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Contractual Obligations, Commercial Commitments and Contingencies (Narative) (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2015
Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]    
Third party guarantees $ 19.3  
Due to third parties, current $ 1.1  
THE DAYTON POWER AND LIGHT COMPANY [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Debt maturity, earliest 2018  
Debt maturity, latest 2040  
Environmental reserves $ 0.7 $ 0.8
DPLER [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Third party guarantees 2.0  
DPLE [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Third party guarantees $ 17.3  
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 $ 73.9  
Electric Generation Company [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]    
Public Utility, Property, Plant and Equipment [Line Items]    
Debt obligation $ 1,507.9