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Fair Value - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]      
Equity Method Investment, Carrying Amount $ 118.7 $ 135.6  
Goodwill, Carrying Amount 148.3 131.2 $ 130.6
Goodwill, Impairment Charge 0.0 3.3 [1] 0.0
Intangible Assets, Carrying Amount 77.6 82.2  
Intangible Assets, Indicators of Impairment 0.0 0.0 0.0
Impairment of Real Estate 0.0 0.0 36.3
U.S. Water Services [Member]      
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]      
Goodwill, Carrying Amount 148.3 131.2 127.3
Goodwill, Impairment Charge 0.0 0.0 [1] 0.0
ALLETE Clean Energy [Member]      
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]      
Goodwill, Carrying Amount $ 0.0 0.0 3.3
Goodwill, Impairment Charge   3.3 [1] 0.0
ATC [Member]      
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]      
Ownership Percentage 8.00%    
Equity Method Investment, Carrying Amount $ 118.7 135.6 124.5
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member]      
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]      
Intangible Assets, Carrying Amount 77.6 82.2  
Intangible Assets, Indicators of Impairment 0.0    
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | U.S. Water Services [Member]      
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]      
Goodwill, Carrying Amount 148.3 131.2  
Goodwill, Impairment Charge 0.0 0.0 0.0
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | Corporate and Other [Member]      
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]      
Impairment of Real Estate     $ 36.3
Property, Plant and Equipment, Indicators of Impairment $ 0.0    
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | ATC [Member] | Regulated Operations [Member]      
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]      
Ownership Percentage 8.00%    
Equity Method Investment, Carrying Amount $ 118.7 135.6  
Equity Method Investment, Indicators of Impairment $ 0.0 $ 0.0  
[1] The facts and circumstances that led to an impairment of ALLETE Clean Energy’s goodwill primarily relate to lower estimated energy prices in periods not under PSAs. Impairment Charge is included in Operating Expenses – Other on the Consolidated Statement of Income. (See Note 1. Operations and Significant Accounting Policies.) ALLETE Clean Energy’s goodwill was primarily related to the acquisition of Storm Lake II in 2014.