Derivative Instruments and Hedging Activities (Outstanding Derivative Instruments) (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) MMBTU MWh	Dec. 31, 2015 USD ($) MWh	Dec. 31, 2014 USD ($)
Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Purchase of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge | MWh	342,900	1,676,700
Sales of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge | MWh	(9,974,500)	(7,795,800)
Derivative, Nonmonetary Notional Amount MWh | MWh	(9,631,600)	(6,119,100)
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Sale of Derivative Instruments Interest Rate Swap	$ 0
Purchase of Derivative Instruments Interest Rate Swap	200,000,000
Derivative, Notional Amount, Purchase (Sales), Net	$ 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 | MWh	342,900	1,676,700
Sales of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge | MWh	(9,974,500)	(7,795,800)
Derivative, Nonmonetary Notional Amount MWh | MWh	(9,631,600)	(6,119,100)
Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Swap [Member]
Sale of Derivative Instruments Interest Rate Swap	$ 0
Purchase of Derivative Instruments Interest Rate Swap	200,000,000
Derivative, Notional Amount, Purchase (Sales), Net	200,000,000
Not Designated as Hedging Instrument [Member]
Derivative, Gain (Loss) on Derivative, Net	(900,000)	$ (16,000,000)	$ (6,100,000)
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (300,000)	$ 100,000	(100,000)
Purchase of Units Derivative Instruments Financial Transmission Rights | MWh	2,300	10,200
Sale of Units Derivative Instruments Financial Transmission Rights | MWh	0	0
Derivative, Nonmonetary Notional Amount MWh | MWh	2,300	10,200
Not Designated as Hedging Instrument [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 0	$ 100,000	(700,000)
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 2,600,000	0	(200,000)
Purchase of Units Derivative Instruments Natural Gas | MMBTU	1,590,000
Sale of Units Derivative Instruments Natural Gas | MMBTU	0
Derivative, Nonmonetary Notional Amount MWh | MMBTU	1,590,000
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (3,200,000)	$ (16,200,000)	(5,100,000)
Purchase of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged | MWh	2,568,300	5,049,900
Sales of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged | MWh	(2,020,900)	(1,663,000)
Derivative, Nonmonetary Notional Amount MWh | MWh	547,400	3,386,900
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (1,700,000)	$ (16,000,000)	(5,500,000)
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (300,000)	$ 100,000	(100,000)
Purchase of Units Derivative Instruments Financial Transmission Rights | MWh	2,300	10,200
Sale of Units Derivative Instruments Financial Transmission Rights | MWh	0	0
Derivative, Nonmonetary Notional Amount MWh | MWh	2,300	10,200
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 0	$ 100,000	(700,000)
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 2,600,000	0	(200,000)
Purchase of Units Derivative Instruments Natural Gas | MMBTU	1,590,000
Sale of Units Derivative Instruments Natural Gas | MMBTU	0
Derivative, Nonmonetary Notional Amount MWh | MMBTU	1,590,000
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (4,000,000)	$ (16,200,000)	(4,500,000)
Purchase of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged | MWh	2,568,300	5,049,900
Sales of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged | MWh	(2,037,500)	(1,665,700)
Derivative, Nonmonetary Notional Amount MWh | MWh	530,800	3,384,200
Sales [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (17,300,000)	$ 27,400,000
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] | Commodity Contract - Heating Oil [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	(17,300,000)	27,400,000
Sales [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	(18,100,000)	27,400,000	700,000
Sales [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0	0	0
Sales [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	0	0	0
Sales [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	0	0	0
Sales [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(18,100,000)	27,400,000	700,000
Fuel [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Gain (Loss) on Derivative, Net			(600,000)
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			(600,000)
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			0
Fuel [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net			(600,000)
Fuel [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net			0
Fuel [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net			(600,000)
Fuel [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0
Fuel [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net			0
Purchased Power [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Gain (Loss) on Derivative, Net	16,400,000	(43,500,000)	(5,400,000)
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	(300,000)	100,000	(100,000)
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	0	0	0
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	2,600,000	0	(200,000)
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	14,100,000	(43,600,000)	(5,100,000)
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	16,400,000	(43,500,000)	(5,500,000)
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	(300,000)	100,000	(100,000)
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	0	0	0
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	2,600,000	0	(200,000)
Purchased Power [Member] | Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 14,100,000	$ (43,600,000)	$ (5,200,000)