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Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
 6 Months Ended
Jun. 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 $ 374.5 $ 384.7
Current Regulatory Liabilities [1] 39.2 0.0
Non-Current Regulatory Liabilities 516.0 532.0
Total Regulatory Liabilities 555.2 532.0
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 that will be Offset Against Interim Rate Refunds 16.7  
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 that will 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] 32.5 0.0
Provision for Tax Reform Refund [Domain]    
Regulatory Assets and Liabilities [Line Items]    
Current Regulatory Liabilities [1],[3] 6.7 0.0
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 401.4 411.2
Wholesale and Retail Contra AFUDC [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 60.2 57.9
Plant Removal Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 23.0 20.3
Cost Recovery Riders [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 16.8 2.2
North Dakota Investment Tax Credits [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Liabilities 14.4 14.1
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 0.2 2.6
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 216.7 220.3
Income Taxes [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 109.3 112.8
Asset Retirement Obligations [Member]    
Regulatory Assets and Liabilities [Line Items]    
Non-Current Regulatory Assets 31.0 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
Conservation Improvement Program [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 $ 4.5 $ 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 $16.7 million of discounts provided to EITE customers that will be offset against interim rate refunds as of June 30, 2018 ($8.6 million as of December 31, 2017). (See 2016 Minnesota General Rate Case and Energy-Intensive Trade‑Exposed Customer Rates.)
[3] We have recorded the impact of income tax changes for Minnesota Power and SWL&P resulting from the TCJA in 2018 as regulatory liabilities and a reduction in revenue as the benefits are deferred pending the outcome of regulatory proceedings with the MPUC and PSCW. (See Tax Cuts and Jobs Act of 2017.)