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Regulatory Matters - Utility Rates (Details)
$ in Millions
3 Months Ended
Jan. 01, 2013
Jun. 01, 2011
Mar. 31, 2016
USD ($)
Customers
Years
MW
Mar. 31, 2015
USD ($)
Minnesota Power [Member] | Bison Wind Energy Center [Member]        
Regulatory Matters [Line Items]        
Generating Capacity (MW) | MW     497  
PSCW [Member] | 2012 Wisconsin Rate Case [Member] | SWL&P [Member] | Retail [Member]        
Regulatory Matters [Line Items]        
Approved Return on Common Equity 10.90%      
Electric Rates [Member] | MPUC [Member] | 2010 Minnesota Rate Case [Member] | Minnesota Power [Member] | Retail [Member]        
Regulatory Matters [Line Items]        
Approved Return on Common Equity   10.38%    
Approved Equity Ratio   54.29%    
Electric Rates [Member] | MPUC [Member] | Minnesota Cost Recovery Riders [Member] | Minnesota Power [Member] | Retail [Member]        
Regulatory Matters [Line Items]        
Revenue from Cost Recovery Riders | $     $ 25.4 $ 20.7
Electric Rates [Member] | MPUC [Member] | Renewable Cost Recovery Rider [Member] | Minnesota Power [Member] | Retail [Member]        
Regulatory Matters [Line Items]        
North Dakota Investment Tax Credits | $     8.0  
Electric Rates [Member] | MPUC [Member] | Annual Automatic Adjustment of Charges [Member] | Minnesota Power [Member] | Retail [Member]        
Regulatory Matters [Line Items]        
Retail Fuel Cost Recovery Collected but Subject to Refund | $     $ 350.0  
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  
Length of Notice Required to Terminate Contract (Years) | Years     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] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Expire December 2024) [Member] | Maximum [Member]        
Regulatory Matters [Line Items]        
Change in Annual Capacity Charge Per Contract - Percentage     2.00%  
Electric Rates [Member] | FERC [Member] | FERC-Approved Wholesale Rates [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Wholesale Electric Contracts (Expire December 2024) [Member] | Minimum [Member]        
Regulatory Matters [Line Items]        
Change in Annual Capacity Charge Per Contract - Percentage     (1.00%)  
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]        
Number of Customers | Customers     1  
Average Monthly Demand (MW) | MW     29