Derivative Instruments and Hedging Activities (Narrative) (Details) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Number_of_interest_rate_swaps	Mar. 29, 2018 USD ($)
Sale of Derivative Instruments Interest Rate Swap				$ 60.0
Gain on sale of Derivative Instruments Interest Rate Swap		$ 0.8
THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Number of Financial Covenants | Number_of_interest_rate_swaps			2
Sale of Derivative Instruments Interest Rate Swap				$ 60.0
Gain on sale of Derivative Instruments Interest Rate Swap	$ 0.8
One Point One Three To One Point One Seven Bonds Maturing In August Two Thousand Twenty [Member]
Long-term Debt, Gross			$ 140.0
One Point One Three To One Point One Seven Bonds Maturing In August Two Thousand Twenty [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Long-term Debt, Gross			140.0
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative, Notional Amount, Purchase (Sales), Net			140.0
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Notional Amount, Purchase (Sales), Net			$ 140.0