Derivative Instruments and Hedging Activities (Outstanding Derivative Instruments) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Mar. 29, 2018	Dec. 31, 2017
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]
Sale of Derivative Instruments Interest Rate Swap		$ (60.0)
Gain on sale of Derivative Instruments Interest Rate Swap	0.8
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount, Purchase (Sales), Net	140.0		$ 200.0
Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Swap [Member]
Derivative, Notional Amount, Purchase (Sales), Net	$ 140.0		$ 200.0