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
	Mar. 31, 2017	Mar. 31, 2016
Change in unrealized gain / (loss)	$ (0.2)	$ (1.6)
Realized loss	(2.6)	(0.4)
Derivative, Gain (Loss) on Derivative, Net	(2.8)	(2.0)
Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.0	0.1
Realized loss	0.2	0.2
Derivative, Gain (Loss) on Derivative, Net	0.2	0.3
Forward Contract Power [Member]
Change in unrealized gain / (loss)	(0.1)	(1.5)
Realized loss	(2.6)	(0.4)
Derivative, Gain (Loss) on Derivative, Net	(2.7)	(1.9)
Natural Gas [Member]
Change in unrealized gain / (loss)	(0.1)	(0.2)
Realized loss	(0.2)	(0.2)
Derivative, Gain (Loss) on Derivative, Net	(0.3)	(0.4)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Change in unrealized gain / (loss)	(0.2)	(2.0)
Realized loss	(2.6)	(0.3)
Derivative, Gain (Loss) on Derivative, Net	(2.8)	(2.3)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	0.0	0.1
Realized loss	0.2	0.2
Derivative, Gain (Loss) on Derivative, Net	0.2	0.3
THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Change in unrealized gain / (loss)	(0.1)	(1.9)
Realized loss	(2.6)	(0.3)
Derivative, Gain (Loss) on Derivative, Net	(2.7)	(2.2)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Change in unrealized gain / (loss)	(0.1)	(0.2)
Realized loss	(0.2)	(0.2)
Derivative, Gain (Loss) on Derivative, Net	(0.3)	(0.4)
Revenue [Member]
Derivative, Gain (Loss) on Derivative, Net	(6.7)	(1.1)
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	(6.7)	(1.1)
Revenue [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	(6.7)	(1.4)
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(6.7)	(1.4)
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	3.9	(0.9)
Purchased Power [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.2	0.3
Purchased Power [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	4.0	(0.8)
Purchased Power [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.3)	(0.4)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	3.9	(0.9)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.2	0.3
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	4.0	(0.8)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net	$ (0.3)	$ (0.4)