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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Jan. 01, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Sep. 30, 2019
Proceeds from Sale of Debt Securities, Available-for-sale   $ 0.5      
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax   0.4      
THE DAYTON POWER AND LIGHT COMPANY [Member]          
Asset Retirement Obligation   4.7 $ 8.0 $ 8.2 $ 4.7
Asset Retirement Obligations, Noncurrent   4.7 8.0    
Unrealized gains and immaterial losses on Master Trust assets in AOCI, after tax   0.0 $ 0.4 $ 0.2  
AOCI reclassed to Retained Earnings before tax $ 1.7        
AOCI reclassed to Retained Earnings, net of tax $ 1.1 (1.1)      
Carrying Value [Member] | Debt [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]          
Lines of Credit, Fair Value Disclosure   $ 586.1     $ 574.2