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Pension and Other Postretirement Benefit Plans - Contributions (Details) - USD ($) shares in Thousands, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items] |  |  |
ESOP Compensation Expense | $ 11.5 | $ 11.2 | $ 10.8
Common Stock [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer Contributions to Defined Benefit Pension Plans - Common Stock Shares |  |  | 0
Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer Contributions to Defined Benefit Pension Plans - Cash | $ 10.3 | 10.7 | $ 10.4
Defined Benefit Plan - Amendments | The non-union defined benefit pension plan was frozen in 2018, and does not allow further crediting of service or earnings to the plan. Further, it is closed to new participants. The Minnesota Power union defined benefit pension plan is also closed to new participants, and the SWL&P union defined benefit pension plan was closed to new participants effective February 1, 2022. |  |
Expected Employer Contributions in 2022 | $ 0.0 |  |
Postretirement Health and Life [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Expected Employer Contributions in 2022 | 0.0 |  |
Postretirement Health and Life [Member] | VEBA [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer Contributions to Other Postretirement Benefit Plans | 0.0 | 0.0 | 0.0
Postretirement Health and Life [Member] | Irrevocable Grantor Trust [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Employer Contributions to Other Postretirement Benefit Plans | $ 0.0 | $ 0.0 | $ 0.0

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- Definition Amount of contribution expected to be received by defined benefit plan from employer in next fiscal year following current fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=123450688&loc=d3e4587-114921 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3536-108585 + Details
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