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Contractual Obligations, Commercial Commitments and Contingencies (Narrative) (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]    
Due to third parties, current $ 1.4 $ 0.9
THE DAYTON POWER AND LIGHT COMPANY [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Debt maturity, earliest 2019  
Debt maturity, latest 2040  
AES Ohio Generation [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Third party guarantees $ 35.0  
Debt Obligation on 4.9% Equity Ownership [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Equity ownership interest 4.90%  
Equity ownership interest aggregate cost $ 69.4  
Electric Generation Company [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Debt obligation $ 1,415.5