Derivative Instruments and Hedging Activities (Narrative) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) Number_of_interest_rate_swaps	Mar. 29, 2018 USD ($)	Dec. 31, 2017 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
Long-term Debt, Gross	$ 594.9
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		$ 200.0
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Notional Amount, Purchase (Sales), Net	$ 140.0		$ 200.0