporate and Other – Net123.6 122.4 
Property, Plant and Equipment – Net$4,840.8 $4,377.0 
(a)Primarily includes BNI Energy and a small amount of non-rate base generation.

Depreciation is computed using the straight-line method over the estimated useful lives of the various classes of assets.
Estimated Useful Lives of Property, Plant and Equipment (Years)
Regulated Operations
   Generation3 to 50ALLETE Clean Energy5 to 35
   Transmission52 to 71Corporate and Other3 to 50
   Distribution19 to 68
NOTE 2. PROPERTY, PLANT AND EQUIPMENT (Continued)

Asset Retirement Obligations. We recognize, at fair value, obligations associated with the retirement of certain tangible, long‑lived assets that result from the acquisition, construction, development or normal operation of the asset. Asset retirement obligations (AROs) relate primarily to the decommissioning of our coal-fired and wind energy facilities, and land reclamation at BNI Energy. AROs are included in Other Non-Current Liabilities on the Consolidated Balance Sheet. The associated retirement costs are capitalized as part of the related long-lived asset and depreciated over the useful life of the asset. Removal costs associated with certain distribution and transmission assets have not been recognized, as these facilities have indeterminate useful lives.

Conditional asset retirement obligations have been identified for treated wood poles and remaining polychlorinated biphenyl and asbestos-containing assets; however, the period of remediation is indeterminable and removal liabilities have not been recognized.

Long-standing ratemaking practices approved by applicable state and federal regulatory authorities have allowed provisions for future plant removal costs in depreciation rates. These plant removal cost recoveries are classified either as AROs or as a regulatory liability for non-AROs. To the extent annual accruals for plant removal costs differ from accruals under approved depreciation rates, a regulatory asset has been established in accordance with GAAP for AROs. (See Note 4. Regulatory Matters.)
Asset Retirement Obligations
Millions
Obligation as of December 31, 2018$138.6 
Accretion7.2 
Liabilities Recognized1.4 
Liabilities Settled(4.6)
Revisions in Estimated Cash Flows17.7 
Obligation as of December 31, 2019160.3 
Accretion8.3 
Liabilities Recognized1.4 
Liabilities Settled(3.6)
Revisions in Estimated Cash Flows0.2 
Obligation as of December 31, 2020$166.6