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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | Jun. 30, 2023 | Dec. 31, 2022
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Regulatory Assets and Liabilities [Line Items] |  |  |  |
Current Regulatory Assets | [1] | $ 15.7 |  | $ 25.6
Non-Current Regulatory Assets |  | 454.5 |  | 441.0
Current Regulatory Liabilities | [2] | 36.4 |  | 23.4
Non-Current Regulatory Liabilities |  | 538.0 |  | 526.1
Transmission Formula Rates [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Current Regulatory Liabilities | [2] | 2.5 |  | 4.9
Income Taxes [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liabilities |  | 320.7 |  | 332.5
Wholesale and Retail Contra AFUDC [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liabilities |  | 79.3 |  | 80.7
Plant Removal Obligations [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liabilities |  | 63.7 |  | 60.0
Pension and Other Postretirement Plan Costs [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liabilities |  | 13.8 |  | 17.6
North Dakota Investment Tax Credits [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liabilities |  | 16.6 |  | 16.9
Other (Member) |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Current Regulatory Liabilities | [2] | 2.1 |  | 0.1
Non-Current Regulatory Liabilities |  | 6.7 |  | 4.2
Boswell Units 1 and 2 Net Plant and Equipment |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liabilities |  | 6.7 |  | 6.7
Fuel Adjustment Clause [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liabilities |  | 17.8 |  | 0.0
Non-Jurisdictional Land Sales |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Liabilities |  | 12.7 |  | 7.5
Public Utilities Reserve for Interim Rates |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Current Regulatory Liabilities | [2],[3] | 31.8 |  | 18.4
Pension and Other Postretirement Plan Costs [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Assets |  | 223.6 |  | 225.9
Income Taxes [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Assets |  | 93.3 |  | 97.6
Cost Recovery Riders [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Assets |  | 42.4 |  | 41.2
Asset Retirement Obligations [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Assets |  | 36.6 |  | 35.6
Manufactured Gas Plant [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Assets |  | 14.3 |  | 15.1
PPACA Income Tax Deferral [Member] |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Assets |  | 4.0 |  | 4.1
Other (Member) |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Assets |  | 7.8 |  | 7.0
Fuel Adjustment Clause (Member) |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Current Regulatory Assets | [1] | 15.7 |  | 25.6
Non-Current Regulatory Assets |  | 5.4 |  | 14.5
Taconite Harbor Energy Center |  |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |  |
Non-Current Regulatory Assets |  | $ 27.1 | [4] | $ 0.0
[1] | Current regulatory assets are presented within Prepayments and Other on the Consolidated Balance Sheet.
[2] | Current regulatory liabilities are presented within Other Current Liabilities on the Consolidated Balance Sheet.
[3] | See 2022 Minnesota General Rate Case.
[4] | In the first quarter of 2023, Minnesota Power retired Taconite Harbor Units 1 and 2. The remaining net book value was reclassified from property, plant and equipment to a regulatory asset on the Consolidated Balance Sheet when the units were retired. Minnesota Power expects to receive recovery of the remaining net book value from customers.

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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 340 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481878/980-340-50-1 + Details
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