Operations and Significant Accounting Policies - Other Non-Current Assets (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Other Postretirement Benefit Plans	$ 104.6	$ 106.3
Contract Assets	17.9	18.5
Operating Lease Right-of-use Assets	11.3	10.7
ALLETE Properties	10.7	10.8
Restricted Cash	2.4	2.4	$ 2.3	$ 2.3
Finance Lease Right-of-use Assets	2.0	2.1
Other	115.9	112.1
Total Other Non-Current Assets	$ 264.8	$ 262.9

Other Postretirement Benefit Plans

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- Definition

Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as noncurrent.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
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Amount, after accumulated amortization, of right-of-use asset from finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
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Amount of lessee's right to use underlying asset under operating lease.

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Amount of other miscellaneous assets expected to be realized or consumed after one year or normal operating cycle, if longer.

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No definition available.

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Amount of noncurrent assets classified as other.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
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Amount of real estate investments, net of accumulated depreciation, which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; (7) other real estate investments; (8) real estate joint ventures; and (9) unconsolidated real estate and other joint ventures not separately presented.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of cash restricted as to withdrawal or usage, classified as noncurrent. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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