Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments 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)
Short-term Derivative Positions [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Fair Value, Net		(3.0)
Short-term Derivative Positions [Member] | Not Designated as Hedging Instrument [Member] | DP&L [Member]
Derivative, Fair Value, Net	(3.9)	(6.3)
Long-term Derivative Positions [Member] | Not Designated as Hedging Instrument [Member]
Derivative, Fair Value, Net		(4.4)
Long-term Derivative Positions [Member] | Not Designated as Hedging Instrument [Member] | DP&L [Member]
Derivative, Fair Value, Net	(1.2)	(6.0)
Cash Flow Hedge [Member]
Derivative, Fair Value, Net	(44.9)	(33.7)
Cash Flow Hedge [Member] | DP&L [Member]
Derivative, Fair Value, Net	(9.2)	(1.2)
Cash Flow Hedge [Member] | Short-term Derivative Positions [Member]
Derivative, Fair Value, Net	(6.9)	1.3
Cash Flow Hedge [Member] | Short-term Derivative Positions [Member] | DP&L [Member]
Derivative, Fair Value, Net	(6.9)	1.3
Cash Flow Hedge [Member] | Long-term Derivative Positions [Member]
Derivative, Fair Value, Net	(38.0)	(35.0)
Cash Flow Hedge [Member] | Long-term Derivative Positions [Member] | DP&L [Member]
Derivative, Fair Value, Net	(2.3)	(2.5)
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
Interest Rate Swap [Member] | Cash Flow Hedge [Member] | Long-term Derivative Positions [Member]
Derivative Liability, Fair Value		(32.5)
Interest Rate Swap [Member] | Cash Flow Hedge [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value	(35.7)
NYMEX Coal [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	(1.1)	(8.3)
NYMEX Coal [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative Liability, Fair Value	(1.1)	(8.3)
NYMEX Coal [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value		(6.2)
NYMEX Coal [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member] | DP&L [Member]
Derivative Liability, Fair Value		(6.2)
Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	0.3	1.8
Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | DP&L [Member]
Derivative Asset, Fair Value	0.3	1.8
Heating Oil [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	0.1
Heating Oil [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member] | DP&L [Member]
Derivative Asset, Fair Value	0.1
FTR's [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	0.1	(0.1)
FTR's [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | DP&L [Member]
Derivative Asset, Fair Value	0.1	0.1
FTR's [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	(0.1)
FTR's [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative Liability, Fair Value	(0.1)
Forward Power Contracts [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	12.0	9.9
Forward Power Contracts [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | DP&L [Member]
Derivative Asset, Fair Value	3.0	1.0
Forward Power Contracts [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	(8.3)	(6.5)
Forward Power Contracts [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative Liability, Fair Value	(6.1)	(0.9)
Forward Power Contracts [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value	(2.4)	(4.0)
Forward Power Contracts [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member] | DP&L [Member]
Derivative Liability, Fair Value	(2.2)	(1.3)
Forward Power Contracts [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	3.7	5.8
Forward Power Contracts [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member] | DP&L [Member]
Derivative Asset, Fair Value	0.9	1.5
Forward Power Contracts [Member] | Cash Flow Hedge [Member] | Short-term Derivative Positions [Member]
Derivative Asset, Fair Value		1.5
Derivative Liability, Fair Value		(0.2)
Forward Power Contracts [Member] | Cash Flow Hedge [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	0.4
Forward Power Contracts [Member] | Cash Flow Hedge [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | DP&L [Member]
Derivative Asset, Fair Value	0.4	1.5
Forward Power Contracts [Member] | Cash Flow Hedge [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	(7.3)
Forward Power Contracts [Member] | Cash Flow Hedge [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative Liability, Fair Value	(7.3)	(0.2)
Forward Power Contracts [Member] | Cash Flow Hedge [Member] | Long-term Derivative Positions [Member]
Derivative Asset, Fair Value		0.1
Derivative Liability, Fair Value		(2.6)
Forward Power Contracts [Member] | Cash Flow Hedge [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value	(3.0)
Forward Power Contracts [Member] | Cash Flow Hedge [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member] | DP&L [Member]
Derivative Liability, Fair Value	(3.0)	(2.6)
Forward Power Contracts [Member] | Cash Flow Hedge [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	0.7
Forward Power Contracts [Member] | Cash Flow Hedge [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member] | DP&L [Member]
Derivative Asset, Fair Value	$ 0.7	$ 0.1