Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments Not Designated as Hedging Instruments) (Details) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
Derivative Liability, Fair Value	$ (13.1)
DP&L [Member] | MTM position [Member]
Derivative Assets (Liabilities), at Fair Value, Net	(2.3)	[1]
Derivative Asset, Fair Value, Not Offset Amount	1.1
Derivative, Fair Value Counterparty and Collateral, Netting	(3.4)
DP&L [Member] | MTM position [Member] | Short-term Derivative Positions [Member]
Derivative Assets (Liabilities), at Fair Value, Net	1.6	[1]
Derivative Asset, Fair Value, Not Offset Amount	0.8
Derivative, Fair Value Counterparty and Collateral, Netting	(0.8)
DP&L [Member] | MTM position [Member] | Long-term Derivative Positions [Member]
Derivative Liability, Fair Value	3.9	[1]
Derivative Asset, Fair Value, Not Offset Amount	0.3
Derivative, Fair Value Counterparty and Collateral, Netting	4.2
MTM position [Member]
Derivative Assets (Liabilities), at Fair Value, Net	3.6	[1]
Derivative Asset, Fair Value, Not Offset Amount	8.6
Derivative, Fair Value Counterparty and Collateral, Netting	(5.0)
MTM position [Member] | Short-term Derivative Positions [Member]
Derivative Assets (Liabilities), at Fair Value, Net	4.2	[1]
Derivative Asset, Fair Value, Not Offset Amount	4.3
Derivative, Fair Value Counterparty and Collateral, Netting	(0.1)
MTM position [Member] | Long-term Derivative Positions [Member]
Derivative Assets (Liabilities), at Fair Value, Net	0.6	[1]
Derivative Asset, Fair Value, Not Offset Amount	4.3
Derivative, Fair Value Counterparty and Collateral, Netting	(4.9)
MTM position [Member] | Long-term Derivative Positions [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member]
Derivative, Fair Value, Not Offset, Net			24.6	[1]
Derivative, Fair Value Counterparty and Collateral, Netting			15.6
Derivative, Fair Value, Offset, Net			9.0
MTM position [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member]
Derivative, Fair Value, Not Offset, Net			39.3	[1]
Derivative, Fair Value Counterparty and Collateral, Netting			23.5
Derivative, Fair Value, Offset, Net			15.8
Futures [Member] | Short-term Derivative Positions [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member]
Derivative Asset, Fair Value			0.5	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting			(0.5)
Futures [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value, Not Offset Amount
DP&L [Member] | Heating Oil [Member] | Short-term Derivative Positions [Member]
Derivative Asset, Fair Value	1.7	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting	1.7
DP&L [Member] | Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value, Not Offset Amount
Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	1.7	[1]
Derivative Asset, Fair Value, Not Offset Amount
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting	(1.7)
Heating Oil [Member] | Long-term Derivative Positions [Member] | Not Designated as Hedging Instrument [Member]
Derivative Asset, Fair Value			1.1	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting			(1.1)
Heating Oil [Member] | Long-term Derivative Positions [Member] | Other Deferred Assets [Member]
Derivative Asset, Fair Value, Not Offset Amount
DP&L [Member] | Short-term Derivative Positions [Member] | FTRs [Member]
Derivative Asset, Fair Value	0.1	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting
DP&L [Member] | Short-term Derivative Positions [Member] | FTRs [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value, Not Offset Amount	0.1
Short-term Derivative Positions [Member] | FTRs [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member]
Derivative Asset, Fair Value			0.3	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting
Short-term Derivative Positions [Member] | FTRs [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	0.1	[1]
Derivative Asset, Fair Value, Not Offset Amount	0.1		0.3
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting
DP&L [Member] | Short-term Derivative Positions [Member] | Forward Power Contracts [Member]
Derivative Asset, Fair Value	0.7	[1]
Derivative Liability, Fair Value	(0.7)	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting	(0.6)
Derivative, Fair Value Counterparty and Collateral, Netting	0.6
DP&L [Member] | Short-term Derivative Positions [Member] | Forward Power Contracts [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value, Not Offset Amount	0.1
DP&L [Member] | Short-term Derivative Positions [Member] | Forward Power Contracts [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value, Not Offset Amount	(0.1)
DP&L [Member] | Long-term Derivative Positions [Member] | Forward Power Contracts [Member]
Derivative Asset, Fair Value	0.8	[1]
Derivative Liability, Fair Value	(0.9)	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting	0.5
Derivative, Fair Value Counterparty and Collateral, Netting	0.5
DP&L [Member] | Long-term Derivative Positions [Member] | Forward Power Contracts [Member] | Other Deferred Credits [Member]
Derivative Liability, Fair Value, Not Offset Amount	(0.4)
DP&L [Member] | Long-term Derivative Positions [Member] | Forward Power Contracts [Member] | Other Deferred Assets [Member]
Derivative Asset, Fair Value, Not Offset Amount	0.3
Short-term Derivative Positions [Member] | Forward Power Contracts [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member]
Derivative Liability, Fair Value			(0.1)	[1]
Derivative, Fair Value Counterparty and Collateral, Netting
Short-term Derivative Positions [Member] | Forward Power Contracts [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	6.4	[1]
Derivative Asset, Fair Value, Not Offset Amount	6.0
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting	(0.4)
Short-term Derivative Positions [Member] | Forward Power Contracts [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	(3.8)	[1]
Derivative Liability, Fair Value, Not Offset Amount	(2.4)		(0.1)
Derivative, Fair Value Counterparty and Collateral, Netting	1.4
Long-term Derivative Positions [Member] | Forward Power Contracts [Member] | Other Deferred Credits [Member]
Derivative Liability, Fair Value	(1.5)	[1]
Derivative Liability, Fair Value, Not Offset Amount	(0.6)
Derivative, Fair Value Counterparty and Collateral, Netting	0.9
Long-term Derivative Positions [Member] | Forward Power Contracts [Member] | Other Deferred Assets [Member]
Derivative Asset, Fair Value	4.7	[1]
Derivative Asset, Fair Value, Not Offset Amount	4.4
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting	(0.3)
Short-term Derivative Positions [Member] | NYMEX-Quality Coal Forwards [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member]
Derivative Asset, Fair Value			14.0	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting			(7.4)
Short-term Derivative Positions [Member] | NYMEX-Quality Coal Forwards [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value, Not Offset Amount			6.6
Long-term Derivative Positions [Member] | NYMEX-Quality Coal Forwards [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member]
Derivative Asset, Fair Value			23.5	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting			(14.5)
Long-term Derivative Positions [Member] | NYMEX-Quality Coal Forwards [Member] | Other Deferred Assets [Member]
Derivative Asset, Fair Value, Not Offset Amount			9.0
DP&L [Member] | Short-term Derivative Positions [Member] | NYMEX Coal [Member]
Derivative Asset, Fair Value	1.6	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting	0.9
DP&L [Member] | Short-term Derivative Positions [Member] | NYMEX Coal [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value, Not Offset Amount	0.7
DP&L [Member] | Long-term Derivative Positions [Member] | NYMEX Coal [Member]
Derivative Asset, Fair Value	0.7	[1]
Derivative Liability, Fair Value	(1.8)	[1]
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting	0.3
Derivative, Fair Value Counterparty and Collateral, Netting	1.8
DP&L [Member] | Long-term Derivative Positions [Member] | NYMEX Coal [Member] | Other Deferred Credits [Member]
Derivative Liability, Fair Value, Not Offset Amount
DP&L [Member] | Long-term Derivative Positions [Member] | NYMEX Coal [Member] | Other Deferred Assets [Member]
Derivative Asset, Fair Value, Not Offset Amount	0.4
Short-term Derivative Positions [Member] | NYMEX Coal [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	1.6	[1]
Derivative Asset, Fair Value, Not Offset Amount	0.6
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting	(1.0)
Long-term Derivative Positions [Member] | NYMEX Coal [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	(1.8)	[1]
Derivative, Fair Value Counterparty and Collateral, Netting	1.8
Long-term Derivative Positions [Member] | NYMEX Coal [Member] | Other Deferred Credits [Member]
Derivative Liability, Fair Value, Not Offset Amount
Long-term Derivative Positions [Member] | NYMEX Coal [Member] | Other Deferred Assets [Member]
Derivative Asset, Fair Value	0.7	[1]
Derivative Asset, Fair Value, Not Offset Amount	0.5
Derivative, Fair Value of Derivative Asset Counterparty and Collateral, Netting	0.2
Short-term Derivative Positions [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member]
Derivative, Fair Value, Not Offset, Net			14.7	[1]
Derivative, Fair Value Counterparty and Collateral, Netting			7.9
Derivative, Fair Value, Offset, Net			6.8
Other Deferred Credits [Member]
Derivative Liability, Fair Value	(4.5)	[1]
Derivative Liability, Fair Value, Not Offset Amount
Derivative, Fair Value Counterparty and Collateral, Netting	4.5
DP&L [Member]
Derivative Liability, Fair Value	(8.5)
Derivative, Fair Value, Offset, Net	$ 2.1

[1] 1 Includes credit valuation adjustment.