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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Change in unrealized gain / (loss)	$ 2.4	$ 3.3	$ 2.2	$ 1.9
Realized gain / (loss)	(0.6)	(1.5)	(3.2)	(2.0)
Derivative, Gain (Loss) on Derivative, Net	1.8	1.8	(1.0)	(0.1)
Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	(0.7)	0.2	(0.7)	0.4
Realized gain / (loss)	0.1	0.2	0.3	0.4
Derivative, Gain (Loss) on Derivative, Net	(0.6)	0.4	(0.4)	0.8
Forward Contract Power [Member]
Change in unrealized gain / (loss)	3.7	2.6	3.6	1.2
Realized gain / (loss)	(0.7)	(2.4)	(3.3)	(2.9)
Derivative, Gain (Loss) on Derivative, Net	3.0	0.2	0.3	(1.7)
Natural Gas [Member]
Change in unrealized gain / (loss)	(0.6)	0.5	(0.7)	0.3
Realized gain / (loss)	0.0	0.7	(0.2)	0.5
Derivative, Gain (Loss) on Derivative, Net	(0.6)	1.2	(0.9)	0.8
THE DAYTON POWER AND LIGHT COMPANY [Member]
Change in unrealized gain / (loss)	2.4	3.5	2.2	1.6
Realized gain / (loss)	(0.6)	(1.4)	(3.3)	(1.8)
Derivative, Gain (Loss) on Derivative, Net	1.8	2.1	(1.1)	(0.2)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	(0.7)	0.2	(0.7)	0.4
Realized gain / (loss)	0.1	0.2	0.3	0.4
Derivative, Gain (Loss) on Derivative, Net	(0.6)	0.4	(0.4)	0.8
THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Change in unrealized gain / (loss)	3.7	2.8	3.6	0.9
Realized gain / (loss)	(0.7)	(2.3)	(3.4)	(2.7)
Derivative, Gain (Loss) on Derivative, Net	3.0	0.5	0.2	(1.8)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Change in unrealized gain / (loss)	(0.6)	0.5	(0.7)	0.3
Realized gain / (loss)	0.0	0.7	(0.2)	0.5
Derivative, Gain (Loss) on Derivative, Net	(0.6)	1.2	(0.9)	0.8
Revenue [Member]
Derivative, Gain (Loss) on Derivative, Net	(1.0)	(5.3)	(6.5)	(6.6)
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	(1.0)	(5.3)	(6.5)	(6.6)
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	(1.1)	(5.1)	(6.6)	(6.6)
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	(1.1)	(5.1)	(6.6)	(6.6)
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	2.8	7.1	5.5	6.5
Purchased Power [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.6)	0.4	(0.4)	0.8
Purchased Power [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	4.0	5.5	6.8	4.9
Purchased Power [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.6)	1.2	(0.9)	0.8
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	2.9	7.2	5.5	6.4
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.6)	0.4	(0.4)	0.8
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	4.1	5.6	6.8	4.8
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (0.6)	$ 1.2	$ (0.9)	$ 0.8