Derivative Instruments and Hedging Activities (Narrative) (Details)	Jun. 30, 2018 USD ($) Number_of_interest_rate_swaps	Mar. 29, 2018 USD ($)	Dec. 31, 2017 USD ($)
Sale of Derivative Instruments Interest Rate Swap		$ 60,000,000
THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Number of Financial Covenants | Number_of_interest_rate_swaps	2
Long-term Debt, Gross	$ 596,000,000
Sale of Derivative Instruments Interest Rate Swap		$ 60,000,000
One Point One Three To One Point One Seven Bonds Maturing In August Two Thousand Twenty [Member]
Long-term Debt, Gross	140,000,000
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	140,000,000
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative, Notional Amount, Purchase (Sales), Net	140,000,000		$ 200,000,000
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Notional Amount, Purchase (Sales), Net	$ 140,000,000		$ 200,000,000