Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Supplemental Cash Flow Information [Abstract]      
Cash Paid During the Period for Interest – Net of Amounts Capitalized $ 63.5 $ 66.0 $ 64.5
Recognition of Right-of-use Assets and Lease Liabilities [1] 28.7 0.0 0.0
Remeasurement of Deferred Income Taxes Resulting from the TCJA [Abstract]      
Increase to Regulatory Assets 0.0 0.0 80.9
Decrease in Investment in ATC 0.0 0.0 (27.9)
Increase (Decrease) in Deferred Income Taxes 0.0 0.0 (353.6)
Increase to Regulatory Liabilities 0.0 0.0 393.6
Noncash Investing and Financing Activities [Abstract]      
Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment 33.9 (0.1) 67.2
Reclassification of Property, Plant and Equipment to Inventory [2] 0.0 46.3 0.0
Costs Incurred, Capitalized Asset Retirement Obligation Costs 20.7 14.2 (15.6)
AFUDC–Equity 2.3 1.2 1.2
ALLETE Common Stock Contributed to the Defined Benefit Pension Plan $ 0.0 $ 0.0 $ 13.5
[1]
See Leases.
[2]
In February 2018, Montana-Dakota Utilities exercised its option to purchase the Thunder Spirit II wind energy facility upon completion, resulting in a reclassification from Property, Plant and Equipment – Net to Inventories – Net for project costs incurred in the prior year. On the Consolidated Statement of Cash Flows, the sale of the wind energy facility in the fourth quarter of 2018 resulted in Operating Activities – Inventories increasing by $46.3 million in 2018 due to the project costs incurred in the prior year.