Derivative Instruments and Hedging Activities (Outstanding Derivative Instruments) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) MMBTU MWh	Dec. 31, 2016 USD ($) MMBTU MWh	Dec. 31, 2015 USD ($)
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	342,900
Sales of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge | MWh	(1,667,000)	(9,974,500)
Derivative, Nonmonetary Notional Amount MWh | MWh	(988,500)	(9,631,600)
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	200,000,000	200,000,000
Derivative, Notional Amount, Purchase (Sales), Net	200,000,000	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	200,000,000
Not Designated as Hedging Instrument [Member]
Derivative, Gain (Loss) on Derivative, Net	3,200,000	(900,000)	$ (16,000,000)
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 400,000	$ (300,000)	100,000
Purchase of Units Derivative Instruments Financial Transmission Rights | MWh	2,100	2,300
Sale of Units Derivative Instruments Financial Transmission Rights | MWh	0	0
Derivative, Nonmonetary Notional Amount MWh | MWh	2,100	2,300
Not Designated as Hedging Instrument [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net			100,000
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 1,600,000	$ 2,600,000	0
Purchase of Units Derivative Instruments Natural Gas | MMBTU	3,322,500	1,590,000
Sale of Units Derivative Instruments Natural Gas | MMBTU	(390,000)	0
Derivative, Nonmonetary Notional Amount MWh | MMBTU	2,932,500	1,590,000
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 1,200,000	$ (3,200,000)	(16,200,000)
Purchase of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged | MWh	871,000	2,568,300
Sales of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged | MWh	(765,600)	(2,020,900)
Derivative, Nonmonetary Notional Amount MWh | MWh	105,400	547,400
Sales [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (1,200,000)	$ (17,300,000)
Sales [Member] | Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0	0
Sales [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	0	0
Sales [Member] | Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(1,200,000)	(17,300,000)
Fuel [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Gain (Loss) on Derivative, Net			27,400,000
Fuel [Member] | Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net			0
Fuel [Member] | Not Designated as Hedging Instrument [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net			0
Fuel [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0
Fuel [Member] | Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net			27,400,000
Purchased Power [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Gain (Loss) on Derivative, Net	4,400,000	16,400,000	(43,500,000)
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	400,000	(300,000)	100,000
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net			0
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	1,600,000	2,600,000	0
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 2,400,000	$ 14,100,000	$ (43,600,000)