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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | 9 Months Ended |
--- | --- | ---
Sep. 30, 2021 | Dec. 31, 2020
Regulatory Assets and Liabilities [Line Items] |  |  |
Regulatory Assets Currently Earning a Return |  | With the exception of the regulatory asset for Boswell Units 1 and 2 net plant and equipment, no other regulatory assets are currently earning a return. |
Non-Current Regulatory Assets |  | $ 502.9 | $ 480.9
Current Regulatory Liabilities | [1] | 9.0 | 7.6
Non-Current Regulatory Liabilities |  | 509.5 | 524.8
Total Regulatory Liabilities |  | 518.5 | 532.4
Transmission Formula Rates [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Current Regulatory Liabilities | [1] | 3.1 | 2.9
Income Taxes [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 359.3 | 375.3
Wholesale and Retail Contra AFUDC [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 84.4 | 86.6
Plant Removal Obligations [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 47.4 | 41.2
Pension and Other Postretirement Plan Costs [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 1.4 | 4.4
North Dakota Investment Tax Credits [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 12.3 | 12.0
Conservation Improvement Program [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 0.0 | 1.5
Other (Member) |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Current Regulatory Liabilities | [1] | 3.3 | 1.0
Non-Current Regulatory Liabilities |  | 4.7 | 3.8
Fuel Adjustment Clause [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Current Regulatory Liabilities | [1] | 2.6 | 3.7
Pension and Other Postretirement Plan Costs [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 247.6 | 259.7
Income Taxes [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 107.2 | 113.7
Cost Recovery Riders [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 61.8 | 54.0
Asset Retirement Obligations [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 31.4 | 31.6
Manufactured Gas Plant [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 11.9 | 8.8
Boswell Units 1 and 2 Net Plant and Equipment [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 3.2 | 5.0
PPACA Income Tax Deferral [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 4.4 | 4.5
Other (Member) |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 5.3 | 3.6
Fuel Adjustment Clause (Member) |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | $ 30.1 | $ 0.0
[1] | Current regulatory liabilities are presented within Other Current Liabilities on the Consolidated Balance Sheet

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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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