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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Regulatory Assets and Liabilities [Line Items] |  |
Total Current Regulatory Assets | $ 13.9 | $ 25.6
Total Non-Current Regulatory Assets | 443.3 | 441.0
Total Current Regulatory Liabilities | 41.9 | 23.4
Total Non-Current Regulatory Liabilities | 549.3 | 526.1
Provision for Interim Rate Refund |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 39.4 | 18.4
Transmission Formula Rates Refund |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 1.2 | 4.9
Other |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 1.3 | 0.1
Total Non-Current Regulatory Liabilities | 5.2 | 4.2
Income Taxes |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Liabilities | 315.7 | 332.5
Wholesale and Retail Contra AFUDC |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Liabilities | 78.6 | 80.7
Plant Removal Obligations |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Liabilities | 65.9 | 60.0
Fuel Adjustment Clause |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Liabilities | 28.3 | 0.0
Non-Jurisdictional Land Sales |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Liabilities | 20.7 | 7.5
North Dakota Investment Tax Credits |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Liabilities | 16.4 | 16.9
Defined Benefit Pension and Other Postretirement Benefit Plans |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Liabilities | 11.8 | 17.6
Boswell Units 1 and 2 Net Plant and Equipment |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Liabilities | 6.7 | 6.7
Fuel Adjustment Clause |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Current Regulatory Assets | 12.1 | 25.6
Total Non-Current Regulatory Assets | 4.1 | 14.5
Other |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Current Regulatory Assets | 1.8 | 0.0
Total Non-Current Regulatory Assets | 4.0 | 7.0
Defined Benefit Pension and Other Postretirement Benefit Plans |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Assets | 223.6 | 225.9
Income Taxes |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Assets | 91.4 | 97.6
Cost Recovery Riders |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Assets | 40.8 | 41.2
Asset Retirement Obligations |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Assets | 37.1 | 35.6
Taconite Harbor |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Assets | 24.2 | 0.0
Manufactured Gas Plant |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Assets | 14.1 | 15.1
PPACA Income Tax Deferral |  |
Regulatory Assets and Liabilities [Line Items] |  |
Total Non-Current Regulatory Assets | $ 4.0 | $ 4.1

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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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