Derivative Instruments and Hedging Activities (Gains or Losses Recognized in AOCI for the Cash Flow Hedges) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	$ 1.1	$ (12.4)	$ 0.2	$ 1.4
Net gains / (losses) associated with current period hedging transactions	7.8	1.2	9.6	(23.8)
Ending accumulated derivative gain / (loss) in AOCI	7.0	(7.6)	7.0	(7.6)
Portion expected to be reclassified to earnings in the next twelve months	3.3		$ 3.3
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)			39 months
Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	17.8	18.8	$ 18.3	19.2
Net gains / (losses) associated with current period hedging transactions	0.0	0.0	0.0	0.0
Ending accumulated derivative gain / (loss) in AOCI	17.7	18.6	17.7	18.6
Portion expected to be reclassified to earnings in the next twelve months	(0.8)		$ (0.8)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)			0 months
THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	1.1	(14.0)	$ 0.2	1.0
Net gains / (losses) associated with current period hedging transactions	7.8	1.4	9.6	(26.3)
Ending accumulated derivative gain / (loss) in AOCI	7.0	(9.2)	7.0	(9.2)
Portion expected to be reclassified to earnings in the next twelve months	3.3		$ 3.3
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)			39 months
THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	2.2	4.6	$ 2.6	5.2
Net gains / (losses) associated with current period hedging transactions	0.0	0.0	0.0	0.0
Ending accumulated derivative gain / (loss) in AOCI	2.1	4.4	2.1	4.4
Portion expected to be reclassified to earnings in the next twelve months	(0.8)		$ (0.8)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)			0 months
Interest Expense [Member] | Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.0	0.0	$ 0.0	0.0
Interest Expense [Member] | Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(0.1)	(0.2)	(0.6)	(0.6)
Interest Expense [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.0	0.0	0.0	0.0
Interest Expense [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(0.1)	(0.2)	(0.5)	(0.8)
Revenue [Member] | Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(2.5)	3.4	(4.5)	15.4
Revenue [Member] | Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.0	0.0	0.0	0.0
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(2.5)	3.2	(4.5)	16.6
Revenue [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.0	0.0	0.0	0.0
Purchased Power [Member] | Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.6	0.2	1.7	(0.6)
Purchased Power [Member] | Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.0	0.0	0.0	0.0
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.6	0.2	1.7	(0.5)
Purchased Power [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	$ 0.0	$ 0.0	$ 0.0	$ 0.0