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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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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -Publisher FASB -URI https://asc.fasb.org/1943274/2147482790/220-10-45-14A Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (j) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-6 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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