Derivative Instruments and Hedging Activities (Outstanding Derivative Instruments) (Details)	12 Months Ended
	Dec. 31, 2014 MWh MMBTU gal	Sep. 30, 2015 MWh
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Purchase of Units Derivative Instruments Financial Transmission Rights	10,500	24,700
Sale of Units Derivative Instruments Financial Transmission Rights	0	0
Derivative, Nonmonetary Notional Amount MWh	10,500	24,700
Not Designated as Hedging Instrument [Member] | Commodity Contract - Heating Oil [Member]
Purchase of Volume Units Derivative Instruments Heating Oil Futures | gal	378,000
Sale of Volume Units Derivative Instruments Heating Oil Futures | gal	0
Derivative, Nonmonetary Notional Amount, Volume | gal	378,000
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Purchase of Units Derivative Instruments Natural Gas	200,000
Sale of Units Derivative Instruments Natural Gas	0
Derivative, Nonmonetary Notional Amount, Energy Measure	200,000
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Purchase of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged	1,725,200	5,309,700
Sales of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged	(2,707,800)	(1,850,300)
Derivative, Nonmonetary Notional Amount MWh	(982,600)	3,459,400
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	10,500	24,700
Sale of Units Derivative Instruments Financial Transmission Rights	0	0
Derivative, Nonmonetary Notional Amount MWh	10,500	24,700
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Purchase of Volume Units Derivative Instruments Heating Oil Futures | gal	378,000
Sale of Volume Units Derivative Instruments Heating Oil Futures | gal	0
Derivative, Nonmonetary Notional Amount, Volume | gal	378,000
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Purchase of Units Derivative Instruments Natural Gas | MMBTU	200,000
Sale of Units Derivative Instruments Natural Gas | MMBTU	0
Derivative, Nonmonetary Notional Amount, Energy Measure	200,000
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	1,725,200	5,309,700
Sales of Units Derivative Instruments Forward Power Contracts Not Designated as Hedged	(2,804,000)	(1,916,500)
Derivative, Nonmonetary Notional Amount MWh	(1,078,800)	3,393,200
Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Purchase of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge	175,000	1,361,200
Sales of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge	(2,991,000)	(7,857,700)
Derivative, Nonmonetary Notional Amount MWh	(2,816,000)	(6,496,500)
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	175,000	1,361,200
Sales of Units Derivative Instruments Forward Power Contracts Designated as Cash Flow Hedge	(2,991,000)	(7,857,700)
Derivative, Nonmonetary Notional Amount MWh	(2,816,000)	(6,496,500)