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, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Change in unrealized gain / (loss)	$ 12.7	$ (29.9)	$ 11.2	$ (41.1)
Derivative, Gain (Loss) on Derivative, Net	(12.1)	6.0	(29.0)	8.2
Gain (Loss) on Derivative Instruments, Net, Pretax	0.6	(23.9)	(17.8)	(32.9)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.6	(23.9)	(17.8)	(32.9)
DP&L [Member]
Change in unrealized gain / (loss)	(5.5)	(29.3)	7.2	(41.7)
Derivative, Gain (Loss) on Derivative, Net	4.2	4.5	(20.6)	8.2
Gain (Loss) on Derivative Instruments, Net, Pretax	(1.3)	(24.8)	(13.4)	(33.5)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(1.3)	(24.8)	(13.4)	(33.5)
Wholesale Revenue [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(2.4)	(1.6)	(1.7)	(6.3)
Wholesale Revenue [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.3	(0.1)	2.0	(0.2)
Purchased Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.2)	2.4	(2.7)	5.4
Purchased Power [Member] | DP&L [Member]
Derivative, Gain (Loss) on Derivative, Net	(1.6)
Derivative Instruments, Gain (Loss) Recognized in Income, Net			(2.0)	(1.3)
Fuel [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(2.7)	(6.3)	(17.4)	(5.1)
Fuel [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net		(6.3)	(17.4)	(5.1)
O&M [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.1		0.2	0.1
O&M [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net			0.2	0.1
Partners Share of Gain Loss [Member]
Partner's share of gain (loss)	4.7	(13.8)	3.5	(21.2)
Partners Share of Gain Loss [Member] | DP&L [Member]
Partner's share of gain (loss)		(13.8)	3.5	(21.2)
Regulatory Asset Liability [Member]
Regulatory (asset)/liability	1.1	(4.6)	0.3	(5.8)
Regulatory Asset Liability [Member] | DP&L [Member]
Regulatory (asset)/liability		(4.6)	0.3	(5.8)
NYMEX Coal [Member]
Change in unrealized gain / (loss)	15.5	(27.9)	13.4	(41.6)
Derivative, Gain (Loss) on Derivative, Net	(12.8)	4.3	(27.2)	8.1
Gain (Loss) on Derivative Instruments, Net, Pretax	2.7	(23.6)	(13.8)	(33.5)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	2.7	(23.6)	(13.8)	(33.5)
NYMEX Coal [Member] | DP&L [Member]
Change in unrealized gain / (loss)	15.5	(27.9)	13.4	(41.6)
Derivative, Gain (Loss) on Derivative, Net	(12.8)	4.3	(27.2)	8.1
Gain (Loss) on Derivative Instruments, Net, Pretax	2.7	(23.6)	(13.8)	(33.5)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	2.7	(23.6)	(13.8)	(33.5)
NYMEX Coal [Member] | Fuel [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(3.2)	(5.8)	(18.2)	(6.4)
NYMEX Coal [Member] | Fuel [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(3.2)	(5.8)	(18.2)	(6.4)
NYMEX Coal [Member] | Partners Share of Gain Loss [Member]
Partner's share of gain (loss)	4.7	(13.8)	3.5	(21.2)
NYMEX Coal [Member] | Partners Share of Gain Loss [Member] | DP&L [Member]
Partner's share of gain (loss)	4.7	(13.8)	3.5	(21.2)
NYMEX Coal [Member] | Regulatory Asset Liability [Member]
Regulatory (asset)/liability	1.2	(4.0)	0.9	(5.9)
NYMEX Coal [Member] | Regulatory Asset Liability [Member] | DP&L [Member]
Regulatory (asset)/liability	1.2	(4.0)	0.9	(5.9)
Heating Oil [Member]
Change in unrealized gain / (loss)		(1.6)	(1.5)
Derivative, Gain (Loss) on Derivative, Net	0.5	0.5	1.9	1.5
Gain (Loss) on Derivative Instruments, Net, Pretax	0.5	(1.1)	0.4	1.5
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.5	(1.1)	0.4	1.5
Heating Oil [Member] | DP&L [Member]
Change in unrealized gain / (loss)		(1.6)	(1.5)
Derivative, Gain (Loss) on Derivative, Net	0.5	0.5	1.9	1.5
Gain (Loss) on Derivative Instruments, Net, Pretax	0.5	(1.1)	0.4	1.5
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.5	(1.1)	0.4	1.5
Heating Oil [Member] | Fuel [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.5	(0.5)	0.8	1.3
Heating Oil [Member] | Fuel [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.5	(0.5)	0.8	1.3
Heating Oil [Member] | O&M [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.1		0.2	0.1
Heating Oil [Member] | O&M [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.1		0.2	0.1
Heating Oil [Member] | Regulatory Asset Liability [Member]
Regulatory (asset)/liability	(0.1)	(0.6)	(0.6)	0.1
Heating Oil [Member] | Regulatory Asset Liability [Member] | DP&L [Member]
Regulatory (asset)/liability	(0.1)	(0.6)	(0.6)	0.1
FTR's [Member]
Change in unrealized gain / (loss)	0.1	(0.1)	(0.1)	(0.1)
Derivative, Gain (Loss) on Derivative, Net	0.1		0.5	(0.6)
Gain (Loss) on Derivative Instruments, Net, Pretax	0.2	(0.1)	0.4	(0.7)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.2	(0.1)	0.4	(0.7)
FTR's [Member] | DP&L [Member]
Change in unrealized gain / (loss)	0.1	(0.1)	(0.1)	(0.1)
Derivative, Gain (Loss) on Derivative, Net	0.1		0.5	(0.6)
Gain (Loss) on Derivative Instruments, Net, Pretax	0.2	(0.1)	0.4	(0.7)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.2	(0.1)	0.4	(0.7)
FTR's [Member] | Purchased Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.2	(0.1)	0.4	(0.7)
FTR's [Member] | Purchased Power [Member] | DP&L [Member]
Derivative, Gain (Loss) on Derivative, Net	0.2
Derivative Instruments, Gain (Loss) Recognized in Income, Net		(0.1)	0.4	(0.7)
Forward Power Contracts [Member]
Change in unrealized gain / (loss)	(2.9)	(0.3)	(0.6)	0.6
Derivative, Gain (Loss) on Derivative, Net	0.1	1.2	(4.2)	(0.8)
Gain (Loss) on Derivative Instruments, Net, Pretax	(2.8)	0.9	(4.8)	(0.2)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(2.8)	0.9	(4.8)	(0.2)
Forward Power Contracts [Member] | DP&L [Member]
Change in unrealized gain / (loss)	10.1	0.3	(4.6)
Derivative, Gain (Loss) on Derivative, Net	(8.0)	(0.3)	4.2	(0.8)
Gain (Loss) on Derivative Instruments, Net, Pretax	2.1		(0.4)	(0.8)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	2.1		(0.4)	(0.8)
Forward Power Contracts [Member] | Wholesale Revenue [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(2.4)	(1.6)	(1.7)	(6.3)
Forward Power Contracts [Member] | Wholesale Revenue [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.3	(0.1)	2.0	(0.2)
Forward Power Contracts [Member] | Purchased Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.4)	2.5	(3.1)	6.1
Forward Power Contracts [Member] | Purchased Power [Member] | DP&L [Member]
Derivative, Gain (Loss) on Derivative, Net	(1.4)
Derivative Instruments, Gain (Loss) Recognized in Income, Net		0.1	(2.4)	(0.6)
Forward Power Contracts [Member] | Fuel [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(2.7)
Forward Power Contracts [Member] | O&M [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.1
Forward Power Contracts [Member] | Partners Share of Gain Loss [Member] | DP&L [Member]
Partner's share of gain (loss)	4.7
Forward Power Contracts [Member] | Regulatory Asset Liability [Member] | DP&L [Member]
Regulatory (asset)/liability	$ 1.1