Derivative Instruments and Hedging Activities (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2019 USD ($) 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