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Regulatory Matters - Ohio (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Jul. 15, 2019
Nov. 09, 2018
Jun. 01, 2016
Mar. 31, 2020
Dec. 31, 2018
Dec. 31, 2017
Regulatory Matters [Line Items]            
Proposed goal to reduce CO2 pollution (percent)       90.00%    
OHIO | PUCO            
Regulatory Matters [Line Items]            
Proposed goal to reduce CO2 pollution (percent)     90.00%      
OHIO | PUCO | Energy Conservation, Economic Development and Job Retention            
Regulatory Matters [Line Items]            
Contribution amount     $ 51.0      
OHIO | Distribution Modernization Rider | PUCO            
Regulatory Matters [Line Items]            
Annual revenue cap for rider           $ 132.5
Recovery period           3 years
Approved amount for rider         $ 168.0  
OHIO | Delivery Capital Recovery Rider | PUCO            
Regulatory Matters [Line Items]            
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 | Distribution Platform Modernization Plan | PUCO            
Regulatory Matters [Line Items]            
Approved amount of annual increase (decrease)   $ 516.0        
Period of grid modernization plan   3 years        
The Ohio Companies | OHIO | OCC DMR Refund | PUCO            
Regulatory Matters [Line Items]            
Loss contingency, damages sought $ 42.0