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Commitments, Guarantees and Contingencies - Other Matters (Details)
$ in Millions
12 Months Ended
Dec. 31, 2015
USD ($)
Years
Dec. 31, 2014
U.S. Water Services [Member] | Letters of Credit [Member]    
Guarantor Obligations [Line Items]    
Collateral $ 0.8  
BNI Energy Reclamation Liability [Member]    
Guarantor Obligations [Line Items]    
Estimated Obligation 47.5  
BNI Energy Reclamation Liability [Member] | Letters of Credit [Member]    
Guarantor Obligations [Line Items]    
Collateral 0.6  
BNI Energy Reclamation Liability [Member] | Surety Bonds [Member]    
Guarantor Obligations [Line Items]    
Collateral 49.9  
ALLETE Properties Development and Maintenance Obligations [Member]    
Guarantor Obligations [Line Items]    
Estimated Obligation 6.3  
ALLETE Properties Development and Maintenance Obligations [Member] | Surety Bonds and Letters of Credit [Member]    
Guarantor Obligations [Line Items]    
Collateral 10.3  
Town Center Community Development District Capital Improvement Bonds [Member]    
Guarantor Obligations [Line Items]    
Bonds $ 26.4  
Bond Interest Rate 6.00%  
Bond Term (Years) | Years 31  
Ownership Percentage of Benefited Property 72.00% 72.00%
Annual Assessment $ 1.4  
Palm Coast Park Community Development District Special Assessment Bonds[Member]    
Guarantor Obligations [Line Items]    
Bonds $ 31.8  
Bond Interest Rate 5.70%  
Bond Term (Years) | Years 31  
Ownership Percentage of Benefited Property 89.00% 93.00%
Annual Assessment $ 2.1