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OPERATING REVENUES - DISAGGREGATION OF ELECTRIC UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
contract
performance_obligations
Jun. 30, 2017
USD ($)
Disaggregation of Operating Revenues        
Revenues $ 1,672.5 $ 1,631.5 $ 3,959.0 $ 3,936.0
Electric        
Disaggregation of Operating Revenues        
Number of performance obligations | performance_obligations     1  
Number of contracts | contract     1  
Number of days payment is due     30 days  
Electric | Wholesale        
Disaggregation of Operating Revenues        
Number of performance obligations | performance_obligations     2  
Public Utility        
Disaggregation of Operating Revenues        
Revenues 1,665.9   $ 3,932.2  
Revenues from contracts with customers        
Disaggregation of Operating Revenues        
Revenues 1,673.6   3,963.6  
Revenues from contracts with customers | Electric        
Disaggregation of Operating Revenues        
Revenues 1,084.2   2,151.9  
Revenues from contracts with customers | Public Utility | Transferred over time        
Disaggregation of Operating Revenues        
Revenues 1,667.3   3,937.2  
Revenues from contracts with customers | Public Utility | Electric | Transferred over time        
Disaggregation of Operating Revenues        
Revenues 1,084.2   2,151.9  
Wisconsin | Public Utility        
Disaggregation of Operating Revenues        
Revenues 1,325.5   2,914.6  
Wisconsin | Revenues from contracts with customers | Public Utility | Transferred over time        
Disaggregation of Operating Revenues        
Revenues 1,320.6   2,906.3  
Wisconsin | Revenues from contracts with customers | Public Utility | Electric | Transferred over time        
Disaggregation of Operating Revenues        
Revenues 1,084.2   2,151.9  
Wisconsin | Revenues from contracts with customers | Public Utility | Electric | Transferred over time | Residential        
Disaggregation of Operating Revenues        
Revenues 393.7   778.0  
Wisconsin | Revenues from contracts with customers | Public Utility | Electric | Transferred over time | Small commercial and industrial        
Disaggregation of Operating Revenues        
Revenues 353.3   684.0  
Wisconsin | Revenues from contracts with customers | Public Utility | Electric | Transferred over time | Large commercial and industrial        
Disaggregation of Operating Revenues        
Revenues 241.6   445.5  
Wisconsin | Revenues from contracts with customers | Public Utility | Electric | Transferred over time | Other        
Disaggregation of Operating Revenues        
Revenues 7.2   14.9  
Wisconsin | Revenues from contracts with customers | Public Utility | Electric | Transferred over time | Total retail revenues        
Disaggregation of Operating Revenues        
Revenues 995.8   1,922.4  
Wisconsin | Revenues from contracts with customers | Public Utility | Electric | Transferred over time | Wholesale        
Disaggregation of Operating Revenues        
Revenues 58.4   113.3  
Wisconsin | Revenues from contracts with customers | Public Utility | Electric | Transferred over time | Resale        
Disaggregation of Operating Revenues        
Revenues 25.1   98.9  
Wisconsin | Revenues from contracts with customers | Public Utility | Electric | Transferred over time | Steam        
Disaggregation of Operating Revenues        
Revenues 4.5   14.2  
Wisconsin | Revenues from contracts with customers | Public Utility | Electric | Transferred over time | Other utility revenues        
Disaggregation of Operating Revenues        
Revenues $ 0.4   $ 3.1