Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments Not Designated as Hedging Instruments) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivative Liability, Fair Value	$ (22.2)
DP&L [Member]
Derivative Liability, Fair Value	(19.8)
Not Designated as Hedging Instrument [Member]
Derivative, Fair Value, Net		(7.4)
Not Designated as Hedging Instrument [Member] | DP&L [Member]
Derivative, Fair Value, Net	(5.1)	(12.3)
Not Designated as Hedging Instrument [Member] | Short-term Derivative Positions [Member]
Derivative, Fair Value, Net		(3.0)
Not Designated as Hedging Instrument [Member] | Short-term Derivative Positions [Member] | DP&L [Member]
Derivative, Fair Value, Net	(3.9)	(6.3)
Not Designated as Hedging Instrument [Member] | Long-term Derivative Positions [Member]
Derivative, Fair Value, Net		(4.4)
Not Designated as Hedging Instrument [Member] | Long-term Derivative Positions [Member] | DP&L [Member]
Derivative, Fair Value, Net	(1.2)	(6.0)
Not Designated as Hedging Instrument [Member]
Derivative, Fair Value, Net	4.3
Not Designated as Hedging Instrument [Member] | Short-term Derivative Positions [Member]
Derivative, Fair Value, Net	2.9
Not Designated as Hedging Instrument [Member] | Long-term Derivative Positions [Member]
Derivative, Fair Value, Net	1.4
NYMEX Coal [Member] | Other Current Liabilities [Member] | Short-term Derivative Positions [Member]
Derivative Liability, Fair Value	(1.1)	(8.3)
NYMEX Coal [Member] | Other Current Liabilities [Member] | Short-term Derivative Positions [Member] | DP&L [Member]
Derivative Liability, Fair Value	(1.1)	(8.3)
Heating Oil [Member] | Other Prepayments and Current Assets [Member] | Short-term Derivative Positions [Member]
Derivative Asset, Fair Value	0.3	1.8
Heating Oil [Member] | Other Prepayments and Current Assets [Member] | Short-term Derivative Positions [Member] | DP&L [Member]
Derivative Asset, Fair Value	0.3	1.8
FTR's [Member] | Other Prepayments and Current Assets [Member] | Short-term Derivative Positions [Member]
Derivative Asset, Fair Value	0.1	(0.1)
FTR's [Member] | Other Prepayments and Current Assets [Member] | Short-term Derivative Positions [Member] | DP&L [Member]
Derivative Asset, Fair Value	0.1	0.1
FTR's [Member] | Other Current Liabilities [Member] | Short-term Derivative Positions [Member]
Derivative Liability, Fair Value	(0.1)
FTR's [Member] | Other Current Liabilities [Member] | Short-term Derivative Positions [Member] | DP&L [Member]
Derivative Liability, Fair Value	(0.1)
Forward Power Contracts [Member] | Other Prepayments and Current Assets [Member] | Short-term Derivative Positions [Member]
Derivative Asset, Fair Value	12.0	9.9
Forward Power Contracts [Member] | Other Prepayments and Current Assets [Member] | Short-term Derivative Positions [Member] | DP&L [Member]
Derivative Asset, Fair Value	3.0	1.0
Forward Power Contracts [Member] | Other Current Liabilities [Member] | Short-term Derivative Positions [Member]
Derivative Liability, Fair Value	(8.3)	(6.5)
Forward Power Contracts [Member] | Other Current Liabilities [Member] | Short-term Derivative Positions [Member] | DP&L [Member]
Derivative Liability, Fair Value	$ (6.1)	$ (0.9)