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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Change in unrealized gain / (loss)	$ (1.2)	$ 0.9	$ 1.0	$ 2.7
Realized gain / (loss)	1.9	(2.3)	(1.2)	(5.0)
Derivative, Gain (Loss) on Derivative, Net	0.7	(1.4)	(0.2)	(2.3)
Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.2	0.0	(0.5)	0.4
Realized gain / (loss)	0.2	(0.1)	0.5	(0.4)
Derivative, Gain (Loss) on Derivative, Net	0.4	(0.1)	0.0	0.0
Forward Contract Power [Member]
Change in unrealized gain / (loss)	(1.4)	1.2	2.3	2.3
Realized gain / (loss)	1.9	(2.4)	(1.4)	(5.3)
Derivative, Gain (Loss) on Derivative, Net	0.5	(1.2)	0.9	(3.0)
Natural Gas [Member]
Change in unrealized gain / (loss)	0.0	(0.3)	(0.8)	0.0
Realized gain / (loss)	(0.2)	0.2	(0.3)	0.7
Derivative, Gain (Loss) on Derivative, Net	(0.2)	(0.1)	(1.1)	0.7
THE DAYTON POWER AND LIGHT COMPANY [Member]
Change in unrealized gain / (loss)	(1.2)	0.9	0.9	2.7
Realized gain / (loss)	1.9	(2.3)	(1.3)	(5.0)
Derivative, Gain (Loss) on Derivative, Net	0.7	(1.4)	(0.4)	(2.3)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.2	0.0	(0.5)	0.4
Realized gain / (loss)	0.2	(0.1)	0.5	(0.4)
Derivative, Gain (Loss) on Derivative, Net	0.4	(0.1)	0.0	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Change in unrealized gain / (loss)	(1.4)	1.2	2.2	2.3
Realized gain / (loss)	1.9	(2.4)	(1.5)	(5.3)
Derivative, Gain (Loss) on Derivative, Net	0.5	(1.2)	0.7	(3.0)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Change in unrealized gain / (loss)	0.0	(0.3)	(0.8)	0.0
Realized gain / (loss)	(0.2)	0.2	(0.3)	0.7
Derivative, Gain (Loss) on Derivative, Net	(0.2)	(0.1)	(1.1)	0.7
Revenue [Member]
Derivative, Gain (Loss) on Derivative, Net	3.3	(10.4)	(2.6)	(13.1)
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	3.3	(10.4)	(2.6)	(13.1)
Revenue [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0	0.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	3.3	(10.4)	(2.7)	(13.1)
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	3.3	(10.4)	(2.7)	(13.1)
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0	0.0	0.0
Purchased Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(2.6)	9.0	2.4	10.8
Purchased Power [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.4	(0.1)	0.0	0.0
Purchased Power [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(2.8)	9.2	3.5	10.1
Purchased Power [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.2)	(0.1)	(1.1)	0.7
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	(2.6)	9.0	2.3	10.8
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.4	(0.1)	0.0	0.0
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(2.8)	9.2	3.4	10.1
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (0.2)	$ (0.1)	$ (1.1)	$ 0.7