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Regulatory Matters - Ohio (Details)
$ in Millions
 3 Months Ended
Nov. 14, 2016
USD ($)
Oct. 12, 2016
USD ($)
Apr. 15, 2016
Aug. 07, 2013
USD ($)
auction
Mar. 31, 2017
USD ($)
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