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Acquisitions - Impacts of Integrys Acquisition (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Jun. 29, 2015
ATC        
Business Combinations [Abstract]        
Equity interest in ATC       60.00%
Integrys        
Business Acquisition [Line Items]        
Acquisition costs $ 3.5 $ 107.6 $ 12.5  
Severance expense   24.9    
Severance payments $ 7.5 16.9    
Revenue attributable to Integrys   1,416.8    
Net income attributable to Integrys   65.9    
Integrys | ATC        
Business Combinations [Abstract]        
Equity interest in ATC 34.00%      
Integrys | Wisconsin        
Business Acquisition [Line Items]        
Severance expense   11.1    
Integrys | Illinois        
Business Acquisition [Line Items]        
Severance expense   0.9    
Integrys | Other States        
Business Acquisition [Line Items]        
Severance expense   0.1    
Integrys | Corporate and Other        
Business Acquisition [Line Items]        
Severance expense   $ 12.8