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Contractual Obligations, Commercial Commitments and Contingencies (Narative) (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]      
Due to third parties, current $ 0.0 $ 0.0 $ 0.9
AES Ohio Generation [Member]      
Public Utility, Property, Plant and Equipment [Line Items]      
Third party guarantees $ 21.0    
DPLE [Member]      
Public Utility, Property, Plant and Equipment [Line Items]      
Third party guarantees   $ 23.6  
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% 4.90%  
Equity Ownership Interest - Guaranteed Portion of Long-term Debt Obligation $ 67.2    
Equity ownership interest aggregate cost   $ 68.1  
Electric Generation Company [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]      
Public Utility, Property, Plant and Equipment [Line Items]      
Debt obligation $ 1,371.3 $ 1,389.6  
Other OVEC Member [Member]      
Public Utility, Property, Plant and Equipment [Line Items]      
Equity ownership interest 4.85% 4.85%