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		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Change in unrealized gain / (loss)	$ 3.3	$ 0.5	$ 1.9	$ (1.3)
Realized loss	(1.5)	(1.6)	(2.0)	(4.1)
Derivative, Gain (Loss) on Derivative, Net	1.8	(1.1)	(0.1)	(5.4)
Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)		0.1		0.2
Realized loss		(0.1)		(0.1)
Derivative, Gain (Loss) on Derivative, Net		0.0		0.1
Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.2	(0.8)	0.4	0.1
Realized loss	0.2	0.0	0.4	(0.1)
Derivative, Gain (Loss) on Derivative, Net	0.4	(0.8)	0.8	0.0
Forward Contract Power [Member]
Change in unrealized gain / (loss)	2.6	1.2	1.2	(1.7)
Realized loss	(2.4)	(1.5)	(2.9)	(3.8)
Derivative, Gain (Loss) on Derivative, Net	0.2	(0.3)	(1.7)	(5.5)
Natural Gas [Member]
Change in unrealized gain / (loss)	0.5		0.3	0.1
Realized loss	0.7		0.5	(0.1)
Derivative, Gain (Loss) on Derivative, Net	1.2		0.8	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Change in unrealized gain / (loss)	3.5	0.4	1.6	(1.4)
Realized loss	(1.4)	(1.6)	(1.8)	(4.1)
Derivative, Gain (Loss) on Derivative, Net	2.1	(1.2)	(0.2)	(5.5)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)		0.1		0.2
Realized loss		(0.1)		(0.1)
Derivative, Gain (Loss) on Derivative, Net		0.0		0.1
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.2	(0.8)	0.4	0.1
Realized loss	0.2	0.0	0.4	(0.1)
Derivative, Gain (Loss) on Derivative, Net	0.4	(0.8)	0.8	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Change in unrealized gain / (loss)	2.8	1.1	0.9	(1.8)
Realized loss	(2.3)	(1.5)	(2.7)	(3.8)
Derivative, Gain (Loss) on Derivative, Net	0.5	(0.4)	(1.8)	(5.6)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Change in unrealized gain / (loss)	0.5		0.3	0.1
Realized loss	0.7		0.5	(0.1)
Derivative, Gain (Loss) on Derivative, Net	1.2		0.8	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	(5.3)	(3.6)	(6.6)	(7.7)
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	(0.8)	0.0	0.0
Revenue [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(5.3)	(2.8)	(6.6)	(7.7)
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	(5.1)	(3.7)	(6.6)	2.2
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net		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	(0.8)	0.0	0.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(5.1)	(2.9)	(6.6)	2.2
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0		0.0	0.0
Purchased Power [Member]
Derivative, Gain (Loss) on Derivative, Net	7.1	2.5	6.5	2.2
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	0.4	0.0	0.8	0.0
Purchased Power [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	5.5	2.5	4.9	2.2
Purchased Power [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	1.2		0.8	0.0
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	7.2	2.5	6.4	(7.8)
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	0.4	0.0	0.8	0.0
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	5.6	$ 2.5	4.8	(7.8)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 1.2		$ 0.8	$ 0.0