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Regulatory Matters - Electric Rates (Details)
$ in Millions
1 Months Ended 12 Months Ended
Dec. 20, 2018
USD ($)
May 25, 2018
Jan. 18, 2018
USD ($)
Aug. 09, 2017
Nov. 30, 2016
Dec. 31, 2018
USD ($)
Customers
Years
MW
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Regulatory Matters [Line Items]                
Charge to Fuel, Purchased Power and Gas - Utility           $ 407.5 $ 396.9 $ 339.9
MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member]                
Regulatory Matters [Line Items]                
Charge to Fuel, Purchased Power and Gas - Utility           19.5    
MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Boswell Energy Center [Member]                
Regulatory Matters [Line Items]                
Decrease in Annual Depreciation Expense Based on Regulatory Order           $ 25.0    
PSCW [Member] | 2016 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Approved Return on Common Equity       10.50%        
Approved Equity Ratio       55.00%        
PSCW [Member] | 2012 Wisconsin Rate Case [Member] | SWL&P [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Approved Return on Common Equity           10.90%    
PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Requested Average Rate Increase   2.70%            
Requested Return on Equity   10.50%            
Requested Equity Ratio   55.41%            
Approved Return on Common Equity 10.40%              
Approved Equity Ratio 55.00%              
Approved Rate Increase - Amount $ 1.3              
Electric Rates [Member] | MPUC [Member] | Minnesota Cost Recovery Riders [Member] | Minnesota Power [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Revenue from Cost Recovery Riders           $ 103.8 96.9 97.1
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Requested Average Rate Increase         9.00%      
Requested Return on Equity         10.25%      
Requested Equity Ratio         53.81%      
Approved Return on Common Equity     9.25%          
Approved Equity Ratio     53.81%          
Annual Additional Revenue Generated from Requested Final Rate Increase     $ 13.0          
Reserve for Interim Rate Refund           40.0 23.7  
Electric Rates [Member] | MPUC [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member] | Minnesota Power [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Estimated Discount to be Provided During the Next Twelve Months           16.0    
Discount Provided to EITE Customers           $ 16.7 8.6 0.0
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Retail Customers [Member] | Corporate and Other [Member]                
Regulatory Matters [Line Items]                
After-Tax Change for North Dakota Investment Tax Credits Reversed             7.9 8.8
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Minnesota Power [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Approved Rate Increase - Amount             $ 14.0 $ 15.0
Electric Rates [Member] | MPUC [Member] | Tax Cuts and Jobs Act [Member] | Minnesota Power [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Amortization Period of the Income Tax Benefits from the Remeasurement of Deferred Income Tax Assets and Liabilities Resulting from the TCJA Excluding the Related Property Plant and Equipment           10 years    
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member]                
Regulatory Matters [Line Items]                
Number of Customers | Customers           16    
Notice Required to Terminate (Years) | Years           3    
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contract (Termination Effective June 2019) [Member]                
Regulatory Matters [Line Items]                
Average Monthly Demand (MW) | MW           29    
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member]                
Regulatory Matters [Line Items]                
Number of Customers | Customers           3    
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (expire December 2024) [Member]                
Regulatory Matters [Line Items]                
Number of Customers | Customers           14    
Electric Rates [Member] | PSCW [Member] | Tax Cuts and Jobs Act [Member] | SWL&P [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Amortization Period of the Income Tax Benefits from the Remeasurement of Deferred Income Tax Assets and Liabilities Resulting from the TCJA Excluding the Related Property Plant and Equipment 4 years              
Electric Rates [Member] | PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Requested Average Rate Increase   2.00%            
Natural Gas Rates [Member] | PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Requested Average Rate Increase   2.30%            
Water Rates [Member] | PSCW [Member] | 2018 Wisconsin General Rate Case [Member] | SWL&P [Member] | Retail Customers [Member]                
Regulatory Matters [Line Items]                
Requested Average Rate Increase   8.30%