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Acquisitions - Impacts of Integrys Acquisition (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 29, 2015
Business Acquisition [Line Items]        
Severance expense $ 29.4      
ATC        
Business Combinations [Abstract]        
Equity interest in ATC       60.00%
Integrys        
Business Acquisition [Line Items]        
Acquisition costs   $ 3.5 $ 107.6  
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