Derivative Instruments and Hedging Activities (Outstanding Derivative Instruments) (Details)	Mar. 31, 2017 USD ($) MMBTU MWh	Dec. 31, 2016 USD ($) MMBTU MWh
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Purchase of Units Derivative Instruments Financial Transmission Rights	900	2,300
Sale of Units Derivative Instruments Financial Transmission Rights	0	0
Derivative, Nonmonetary Notional Amount MWh	900	2,300
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Purchase of Units Derivative Instruments Natural Gas | MMBTU	17,127,500	1,590,000
Sale of Units Derivative Instruments Natural Gas | MMBTU	0	0
Derivative, Nonmonetary Notional Amount MWh | MMBTU	17,127,500	1,590,000
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Purchase of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged	1,727,300	2,568,300
Sales of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged	(2,052,600)	(2,020,900)
Derivative, Nonmonetary Notional Amount MWh	(325,300)	547,400
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Purchase of Units Derivative Instruments Financial Transmission Rights	900	2,300
Sale of Units Derivative Instruments Financial Transmission Rights	0	0
Derivative, Nonmonetary Notional Amount MWh	900	2,300
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Purchase of Units Derivative Instruments Natural Gas | MMBTU	17,127,500	1,590,000
Sale of Units Derivative Instruments Natural Gas | MMBTU	0	0
Derivative, Nonmonetary Notional Amount MWh | MMBTU	17,127,500	1,590,000
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Purchase of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged	1,727,300	2,568,300
Sales of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged	(2,057,900)	(2,037,500)
Derivative, Nonmonetary Notional Amount MWh	(330,600)	530,800
Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Purchase of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge	1,276,600	342,900
Sales of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge	(6,250,900)	(9,974,500)
Derivative, Nonmonetary Notional Amount MWh	(4,974,300)	(9,631,600)
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Purchase of Derivative Instruments Interest Rate Swap | $	$ 200,000,000	$ 200,000,000
Sale of Derivative Instruments Interest Rate Swap | $	0	0
Derivative, Notional Amount, Purchase (Sales), Net | $	$ 200,000,000	$ 200,000,000
Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Purchase of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge	1,276,600	342,900
Sales of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge	(6,250,900)	(9,974,500)
Derivative, Nonmonetary Notional Amount MWh	(4,974,300)	(9,631,600)
Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Swap [Member]
Purchase of Derivative Instruments Interest Rate Swap | $	$ 200,000,000	$ 200,000,000
Sale of Derivative Instruments Interest Rate Swap | $	0	0
Derivative, Notional Amount, Purchase (Sales), Net | $	$ 200,000,000	$ 200,000,000