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
	Mar. 31, 2012	Mar. 31, 2011
Change in unrealized gain / (loss)	$ (6.6)
Derivative, Gain (Loss) on Derivative, Net	(6.6)	1.2
Gain (Loss) on Derivative Instruments, Net, Pretax	(13.2)	1.2
Partner's share of gain (loss)		(2.4)
Regulatory (asset)/liability		1.9
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(13.2)	1.2
DP&L [Member]
Change in unrealized gain / (loss)	(8.0)	(0.7)
Derivative, Gain (Loss) on Derivative, Net	(4.3)	1.7
Gain (Loss) on Derivative Instruments, Net, Pretax	(12.3)	1.0
Partner's share of gain (loss)	(3.5)	(2.4)
Regulatory (asset)/liability	(1.0)	1.9
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(12.3)	1.0
Purchased Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(4.6)	0.5
Purchased Power [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(1.9)	(1.3)
Fuel [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(7.6)	2.7
Fuel [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(7.6)	2.7
O&M [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.1	0.1
O&M [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.1	0.1
Retail Revenue [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	3.4	(1.6)
Retail Revenue [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	1.6
Partners Share of Gain Loss [Member]
Partner's share of gain (loss)	(3.5)
Regulatory Asset Liability [Member]
Regulatory (asset)/liability	(1.0)
NYMEX Coal [Member]
Change in unrealized gain / (loss)	(7.8)	(3.5)
Derivative, Gain (Loss) on Derivative, Net	(5.0)	2.4
Gain (Loss) on Derivative Instruments, Net, Pretax	(12.8)	(1.1)
Partner's share of gain (loss)		(2.4)
Regulatory (asset)/liability		0.3
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(12.8)	(1.1)
NYMEX Coal [Member] | DP&L [Member]
Change in unrealized gain / (loss)	(7.8)	(3.5)
Derivative, Gain (Loss) on Derivative, Net	(5.0)	2.4
Gain (Loss) on Derivative Instruments, Net, Pretax	(12.8)	(1.1)
Partner's share of gain (loss)	(3.5)	(2.4)
Regulatory (asset)/liability	(1.1)	0.3
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(12.8)	(1.1)
NYMEX Coal [Member] | Purchased Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
NYMEX Coal [Member] | Fuel [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(8.2)	1.0
NYMEX Coal [Member] | Fuel [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(8.2)	1.0
NYMEX Coal [Member] | O&M [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
NYMEX Coal [Member] | Retail Revenue [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
NYMEX Coal [Member] | Partners Share of Gain Loss [Member]
Partner's share of gain (loss)	(3.5)
NYMEX Coal [Member] | Regulatory Asset Liability [Member]
Regulatory (asset)/liability	(1.1)
Heating Oil [Member]
Change in unrealized gain / (loss)	(0.1)	3.0
Derivative, Gain (Loss) on Derivative, Net	0.9	0.4
Gain (Loss) on Derivative Instruments, Net, Pretax	0.8	3.4
Regulatory (asset)/liability		1.6
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.8	3.4
Heating Oil [Member] | DP&L [Member]
Change in unrealized gain / (loss)	(0.1)	3.0
Derivative, Gain (Loss) on Derivative, Net	0.9	0.4
Gain (Loss) on Derivative Instruments, Net, Pretax	0.8	3.4
Regulatory (asset)/liability	0.1	1.6
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.8	3.4
Heating Oil [Member] | Purchased Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
Heating Oil [Member] | Fuel [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.6	1.7
Heating Oil [Member] | Fuel [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.6	1.7
Heating Oil [Member] | O&M [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.1	0.1
Heating Oil [Member] | O&M [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0.1	0.1
Heating Oil [Member] | Retail Revenue [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
Heating Oil [Member] | Partners Share of Gain Loss [Member]
Partner's share of gain (loss)
Heating Oil [Member] | Regulatory Asset Liability [Member]
Regulatory (asset)/liability	0.1
FTR's [Member]
Change in unrealized gain / (loss)	(0.1)	(0.1)
Derivative, Gain (Loss) on Derivative, Net	(0.2)	(0.8)
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.3)	(0.9)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.3)	(0.9)
FTR's [Member] | DP&L [Member]
Change in unrealized gain / (loss)	(0.1)	(0.1)
Derivative, Gain (Loss) on Derivative, Net	(0.2)	(0.8)
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.3)	(0.9)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.3)	(0.9)
FTR's [Member] | Purchased Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.3)	(0.9)
FTR's [Member] | Purchased Power [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.3)	(0.9)
FTR's [Member] | Fuel [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
FTR's [Member] | O&M [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
FTR's [Member] | Retail Revenue [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
FTR's [Member] | Partners Share of Gain Loss [Member]
Partner's share of gain (loss)
FTR's [Member] | Regulatory Asset Liability [Member]
Regulatory (asset)/liability
Forward Power Contracts [Member]
Change in unrealized gain / (loss)	1.4	0.6
Derivative, Gain (Loss) on Derivative, Net	(2.3)	(0.8)
Gain (Loss) on Derivative Instruments, Net, Pretax	(0.9)	(0.2)
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(0.9)	(0.2)
Forward Power Contracts [Member] | DP&L [Member]
Change in unrealized gain / (loss)		(0.1)
Derivative, Gain (Loss) on Derivative, Net		(0.3)
Gain (Loss) on Derivative Instruments, Net, Pretax		(0.4)
Derivative Instruments, Gain (Loss) Recognized in Income, Net		(0.4)
Forward Power Contracts [Member] | Purchased Power [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(4.3)	1.4
Forward Power Contracts [Member] | Purchased Power [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(1.6)	(0.4)
Forward Power Contracts [Member] | Fuel [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
Forward Power Contracts [Member] | O&M [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
Forward Power Contracts [Member] | Retail Revenue [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	3.4	(1.6)
Forward Power Contracts [Member] | Retail Revenue [Member] | DP&L [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	1.6
Forward Power Contracts [Member] | Partners Share of Gain Loss [Member]
Partner's share of gain (loss)
Forward Power Contracts [Member] | Regulatory Asset Liability [Member]
Regulatory (asset)/liability