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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Regulatory Assets and Liabilities [Line Items]    
Regulatory Assets and Liabilities Currently Earning a Return No regulatory assets or liabilities are currently earning a return.  
Non-Current Regulatory Assets $ 370.6 $ 384.7
Current Regulatory Liabilities [1] 50.3 0.0
Non-Current Regulatory Liabilities 512.3 532.0
Total Regulatory Liabilities 562.6 532.0
Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member]    
Regulatory Assets and Liabilities [Line Items]    
Provided Discounts to EITE Customers to be Offset Against Interim Rate Refunds 20.8  
Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member] | MPUC [Member]    
Regulatory Assets and Liabilities [Line Items]    
Provided Discounts to EITE Customers to be Offset Against Interim Rate Refunds   8.6
Provision for Interim Rate Refund [Domain]    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1],[2] 36.7 0.0
Provision for Tax Reform Refund [Domain]    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1],[3] 9.2 0.0
Transmission Formula Rates [Member]    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1] 4.4 0.0
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 398.1 411.2
Wholesale and Retail Contra AFUDC [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 62.0 57.9
Plant Removal Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 24.1 20.3
North Dakota Investment Tax Credits [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 14.6 14.1
Cost Recovery Riders [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 11.0 2.2
CIP Financial Incentive [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 1.1 0.0
Provision for Interim Rate Refund [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities [2] 0.0 23.7
Other    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 1.4 2.6
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 214.0 220.3
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 107.9 112.8
Asset Retirement Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 31.9 29.6
Manufactured Gas Plant [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 8.0 8.1
PPACA Income Tax Deferral [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 5.0 5.0
CIP Financial Incentive [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 0.0 3.3
Other    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets $ 3.8 $ 5.6
[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 first quarter of 2019 and includes $20.8 million of discounts provided to EITE customers as of September 30, 2018, that will be offset against interim rate refunds ($8.6 million as of December 31, 2017). (See 2016 Minnesota General Rate Case and Energy-Intensive Trade‑Exposed Customer Rates.)
[3] Provision for Tax Reform Refund is expected to be refunded to Minnesota Power customers in the first quarter of 2019 and SWL&P customers in 2019 pending the outcome of SWL&P’s rate filing with the PSCW. (See Tax Cuts and Jobs Act of 2017.)