OPERATIONS AND SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Operations And Significant Accounting Policies [Line Items]
Prepayments and other	$ 75,300,000	$ 99,700,000
Goodwill	154,900,000	154,900,000
Goodwill change	0
New Energy
Operations And Significant Accounting Policies [Line Items]
Prepayments and other	$ 22,500,000	$ 32,400,000

Amount, after accumulated impairment loss, of asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

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+ Details

ALLETE INC 10-Q