Derivative Instruments and Hedging Activities (Outstanding Derivative Instruments) (Details)	Mar. 31, 2018 USD ($) MWh	Mar. 29, 2018 USD ($)	Dec. 31, 2017 USD ($) MMBTU MWh
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)
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Purchase of Units Derivative Instruments Financial Transmission Rights	100		2,100
Sale of Units Derivative Instruments Financial Transmission Rights	0		0
Derivative, Nonmonetary Notional Amount MWh	100		2,100
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
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]
Purchase of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged	73,200		871,000
Sales of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged	0		(765,600)
Derivative, Nonmonetary Notional Amount MWh	73,200		105,400
Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Purchase of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge			678,500
Sales of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge			(1,667,000)
Derivative, Nonmonetary Notional Amount MWh			(988,500)
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Purchase of Derivative Instruments Interest Rate Swap | $	$ 140,000,000		$ 200,000,000
Sale of Derivative Instruments Interest Rate Swap | $	0		0
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