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Regulatory Matters - Conservation Improvement Program (CIP) (Details) - MPUC [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
CIP Triennial Filing [Member] | Minnesota Power [Member]      
Regulatory Matters [Line Items]      
CIP Investment Goal $ 10.5 $ 10.3  
CIP Actual Spending 8.3 9.0 $ 8.1
CIP Investment Goal, Year Two 10.5    
CIP Consolidated Filing [Member] | Minnesota Power [Member]      
Regulatory Matters [Line Items]      
CIP Financial Incentive $ 2.8 $ 3.0  
Minimum [Member]      
Regulatory Matters [Line Items]      
CIP Spending Minimum - Percentage 1.50%