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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
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 |  | $ 385.1 | $ 389.5
Current Regulatory Liabilities | [1] | 44.4 | 55.1
Non-Current Regulatory Liabilities |  | 504.1 | 512.1
Total Regulatory Liabilities |  | 548.5 | 567.2
Provision for Interim Rate Refund [Domain] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Current Regulatory Liabilities | [1],[2] | 40.6 | 40.0
Transmission Formula Rates [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Current Regulatory Liabilities | [1] | 3.3 | 4.4
Non-Current Regulatory Liabilities |  | 1.6 | 1.6
Provision for Tax Reform Refund [Domain] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Current Regulatory Liabilities | [1],[3] | 0.5 | 10.7
Income Taxes [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 389.4 | 396.4
Wholesale and Retail Contra AFUDC [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 67.3 | 64.4
Plant Removal Obligations [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 26.8 | 25.1
North Dakota Investment Tax Credits [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 12.3 | 14.7
CIP Financial Incentive [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 5.2 | 1.5
Cost Recovery Riders [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 0.0 | 6.9
Other |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Liabilities |  | 1.5 | 1.5
Defined Benefit Pension and Other Postretirement Benefit Plans [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 218.0 | 218.5
Income Taxes [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 103.2 | 105.5
Asset Retirement Obligations [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 32.4 | 32.6
Boswell 1 & 2 [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 14.8 | 16.3
Manufactured Gas Plant [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 8.1 | 8.0
PPACA Income Tax Deferral [Member] |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | 4.9 | 5.0
Other |  |  |
Regulatory Assets and Liabilities [Line Items] |  |  |
Non-Current Regulatory Assets |  | $ 3.7 | $ 3.6
[1] | Current regulatory liabilities are presented within Other Current Liabilities on the Consolidated Balance Sheet.
[2] | This amount is expected to be refunded to Minnesota Power’s regulated retail customers in the second quarter of 2019.
[3] | Provision for Tax Reform Refund related to the income tax benefits of the TCJA in 2018 was refunded to Minnesota Power customers in the first quarter of 2019 and will be refunded to SWL&P customers in 2019 and 2020.

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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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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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