Derivative Instruments and Hedging Activities (Outstanding Derivative Instruments) (Details)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) MMBTU MWh	Dec. 31, 2016 USD ($)	Mar. 29, 2018 USD ($)
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] | Forward Contract Power [Member]
Purchase of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge | MWh		678,500
Sales of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge | MWh		(1,667,000)
Derivative, Nonmonetary Notional Amount MWh | MWh		(988,500)
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Sale of Derivative Instruments Interest Rate Swap	0	$ 0
Purchase of Derivative Instruments Interest Rate Swap	140,000,000	200,000,000
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
Not Designated as Hedging Instrument [Member]
Derivative, Gain (Loss) on Derivative, Net	900,000	3,200,000	$ (900,000)
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	700,000	$ 400,000	(300,000)
Purchase of Units Derivative Instruments Financial Transmission Rights | MWh		2,100
Sale of Units Derivative Instruments Financial Transmission Rights | MWh		0
Derivative, Nonmonetary Notional Amount MWh | MWh		2,100
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	200,000	$ 1,600,000	2,600,000
Purchase of Units Derivative Instruments Natural Gas | MMBTU		3,322,500
Sale of Units Derivative Instruments Natural Gas | MMBTU		(390,000)
Derivative, Nonmonetary Notional Amount MWh | MMBTU		2,932,500
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 0	$ 1,200,000	$ (3,200,000)
Purchase of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged | MWh		871,000
Sales of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged | MWh		(765,600)
Derivative, Nonmonetary Notional Amount MWh | MWh		105,400