Derivative Instruments and Hedging Activities (Classification within the Condensed Consolidated Statements of Results of Operations or Balance Sheets of the Gains and Losses) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Change in unrealized gain / (loss)			$ (1.3)	$ (5.3)
Gain (Loss) on Derivative Instruments, Net, Pretax			(4.1)	(0.8)
Derivative, Gain (Loss) on Derivative, Net			(5.4)	(6.1)
THE DAYTON POWER AND LIGHT COMPANY [Member] | NYMEX Coal Contract [Member]
Change in unrealized gain / (loss)			0.1
Derivative, Gain (Loss) on Derivative, Net			(0.1)
THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Gain (Loss) on Derivative Instruments, Net, Pretax			0.8
Derivative, Gain (Loss) on Derivative, Net				(0.3)
Regulatory Asset [Member]
Derivative, Gain (Loss) on Derivative, Net			0.1
Not Designated as Hedging Instrument [Member]
Change in unrealized gain / (loss)	$ 0.5	$ 0.7
Gain (Loss) on Derivative Instruments, Net, Pretax	(1.6)	1.1
Derivative, Gain (Loss) on Derivative, Net	(1.1)	1.8	(5.4)	(6.1)
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	(0.8)	(1.2)	0.1	(1.5)
Gain (Loss) on Derivative Instruments, Net, Pretax	0.0	0.6	(0.1)	0.6
Derivative, Gain (Loss) on Derivative, Net	(0.8)	(0.6)	0.0	(0.9)
Not Designated as Hedging Instrument [Member] | Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)	0.1	0.1	0.2	(0.1)
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.1)	0.0	(0.1)	0.1
Derivative, Gain (Loss) on Derivative, Net		0.1	0.1
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member]
Change in unrealized gain / (loss)	1.2	1.8	(1.7)	(3.7)
Gain (Loss) on Derivative Instruments, Net, Pretax	(1.5)	0.5	(3.8)	(1.5)
Derivative, Gain (Loss) on Derivative, Net	(0.3)	2.3	(5.5)	(5.2)
Not Designated as Hedging Instrument [Member] | Natural Gas [Member]
Change in unrealized gain / (loss)			0.1
Gain (Loss) on Derivative Instruments, Net, Pretax			(0.1)
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Change in unrealized gain / (loss)	0.4	1.3	(1.4)	(4.7)
Gain (Loss) on Derivative Instruments, Net, Pretax	(1.6)	1.2	(4.1)	(0.1)
Derivative, Gain (Loss) on Derivative, Net	(1.2)	2.5	(5.5)	(4.8)
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Change in unrealized gain / (loss)	(0.8)	(1.2)	0.1	(1.5)
Gain (Loss) on Derivative Instruments, Net, Pretax		0.6	(0.1)	0.6
Derivative, Gain (Loss) on Derivative, Net	(0.8)	(0.6)	0.0	(0.9)
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Change in unrealized gain / (loss)	0.1	0.1	0.2	(0.1)
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.1)		(0.1)	0.1
Derivative, Gain (Loss) on Derivative, Net	0.0	0.1	0.1
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Change in unrealized gain / (loss)	1.1	2.4	(1.8)	(3.1)
Gain (Loss) on Derivative Instruments, Net, Pretax	(1.5)	0.6	(3.8)	(0.8)
Derivative, Gain (Loss) on Derivative, Net	(0.4)	3.0	(5.6)	(3.9)
Not Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Change in unrealized gain / (loss)			0.1
Gain (Loss) on Derivative Instruments, Net, Pretax			(0.1)
Not Designated as Hedging Instrument [Member] | Regulatory Asset [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Not Designated as Hedging Instrument [Member] | Regulatory Asset [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net			0.1
Not Designated as Hedging Instrument [Member] | Regulatory Asset [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Not Designated as Hedging Instrument [Member] | Regulatory Asset [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Not Designated as Hedging Instrument [Member] | Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net			0.1
Not Designated as Hedging Instrument [Member] | Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Not Designated as Hedging Instrument [Member] | Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net			0.1
Not Designated as Hedging Instrument [Member] | Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Not Designated as Hedging Instrument [Member] | Regulatory Asset [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net			0.0
Not Designated as Hedging Instrument [Member] | Purchased Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(3.6)	1.7	(7.7)	(6.1)
Not Designated as Hedging Instrument [Member] | Purchased Power [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.8)	(0.6)	0.0	(0.9)
Not Designated as Hedging Instrument [Member] | Purchased Power [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(2.8)	2.3	(7.7)	(5.2)
Not Designated as Hedging Instrument [Member] | Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	(3.7)	1.7	(7.8)	(6.2)
Not Designated as Hedging Instrument [Member] | Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	(0.8)	(0.6)		(0.9)
Not Designated as Hedging Instrument [Member] | 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
Not Designated as Hedging Instrument [Member] | Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	(2.9)	2.3	(7.8)	(5.3)
Not Designated as Hedging Instrument [Member] | Revenue [Member]
Derivative, Gain (Loss) on Derivative, Net	2.5	0.1	2.2
Not Designated as Hedging Instrument [Member] | Revenue [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	2.5		2.2
Not Designated as Hedging Instrument [Member] | Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net	2.5	0.7	2.2	1.4
Not Designated as Hedging Instrument [Member] | Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Not Designated as Hedging Instrument [Member] | Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Not Designated as Hedging Instrument [Member] | Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net	$ 2.5	0.7	$ 2.2	$ 1.4
Not Designated as Hedging Instrument [Member] | Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Gain (Loss) on Derivative, Net		0.1
Not Designated as Hedging Instrument [Member] | Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Derivative, Gain (Loss) on Derivative, Net		0.0
Not Designated as Hedging Instrument [Member] | Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Derivative, Gain (Loss) on Derivative, Net		0.1
Not Designated as Hedging Instrument [Member] | Fuel [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Derivative, Gain (Loss) on Derivative, Net		$ 0.0