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Regulatory Matters - Electric Rates (Details)
$ in Millions
1 Months Ended 3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
May 25, 2018
USD ($)
Apr. 02, 2018
USD ($)
Jan. 18, 2018
USD ($)
Aug. 09, 2017
Apr. 13, 2017
USD ($)
Dec. 31, 2016
USD ($)
Nov. 30, 2016
USD ($)
Sep. 30, 2018
USD ($)
Customers
Years
MW
Sep. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
Sep. 30, 2018
USD ($)
Customers
Years
MW
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Regulatory Matters [Line Items]                          
Pre-Tax Charge to Fuel, Purchase Power and Gas - Utility               $ 103.2 $ 93.5   $ 300.6 $ 283.2  
MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member]                          
Regulatory Matters [Line Items]                          
Pre-Tax Charge to Fuel, Purchase 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]                          
Expected Decrease in Annual Depreciation Expense               25.0     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%                  
Approved Rate Increase Amount                   $ 2.5      
PSCW [Member] | 2012 Wisconsin General 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%                        
Annual Additional Revenue Generated from Requested Final Rate Increase $ 2.4                        
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               29.1 23.1   81.2 71.7  
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%            
Annual Additional Revenue Generated from Requested Final Rate Increase     $ 13.0       $ 55.0            
Annual Additional Revenue Generated from Requested Interim Rate Increase         $ 32.2 $ 34.7 49.0            
Annual Additional Revenue Generated from Requested Final Rate Increase, Amended             $ 49.0            
Approved Return on Common Equity     9.25%                    
Approved Equity Ratio     53.81%                    
Reserve for Interim Rate Refund               57.5     57.5   32.3
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member] | Minimum [Member]                          
Regulatory Matters [Line Items]                          
Requested Reconsideration of Certain Decisions Representing Annualized Additional Revenue   $ 20.0                      
Electric Rates [Member] | MPUC [Member] | 2016 Minnesota General Rate Case [Member] | Minnesota Power [Member] | Retail Customers [Member] | Maximum [Member]                          
Regulatory Matters [Line Items]                          
Requested Reconsideration of Certain Decisions Representing Annualized Additional Revenue   $ 25.0                      
Electric Rates [Member] | MPUC [Member] | Energy-Intensive Trade-Exposed Customer Rates [Member] | Minnesota Power [Member] | Retail Customers [Member]                          
Regulatory Matters [Line Items]                          
Provided Discounts to EITE Customers               4.1 $ 2.7   12.2 8.6 8.6
Expected Discounts to EITE Customers Annually               $ 15.0     $ 15.0    
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 7.9
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Minnesota Power [Member] | Retail Customers [Member]                          
Regulatory Matters [Line Items]                          
Approved Rate Increase Amount                       $ 14.3 $ 14.3
Electric Rates [Member] | MPUC [Member] | Tax Cuts and Jobs Act [Member] | Minnesota Power [Member] | Retail Customers [Member]                          
Regulatory Matters [Line Items]                          
Amortization Periord 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     16    
Length of Notice Required to Terminate (Years) | Years               3     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     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     3    
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Expire from 2024 through 2029) [Member]                          
Regulatory Matters [Line Items]                          
Number of Customers | Customers               14     14    
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 [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 [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%