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		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended			3 Months Ended		6 Months Ended								3 Months Ended	6 Months Ended	3 Months Ended		6 Months Ended	9 Months Ended	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Forward Contract Power [Member]	Jun. 30, 2013 Forward Contract Power [Member]	Jun. 30, 2014 Forward Contract Power [Member]	Jun. 30, 2013 Forward Contract Power [Member]	Jun. 30, 2013 Interest Rate Contract [Member]	Jun. 30, 2014 Interest Rate Contract [Member]	Jun. 30, 2013 Interest Rate Contract [Member]	Mar. 31, 2014 Interest Rate Contract [Member]	Jun. 30, 2014 DP&L [Member] Forward Contract Power [Member]	Jun. 30, 2013 DP&L [Member] Forward Contract Power [Member]	Jun. 30, 2014 DP&L [Member] Forward Contract Power [Member]	Jun. 30, 2013 DP&L [Member] Forward Contract Power [Member]	Jun. 30, 2014 DP&L [Member] Interest Rate Contract [Member]	Mar. 31, 2014 DP&L [Member] Interest Rate Contract [Member]	Jun. 30, 2013 DP&L [Member] Interest Rate Contract [Member]	Mar. 31, 2013 DP&L [Member] Interest Rate Contract [Member]	Dec. 31, 2012 DP&L [Member] Interest Rate Contract [Member]	Sep. 30, 2012 DP&L [Member] Interest Rate Contract [Member]	Jun. 30, 2014 Interest Expense [Member] Interest Rate Contract [Member]	Jun. 30, 2014 Interest Expense [Member] Interest Rate Contract [Member]	Jun. 30, 2014 Interest Expense [Member] DP&L [Member] Interest Rate Contract [Member]	Jun. 30, 2013 Interest Expense [Member] DP&L [Member] Interest Rate Contract [Member]	Jun. 30, 2014 Interest Expense [Member] DP&L [Member] Interest Rate Contract [Member]	Sep. 30, 2012 Interest Expense [Member] DP&L [Member] Interest Rate Contract [Member]	Jun. 30, 2014 Revenue [Member] Forward Contract Power [Member]	Jun. 30, 2013 Revenue [Member] Forward Contract Power [Member]	Jun. 30, 2014 Revenue [Member] Forward Contract Power [Member]	Jun. 30, 2013 Revenue [Member] Forward Contract Power [Member]	Jun. 30, 2014 Revenue [Member] DP&L [Member] Forward Contract Power [Member]	Jun. 30, 2013 Revenue [Member] DP&L [Member] Forward Contract Power [Member]	Jun. 30, 2014 Revenue [Member] DP&L [Member] Forward Contract Power [Member]	Jun. 30, 2013 Revenue [Member] DP&L [Member] Forward Contract Power [Member]	Jun. 30, 2014 Purchased Power [Member] Forward Contract Power [Member]	Jun. 30, 2013 Purchased Power [Member] Forward Contract Power [Member]	Jun. 30, 2014 Purchased Power [Member] Forward Contract Power [Member]	Jun. 30, 2013 Purchased Power [Member] Forward Contract Power [Member]	Jun. 30, 2013 Purchased Power [Member] DP&L [Member] Forward Contract Power [Member]	Jun. 30, 2014 Purchased Power [Member] DP&L [Member] Forward Contract Power [Member]	Jun. 30, 2013 Purchased Power [Member] DP&L [Member] Forward Contract Power [Member]
Beginning accumulated derivative gain / (loss) in AOCI					$ (5.7)	$ (5.7)	$ 1.4	$ (3.0)	$ 4.6	$ 19.2	$ 0.5	$ 18.9	$ (5.9)	$ (6.9)	$ 1.0	$ (4.7)	$ 5.2	$ 4.9	$ 6.1	$ 6.7	$ 7.3	$ 6.1
Net gains / (losses) associated with current period hedging transactions					(12.1)	3.0	(25.0)	0.1	8.2		12.3		(14.7)	3.1	(27.7)	0.5
Net income (loss)	33.8	22.8	(215.2)	42.8
Ending accumulated derivative gain / (loss) in AOCI					(12.4)	(1.1)	(12.4)	(1.1)	12.8	18.8	12.8	18.9	(14.0)	(1.9)	(14.0)	(1.9)	4.6	4.9	6.1	6.7	7.3	6.1
Portion expected to be reclassified to earnings in the next twelve months					(9.4)		(9.4)			(0.6)			(14.5)		(14.5)		(1.2)
Maximum length of time that we are hedging our exposure to variability in future cash flows related to forecasted transactions (in months)							18 months			0 months					18 months		0 months
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net																							$ (0.1)	$ (0.4)	$ (0.3)	$ (0.6)	$ (0.6)	$ (1.2)	$ 5.5	$ 1.3	$ 12.0	$ 0.9	$ 6.6	$ 1.4	$ 13.3	$ 1.1	$ (0.1)	$ 0.3	$ (0.8)	$ 0.9	$ 0.5	$ (0.6)	$ 1.2