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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Change in unrealized gain / (loss)	$ 0.9	$ (3.0)	$ 2.7	$ (4.2)
Realized loss	(2.3)	(4.6)	(5.0)	(8.6)
Derivative, Gain (Loss) on Derivative, Net	(1.4)	(7.6)	(2.3)	(12.8)
Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)		0.1		0.4
Realized loss		(0.2)		(0.3)
Derivative, Gain (Loss) on Derivative, Net		(0.1)		0.1
Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.0	0.1	0.4	0.2
Realized loss	(0.1)	(0.1)	(0.4)	(0.1)
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0.0	0.0	0.1
Forward Contract Power [Member]
Change in unrealized gain / (loss)	1.2	(3.2)	2.3	(4.9)
Realized loss	(2.4)	(4.3)	(5.3)	(8.1)
Derivative, Gain (Loss) on Derivative, Net	(1.2)	(7.5)	(3.0)	(13.0)
Natural Gas [Member]
Change in unrealized gain / (loss)	(0.3)		0.0	0.1
Realized loss	0.2		0.7	(0.1)
Derivative, Gain (Loss) on Derivative, Net	(0.1)		0.7	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Change in unrealized gain / (loss)	0.9	(3.1)	2.7	(4.3)
Realized loss	(2.3)	(4.6)	(5.0)	(8.6)
Derivative, Gain (Loss) on Derivative, Net	(1.4)	(7.7)	(2.3)	(12.9)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)		0.1		0.4
Realized loss		(0.2)		(0.3)
Derivative, Gain (Loss) on Derivative, Net		(0.1)		0.1
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.0	0.1	0.4	0.2
Realized loss	(0.1)	(0.1)	(0.4)	(0.1)
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0.0	0.0	0.1
THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Change in unrealized gain / (loss)	1.2	(3.3)	2.3	(5.0)
Realized loss	(2.4)	(4.3)	(5.3)	(8.1)
Derivative, Gain (Loss) on Derivative, Net	(1.2)	(7.6)	(3.0)	(13.1)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Change in unrealized gain / (loss)	(0.3)		0.0	0.1
Realized loss	0.2		0.7	(0.1)
Derivative, Gain (Loss) on Derivative, Net	(0.1)		0.7	0.0
Regulatory Asset [Member]
Derivative, Gain (Loss) on Derivative, Net				0.1
Regulatory Asset [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net				0.1
Regulatory Asset [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net				0.0
Regulatory Asset [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net				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
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net				0.1
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net				0.0
Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net				0.0
Regulatory Asset [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	(10.4)	3.5	(13.1)	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	0.0	0.0
Revenue [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(10.4)	3.5	(13.1)	8.9
Revenue [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0		0.0	0.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	(10.4)	3.4	(13.1)
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net		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
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(10.4)	3.4	(13.1)
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0		0.0
Purchased Power [Member]
Derivative, Gain (Loss) on Derivative, Net	9.0	(11.0)	10.8	(21.8)
Purchased Power [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net		0.0		0.0
Purchased Power [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0.0	0.0	0.1
Purchased Power [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	9.2	(11.0)	10.1	(21.9)
Purchased Power [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.1)		0.7	0.0
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	9.0	(11.0)	10.8	(21.8)
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
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0.0	0.0	0.1
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	9.2	(11.0)	10.1	(21.9)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (0.1)	(0.1)	$ 0.7	0.0
Fuel [Member]
Derivative, Gain (Loss) on Derivative, Net		(0.1)
Fuel [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net		(0.1)
Fuel [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net		0.0
Fuel [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net		0.0
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net				8.8
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net		(0.1)		0.0
Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net		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		$ 8.8