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Regulatory Matters - Ohio (Details)
$ in Millions
 9 Months Ended
Oct. 12, 2016
USD ($)
Apr. 15, 2016
Aug. 07, 2013
USD ($)
auction
Sep. 30, 2017
USD ($)
Regulatory Matters [Line Items]        
Proposed goal to reduce CO2 pollution (percent)       90.00%
Ohio        
Regulatory Matters [Line Items]        
Energy efficient portfolio plan term   3 years    
Estimated cost of plans       $ 268.0
Credit to non-shopping customers     $ 43.4  
Ohio | PUCO        
Regulatory Matters [Line Items]        
Proposed goal to reduce CO2 pollution (percent) 90.00%      
Number of renewable energy auctions | auction     1  
Ohio | Distribution Modernization Rider        
Regulatory Matters [Line Items]        
Amount of rider valuation $ 558.0      
Period of rider valuation 8 years      
Ohio | Distribution Modernization Rider | PUCO        
Regulatory Matters [Line Items]        
Annual revenue cap for rider $ 132.5      
Recovery period 3 years      
Possible extension period 2 years      
Approved amount for rider $ 204.0      
Excessive earnings test exclusion period 3 years      
Renewal period for excessive earnings test exclusion period 2 years      
Ohio | Delivery Capital Recovery Rider | PUCO        
Regulatory Matters [Line Items]        
Annual revenue cap for rider $ 30.0      
Annual revenue cap for rider for years three through six 20.0      
Annual revenue cap for rider for years six through eight 15.0      
Ohio | Energy Conservation, Economic Development and Job Retention | PUCO        
Regulatory Matters [Line Items]        
Contribution amount $ 51.0