Derivative Instruments and Hedging Activities (Outstanding Derivative Instruments) (Details) - USD ($)	Jun. 30, 2018	Mar. 29, 2018	Dec. 31, 2017
Sale of Derivative Instruments Interest Rate Swap		$ (60,000,000)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Sale of Derivative Instruments Interest Rate Swap		$ (60,000,000)
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount, Purchase (Sales), Net	$ 140,000,000		$ 200,000,000
Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount, Purchase (Sales), Net	$ 140,000,000		$ 200,000,000