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Business Segments (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2016
USD ($)
a
Mar. 31, 2015
USD ($)
Dec. 31, 2015
USD ($)
Business Segments [Line Items]      
Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments During the third quarter of 2015, management updated our reportable segment presentation to reflect the manner in which we operate, assess, and allocate resources after our recent acquisitions. We now present three reportable segments, Regulated Operations, ALLETE Clean Energy and U.S. Water Services. Prior period amounts have been revised to conform with the current business segment presentation.    
Number of Reportable Segments 3    
Operating Revenue $ 333.8 $ 320.0  
Net Income (Loss) Attributable to ALLETE 45.9 39.9  
Assets [1] $ 4,872.7   $ 4,894.5
Regulated Operations [Member] [Member]      
Business Segments [Line Items]      
Number of Operating Segments 3    
Operating Revenue $ 252.3 262.8  
Net Income (Loss) Attributable to ALLETE [2] 42.4 41.0  
Assets [1] 3,822.3   3,853.1
ALLETE Clean Energy [Member]      
Business Segments [Line Items]      
Operating Revenue 23.6 12.4  
Net Income (Loss) Attributable to ALLETE 6.1 2.5  
Assets [1] 498.1   501.5
U.S. Water Services [Member]      
Business Segments [Line Items]      
Operating Revenue 32.4 15.5  
Net Income (Loss) Attributable to ALLETE (0.5) (0.1)  
Assets $ 256.6   258.3
Corporate and Other [Member]      
Business Segments [Line Items]      
Number of Operating Segments 2    
Land in Minnesota (Acres) | a 5,000    
Operating Revenue $ 25.5 29.3  
Net Income (Loss) Attributable to ALLETE [2] (2.1) $ (3.5)  
Assets $ 295.7   $ 281.6
[1] As a result of revised accounting guidance adopted in the first quarter of 2016, we reclassified unamortized debt issuance costs from Other Non-Current Assets to Long-Term Debt on the Consolidated Balance Sheet. Prior period segment assets have been revised to conform to the current presentation. (See Note 1. Operations and Significant Accounting Policies.)
[2] In 2015, an intercompany loan agreement was entered into and interest expense was allocated to certain subsidiaries which is eliminated in consolidation. Prior period segment results have been revised to conform to the current presentation as if the intercompany loan existed as of January 1, 2015.