Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments Designated as Hedging Instruments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Derivative Liability, Fair Value	$ (38.6)
DP&L [Member]
Derivative Liability, Fair Value	32.0
Not Designated as Hedging Instrument [Member]
Derivative Fair Value Counterparty And Collateral Netting	21.0	12.9
Derivative, Fair Value, Net	7.7
Derivative, Fair Value, Offset, Net	(13.3)	(7.4)
Not Designated as Hedging Instrument [Member] | DP&L [Member]
Derivative Fair Value Counterparty And Collateral Netting	14.3	9.0
Derivative, Fair Value, Net	(6.1)	(3.3)
Derivative, Fair Value, Offset, Net	(20.4)	(12.3)
Short-term Derivative Positions [Member] | Not Designated as Hedging Instrument [Member]
Derivative Fair Value Counterparty And Collateral Netting	14.4
Derivative, Fair Value, Net	5.0
Derivative, Fair Value, Offset, Net	(9.4)
Short-term Derivative Positions [Member] | Not Designated as Hedging Instrument [Member] | DP&L [Member]
Derivative Fair Value Counterparty And Collateral Netting	8.7	2.8
Derivative, Fair Value, Net	(6.2)	(3.5)
Derivative, Fair Value, Offset, Net	(14.9)	(6.3)
Long-term Derivative Positions [Member] | Not Designated as Hedging Instrument [Member]
Derivative Fair Value Counterparty And Collateral Netting	6.6	7.5
Derivative, Fair Value, Net	2.7
Derivative, Fair Value, Offset, Net	(3.9)	(4.4)
Long-term Derivative Positions [Member] | Not Designated as Hedging Instrument [Member] | DP&L [Member]
Derivative Fair Value Counterparty And Collateral Netting	5.6	6.2
Derivative, Fair Value, Net	0.1	0.2
Derivative, Fair Value, Offset, Net	(5.5)	(6.0)
Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	1.0	1.5
Derivative Fair Value Counterparty And Collateral Netting	(0.6)	(0.9)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		0.6
Derivative, Fair Value, Net	0.4
Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | DP&L [Member]
Derivative Asset, Fair Value	1.0	1.5
Derivative Fair Value Counterparty And Collateral Netting	(0.6)	(0.9)
Derivative, Fair Value, Net	0.4	0.6
Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Fair Value Counterparty And Collateral Netting	1.1
Derivative Liability, Fair Value, Not Offset Amount		(0.2)
Derivative Liability, Fair Value	(1.5)	(0.2)
Derivative, Fair Value, Net	(0.4)
Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative Fair Value Counterparty And Collateral Netting	1.1
Derivative Liability, Fair Value	(1.5)	(0.2)
Derivative, Fair Value, Net	(0.4)	(0.2)
Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Fair Value Counterparty And Collateral Netting	3.0
Derivative Fair Value Counterparty And Collateral Netting, Liability		1.7
Derivative Liability, Fair Value	(4.7)	(2.6)
Derivative, Fair Value, Net	(1.7)
Derivative, Fair Value, Offset, Net		0.9
Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member] | DP&L [Member]
Derivative Fair Value Counterparty And Collateral Netting	3.0	1.7
Derivative Liability, Fair Value	(4.7)	(2.6)
Derivative, Fair Value, Net	(1.7)	(0.9)
Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value		0.1
Derivative Fair Value Counterparty And Collateral Netting		(0.1)
Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member] | DP&L [Member]
Derivative Asset, Fair Value		0.1
Derivative Fair Value Counterparty And Collateral Netting		(0.1)
Interest Rate Swap [Member] | Long-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative, Fair Value, Offset, Net		32.5
Interest Rate Swap [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Fair Value Counterparty And Collateral Netting
Derivative Liability, Fair Value	(16.8)	(32.5)
Derivative, Fair Value, Net	(16.8)
Cash Flow Hedge [Member]
Derivative Fair Value Counterparty And Collateral Netting	3.5
Derivative, Fair Value, Net	(18.5)
Derivative, Fair Value, Offset, Net	(22.0)
Cash Flow Hedge [Member] | DP&L [Member]
Derivative Fair Value Counterparty And Collateral Netting		0.7
Derivative, Fair Value, Net		(0.5)
Derivative, Fair Value, Offset, Net		(1.2)
Cash Flow Hedge [Member] | Designated as Hedging Instrument [Member]
Derivative Fair Value Counterparty And Collateral Netting		0.7
Derivative, Fair Value, Net		(33.7)
Derivative, Fair Value, Offset, Net		(33.0)
Cash Flow Hedge [Member] | Designated as Hedging Instrument [Member] | DP&L [Member]
Derivative Fair Value Counterparty And Collateral Netting	3.5
Derivative, Fair Value, Net	(1.7)
Derivative, Fair Value, Offset, Net	(5.2)
Cash Flow Hedge [Member] | Short-term Derivative Positions [Member]
Derivative Fair Value Counterparty And Collateral Netting	0.5
Derivative, Fair Value, Offset, Net	(0.5)
Cash Flow Hedge [Member] | Short-term Derivative Positions [Member] | DP&L [Member]
Derivative Fair Value Counterparty And Collateral Netting	0.5	(0.9)
Derivative, Fair Value, Net		0.4
Derivative, Fair Value, Offset, Net	(0.5)	1.3
Cash Flow Hedge [Member] | Short-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
Derivative Fair Value Counterparty And Collateral Netting		(0.9)
Derivative Liability, Fair Value, Not Offset Amount		0.4
Derivative, Fair Value, Net		1.3
Cash Flow Hedge [Member] | Long-term Derivative Positions [Member]
Derivative Fair Value Counterparty And Collateral Netting	3.0
Derivative, Fair Value, Net	(18.5)
Derivative, Fair Value, Offset, Net	(21.5)
Cash Flow Hedge [Member] | Long-term Derivative Positions [Member] | DP&L [Member]
Derivative Fair Value Counterparty And Collateral Netting	3.0	1.6
Derivative, Fair Value, Net	(1.7)	(0.9)
Derivative, Fair Value, Offset, Net	(4.7)	(2.5)
Cash Flow Hedge [Member] | Long-term Derivative Positions [Member] | Designated as Hedging Instrument [Member]
Derivative Fair Value Counterparty And Collateral Netting		1.6
Derivative, Fair Value, Net		(35.0)
Derivative, Fair Value, Offset, Net		(33.4)
Not Designated as Hedging Instrument [Member] | Short-term Derivative Positions [Member]
Derivative Fair Value Counterparty And Collateral Netting		5.4
Derivative, Fair Value, Offset, Net		$ (3.0)