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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.