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Operations and Significant Accounting Policies - Supplemental Statement of Cash Flows Information (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Supplemental Cash Flow Information [Abstract]    
Cash Paid for Interest – Net of Amounts Capitalized $ 19.3 $ 18.9
Noncash Investing and Financing Activities [Abstract]    
Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment (48.1) (3.5)
Reclassification of Property, Plant and Equipment to Inventory [1] 46.9 0.0
Capitalized Asset Retirement Costs 0.8 19.3
AFUDC–Equity 0.3 0.2
ALLETE Common Stock Contributed to the Pension Plans $ 0.0 $ 13.5
[1] On February 28, 2018, Montana-Dakota Utilities exercised its option to purchase the Thunder Spirit II wind energy facility upon completion, resulting in a reclassification of the project value from Property, Plant and Equipment – Net to Inventory – Net as ALLETE Clean Energy will no longer own and operate the facility upon completion.