Derivative Instruments and Hedging Activities (Gains or Losses Recognized in AOCI for the Cash Flow Hedges) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended									3 Months Ended
	Mar. 31, 2013 Forward Contract Power [Member]	Mar. 31, 2012 Forward Contract Power [Member]	Mar. 31, 2013 Interest Rate Contract [Member]	Mar. 31, 2012 Interest Rate Contract [Member]	Mar. 31, 2013 DP&L [Member] Forward Contract Power [Member]	Mar. 31, 2012 DP&L [Member] Forward Contract Power [Member]	Mar. 31, 2013 DP&L [Member] Interest Rate Contract [Member]	Mar. 31, 2012 DP&L [Member] Interest Rate Contract [Member]	Dec. 31, 2011 DP&L [Member] Interest Rate Contract [Member]	Mar. 31, 2012 Interest Expense [Member] Interest Rate Contract [Member]	Mar. 31, 2013 Interest Expense [Member] DP&L [Member] Interest Rate Contract [Member]	Mar. 31, 2012 Interest Expense [Member] DP&L [Member] Interest Rate Contract [Member]	Mar. 31, 2013 Revenue [Member] Forward Contract Power [Member]	Mar. 31, 2012 Revenue [Member] Forward Contract Power [Member]	Mar. 31, 2013 Revenue [Member] DP&L [Member] Forward Contract Power [Member]	Mar. 31, 2012 Revenue [Member] DP&L [Member] Forward Contract Power [Member]	Mar. 31, 2013 Purchased Power [Member] Forward Contract Power [Member]	Mar. 31, 2012 Purchased Power [Member] Forward Contract Power [Member]	Mar. 31, 2013 Purchased Power [Member] DP&L [Member] Forward Contract Power [Member]	Mar. 31, 2012 Purchased Power [Member] DP&L [Member] Forward Contract Power [Member]
Beginning accumulated derivative gain / (loss) in AOCI	$ (3.0)	$ 0.3	$ 0.5	$ (0.8)	$ (4.7)	$ (0.8)	$ 7.3	$ 9.2	$ 9.8
Net gains / (losses) associated with current period hedging transactions	(2.9)	(1.5)	4.1	9.1	(2.6)	(1.5)
Net gains reclassified to earnings										0.2	(0.6)	(0.6)	(0.3)	(1.2)	(0.3)	(1.2)	0.5	0.1	0.7	0.1
Ending accumulated derivative gain / (loss) in AOCI	(5.7)	(2.3)	4.6	8.5	(6.9)	(3.4)	6.7	9.2	9.8
Net gains / (losses) associated with the ineffective portion of the hedging transaction										(1.6)
Portion expected to be reclassified to earnings in the next twelve months	$ 9.9				$ 8.8
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)	21 months		5 months		21 months		5 months