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Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($)		12 Months Ended
	Jan. 15, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
ESOP compensation expense		$ 15,700,000	$ 13,700,000	$ 12,000,000
Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	$ 19,100,000
Pension
Defined Benefit Plan Disclosure [Line Items]
Employer contributions to defined benefit pension plans - cash		25,000,000	17,300,000	0
Employer contribution		34,000,000.0	26,200,000
Expected employer contributions in 2025		0
Net loss		$ 235,200,000	$ 256,900,000
ALLETE common stock included in pension plan equity securities (in shares)		0	0
Postretirement Health and Life
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions in 2025		$ 0
Net loss		(27,100,000)	$ (24,800,000)
Postretirement Health and Life | VEBA
Defined Benefit Plan Disclosure [Line Items]
Employer contributions to other postretirement benefit plans		0	0	0
Postretirement Health and Life | Irrevocable Grantor Trust
Defined Benefit Plan Disclosure [Line Items]
Employer contributions to other postretirement benefit plans		0	0	$ 0
Long-term investments		$ 13,400,000	$ 12,800,000

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Number of shares of employer stock in which defined benefit plan asset is invested. Includes, but is not limited to, stock issued or managed by related party of employer.

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Amount of cash outflow for pension and other postretirement benefits. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree.

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