Derivative Instruments and Hedging Activities (Classification within the Condensed Consolidated Statements of Results of Operations or Balance Sheets of the Gains and Losses) (Details) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Change in unrealized gain / (loss)	$ (3.0)	$ (2.2)	$ (4.2)	$ (7.5)
Gain (Loss) on Derivative Instruments, Net, Pretax	(4.6)	(2.0)	(8.6)	(2.8)
Derivative, Gain (Loss) on Derivative, Net	(7.6)	(4.2)	(12.8)	(10.3)
Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)	0.1	(0.2)	0.4	(0.3)
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.2)	0.0	(0.3)	0.1
Derivative, Gain (Loss) on Derivative, Net	(0.1)	(0.2)	0.1	(0.2)
Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.1	0.3	0.2	(1.2)
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.1)	0.1	(0.1)	0.7
Derivative, Gain (Loss) on Derivative, Net	0.0	0.4	0.1	(0.5)
Forward Contract Power [Member]
Change in unrealized gain / (loss)	(3.2)	(2.3)	(4.9)	(6.0)
Gain (Loss) on Derivative Instruments, Net, Pretax	(4.3)	(2.1)	(8.1)	(3.6)
Derivative, Gain (Loss) on Derivative, Net	(7.5)	(4.4)	(13.0)	(9.6)
Natural Gas [Member]
Change in unrealized gain / (loss)			0.1
Gain (Loss) on Derivative Instruments, Net, Pretax			(0.1)
Derivative, Gain (Loss) on Derivative, Net			0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Change in unrealized gain / (loss)	(3.1)	(2.6)	(4.3)	(7.2)
Gain (Loss) on Derivative Instruments, Net, Pretax	(4.6)	(2.0)	(8.6)	(2.2)
Derivative, Gain (Loss) on Derivative, Net	(7.7)	(4.6)	(12.9)	(9.4)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)	0.1	(0.2)	0.4	(0.3)
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.2)	0.0	(0.3)	0.1
Derivative, Gain (Loss) on Derivative, Net	(0.1)	(0.2)	0.1	(0.2)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.1	0.3	0.2	(1.2)
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.1)	0.1	(0.1)	0.7
Derivative, Gain (Loss) on Derivative, Net	0.0	0.4	0.1	(0.5)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Change in unrealized gain / (loss)	(3.3)	(2.7)	(5.0)	(5.7)
Gain (Loss) on Derivative Instruments, Net, Pretax	(4.3)	(2.1)	(8.1)	(3.0)
Derivative, Gain (Loss) on Derivative, Net	(7.6)	(4.8)	(13.1)	(8.7)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Change in unrealized gain / (loss)			0.1
Gain (Loss) on Derivative Instruments, Net, Pretax			(0.1)
Derivative, Gain (Loss) on Derivative, Net			0.0
Regulatory Asset [Member]
Derivative, Gain (Loss) on Derivative, Net		(0.1)	0.1	(0.1)
Regulatory Asset [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net		(0.1)	0.1	(0.1)
Regulatory Asset [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net		0.0	0.0	0.0
Regulatory Asset [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net		0.0	0.0	0.0
Regulatory Asset [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net		(0.1)	0.1	(0.1)
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net		(0.1)	0.1	(0.1)
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net		0.0	0.0	0.0
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net		0.0	0.0	0.0
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Purchased Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(11.0)	(4.0)	(21.8)	(10.1)
Purchased Power [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0	0.0
Purchased Power [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.4	0.1	(0.5)
Purchased Power [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(11.0)	(4.4)	(21.9)	(9.6)
Purchased Power [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	(11.0)	(4.1)	(21.8)	(10.2)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0	0.0
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.4	0.1	(0.5)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(11.0)	(4.5)	(21.9)	(9.7)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Revenue [Member]
Derivative, Gain (Loss) on Derivative, Net	3.5		8.9
Revenue [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0		0.0
Revenue [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0		0.0
Revenue [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	3.5		8.9
Revenue [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	3.4	(0.3)	8.8	1.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0	0.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0	0.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	3.4	(0.3)	8.8	1.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Fuel [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.1)	(0.1)		(0.1)
Fuel [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.1)	(0.1)		(0.1)
Fuel [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0		0.0
Fuel [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 0.0	0.0		0.0
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net		(0.1)	(0.1)	(0.1)
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net		(0.1)	(0.1)	(0.1)
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net		0.0	0.0	0.0
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net		$ 0.0	$ 0.0	$ 0.0