Derivative Instruments and Hedging Activities (Gains or Losses Recognized in AOCI for the Cash Flow Hedges) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	$ 1.0	$ (5.7)	$ 0.2	$ 1.4
Net gains / (losses) associated with current period hedging transactions	1.7	(12.1)	1.8	(25.0)
Ending accumulated derivative gain / (loss) in AOCI	1.1	(12.4)	1.1	(12.4)
Portion expected to be reclassified to earnings in the next twelve months	0.9		$ 0.9
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)			23 months
Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Beginning accumulated derivative gain / (loss) in AOCI	18.2	18.9	$ 18.3	19.2
Net gains / (losses) associated with current period hedging transactions	0.0	0.0
Ending accumulated derivative gain / (loss) in AOCI	17.8	18.8	17.8	18.8
Portion expected to be reclassified to earnings in the next twelve months	(0.6)		$ (0.6)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in 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	1.0	(5.9)	$ 0.2	1.0
Net gains / (losses) associated with current period hedging transactions	1.7	(14.7)	1.8	(27.7)
Ending accumulated derivative gain / (loss) in AOCI	1.1	(14.0)	1.1	(14.0)
Portion expected to be reclassified to earnings in the next twelve months	0.9		$ 0.9
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)			23 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	2.4	4.9	$ 2.6	5.2
Net gains / (losses) associated with current period hedging transactions	0.0	0.0
Ending accumulated derivative gain / (loss) in AOCI	2.2	4.6	2.2	4.6
Portion expected to be reclassified to earnings in the next twelve months	(0.7)		$ (0.7)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in 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
Interest Expense [Member] | Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(0.4)	(0.1)	$ (0.5)	(0.4)
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
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	(0.2)	(0.3)	(0.4)	(0.6)
Revenue [Member] | Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	(1.9)	5.5	(2.1)	12.0
Revenue [Member] | Interest Rate Contract [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	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	(1.9)	6.6	(2.1)	13.3
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
Purchased Power [Member] | Forward Contract Power [Member]
Accumulated Derivative Gain/Loss in AOCI [Roll Forward]
Net gains losses reclassified to earnings	0.3	(0.1)	1.2	(0.8)
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
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	0.3	0.0	$ 1.2	$ (0.6)
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