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Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($) |  | 12 Months Ended
--- | --- | ---
Jan. 12, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
ESOP compensation expense |  | $ 13,700,000 | $ 12,000,000 | $ 11,500,000
Subsequent Event |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer contribution | $ 25,000,000 |  |  |
Pension |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer contributions to defined benefit pension plans - cash |  | 17,300,000 | 0 | 10,300,000
Employer contribution |  | 26,200,000 | 12,300,000 |
Expected employer contributions in 2024 |  | 2,000,000 |  |
Net loss |  | 256,900,000 | 260,200,000 |
Prior Service Credit |  | $ 0 | $ 100,000 |
ALLETE common stock included in pension plan equity securities (in shares) |  | 0 | 0 |
Postretirement Health and Life |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Employer contribution |  |  | $ 300,000 |
Expected employer contributions in 2024 |  | $ 0 |  |
Net loss |  | (24,800,000) | (9,200,000) |
Prior Service Credit |  | (33,800,000) | (13,200,000) |
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 |  | $ 12,800,000 | $ 11,800,000 |

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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, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation 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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