Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
Total Short-term Investments	$ 0		$ 69.3
Interest Rate Swap [Member] | Collateral and Counterparty Netting [Member]
Total Derivative Liabilities
Interest Rate Swap [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Liabilities	(28.5)
Interest Rate Swap [Member] | Level 1 [Member]
Total Derivative Liabilities
Interest Rate Swap [Member] | Level 2 [Member]
Total Derivative Liabilities	(28.5)
Interest Rate Swap [Member] | Level 3 [Member]
Total Derivative Liabilities
Interest Rate Swap [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Liabilities	(28.5)
DP&L [Member] | Equity Securities [Member] | Collateral and Counterparty Netting [Member]
Total Master Trust Assets, Fair Value		[1]
DP&L [Member] | Equity Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	33.5	[1]	30.2	[1],[2]
DP&L [Member] | Equity Securities [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	29.5	[1]	25.8	[1]
DP&L [Member] | Equity Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	4.0	[1]	4.4	[1]
DP&L [Member] | Equity Securities [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value		[1]
DP&L [Member] | Equity Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	33.5	[1]	30.2	[1]
Equity Securities [Member] | Collateral and Counterparty Netting [Member]
Total Master Trust Assets, Fair Value
Equity Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	4.0		4.4	[2]
Equity Securities [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value
Equity Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	4.0		4.4
Equity Securities [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value
Equity Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	4.0		4.4
DP&L [Member] | Debt Securities [Member] | Collateral and Counterparty Netting [Member]
Total Master Trust Assets, Fair Value
DP&L [Member] | Debt Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	5.6		5.5	[2]
DP&L [Member] | Debt Securities [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value
DP&L [Member] | Debt Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	5.6		5.5
DP&L [Member] | Debt Securities [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value
DP&L [Member] | Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	5.6		5.5
Debt Securities [Member] | Collateral and Counterparty Netting [Member]
Total Master Trust Assets, Fair Value
Debt Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	5.6		5.5	[2]
Debt Securities [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value
Debt Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	5.6		5.5
Debt Securities [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value
Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	5.6		5.5
DP&L [Member] | Money Market Funds [Member] | Collateral and Counterparty Netting [Member]
Total Master Trust Assets, Fair Value
DP&L [Member] | Money Market Funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2		1.6	[2]
DP&L [Member] | Money Market Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value
DP&L [Member] | Money Market Funds [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.2		1.6
DP&L [Member] | Money Market Funds [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value
DP&L [Member] | Money Market Funds [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2		1.6
Money Market Funds [Member] | Collateral and Counterparty Netting [Member]
Total Master Trust Assets, Fair Value
Money Market Funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2		1.6	[2]
Money Market Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value
Money Market Funds [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.2		1.6
Money Market Funds [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value
Money Market Funds [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2		1.6
DP&L [Member] | Hedge Funds, Multi-strategy [Member] | Collateral and Counterparty Netting [Member]
Total Master Trust Assets, Fair Value
DP&L [Member] | Hedge Funds, Multi-strategy [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2		0.3	[2]
DP&L [Member] | Hedge Funds, Multi-strategy [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value
DP&L [Member] | Hedge Funds, Multi-strategy [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.2		0.3
DP&L [Member] | Hedge Funds, Multi-strategy [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value
DP&L [Member] | Hedge Funds, Multi-strategy [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2		0.3
Multi Strategy Fund [Member] | Collateral and Counterparty Netting [Member]
Total Master Trust Assets, Fair Value
Multi Strategy Fund [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2		0.3	[2]
Multi Strategy Fund [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value
Multi Strategy Fund [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.2		0.3
Multi Strategy Fund [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value
Multi Strategy Fund [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2		0.3
DP&L [Member] | Collateral and Counterparty Netting [Member]
Total Master Trust Assets, Fair Value
Total Derivative Assets	(5.3)		(23.7)
Total Assets	(5.3)		(23.7)
Total Derivative Liabilities	7.7		(1.1)
Total Liabilities	7.7		(1.1)
DP&L [Member] | Collateral and Counterparty Netting [Member] | FTRs [Member]
Total Derivative Assets
DP&L [Member] | Collateral and Counterparty Netting [Member] | Futures [Member]
Total Derivative Assets	(1.7)		(1.6)
DP&L [Member] | Collateral and Counterparty Netting [Member] | Forward NYMEX Coal Contracts [Member]
Total Derivative Assets	(1.2)		(21.9)
DP&L [Member] | Collateral and Counterparty Netting [Member] | Forward Contract Power [Member]
Total Derivative Assets	(2.4)		(0.2)
Total Derivative Liabilities	1.4		(1.1)
Collateral and Counterparty Netting [Member]
Total Master Trust Assets, Fair Value
Total Derivative Assets	(4.9)		(23.7)
Total Assets	(4.9)		(23.7)
Total Short-term Investments
Total Derivative Liabilities	8.9		(1.1)
Total Liabilities	8.9		(1.1)
Collateral and Counterparty Netting [Member] | Short-term Investments - Bonds [Member]
Total Short-term Investments
Collateral and Counterparty Netting [Member] | Short-term Investments - VRDNs [Member]
Total Derivative Assets
Collateral and Counterparty Netting [Member] | FTRs [Member]
Total Derivative Assets
Collateral and Counterparty Netting [Member] | Futures [Member]
Total Derivative Assets	(1.7)		(1.6)
Collateral and Counterparty Netting [Member] | Forward NYMEX Coal Contracts [Member]
Total Derivative Assets	(1.2)		(21.9)
Total Derivative Liabilities	6.3
Collateral and Counterparty Netting [Member] | Interest Rate Hedge [Member]
Total Derivative Assets
Total Derivative Liabilities
Collateral and Counterparty Netting [Member] | Forward Contract Power [Member]
Total Derivative Assets	(2.0)		(0.2)
Total Derivative Liabilities	2.6		(1.1)
DP&L [Member] | FTRs [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Assets	0.1		0.3	[2]
DP&L [Member] | FTRs [Member] | Level 1 [Member]
Total Derivative Assets
DP&L [Member] | FTRs [Member] | Level 2 [Member]
Total Derivative Assets	0.1		0.3
DP&L [Member] | FTRs [Member] | Level 3 [Member]
Total Derivative Assets
DP&L [Member] | FTRs [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets	0.1		0.3
FTRs [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Assets	0.1		0.3	[2]
FTRs [Member] | Level 1 [Member]
Total Derivative Assets
FTRs [Member] | Level 2 [Member]
Total Derivative Assets	0.1		0.3
FTRs [Member] | Level 3 [Member]
Total Derivative Assets
FTRs [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets	0.1		0.3
DP&L [Member] | Futures [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Assets	1.7		1.6	[2]
DP&L [Member] | Futures [Member] | Level 1 [Member]
Total Derivative Assets	1.7		1.6
DP&L [Member] | Futures [Member] | Level 2 [Member]
Total Derivative Assets
DP&L [Member] | Futures [Member] | Level 3 [Member]
Total Derivative Assets
DP&L [Member] | Futures [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets
Futures [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Assets	1.7		1.6	[2]
Futures [Member] | Level 1 [Member]
Total Derivative Assets	1.7		1.6
Futures [Member] | Level 2 [Member]
Total Derivative Assets
Futures [Member] | Level 3 [Member]
Total Derivative Assets
DP&L [Member] | Forward NYMEX Coal Contracts [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Assets	2.3		37.5	[2]
DP&L [Member] | Forward NYMEX Coal Contracts [Member] | Level 1 [Member]
Total Derivative Assets
DP&L [Member] | Forward NYMEX Coal Contracts [Member] | Level 2 [Member]
Total Derivative Assets	2.3		37.5
DP&L [Member] | Forward NYMEX Coal Contracts [Member] | Level 3 [Member]
Total Derivative Assets
DP&L [Member] | Forward NYMEX Coal Contracts [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets	1.1		15.6
Forward NYMEX Coal Contracts [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Assets	2.3		37.5	[2]
Total Derivative Liabilities	(6.3)
Forward NYMEX Coal Contracts [Member] | Level 1 [Member]
Total Derivative Assets
Total Derivative Liabilities
Forward NYMEX Coal Contracts [Member] | Level 2 [Member]
Total Derivative Assets	2.3		37.5
Total Derivative Liabilities	(6.3)
Forward NYMEX Coal Contracts [Member] | Level 3 [Member]
Total Derivative Assets
Total Derivative Liabilities
Forward NYMEX Coal Contracts [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets	1.1		15.6
Interest Rate Hedge [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Assets			20.7	[2]
Total Derivative Liabilities			6.6	[2]
Interest Rate Hedge [Member] | Level 1 [Member]
Total Derivative Assets
Total Derivative Liabilities
Interest Rate Hedge [Member] | Level 2 [Member]
Total Derivative Assets			20.7
Total Derivative Liabilities			6.6
Interest Rate Hedge [Member] | Level 3 [Member]
Total Derivative Assets
Total Derivative Liabilities
Interest Rate Hedge [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets			20.7
Total Derivative Liabilities			6.6
DP&L [Member] | Forward Contract Power [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Assets	3.2		0.2	[2]
Total Derivative Liabilities	(2.8)		3.1	[2]
DP&L [Member] | Forward Contract Power [Member] | Level 1 [Member]
Total Derivative Assets
Total Derivative Liabilities
DP&L [Member] | Forward Contract Power [Member] | Level 2 [Member]
Total Derivative Assets	3.2		0.2
Total Derivative Liabilities	(2.8)		3.1
DP&L [Member] | Forward Contract Power [Member] | Level 3 [Member]
Total Derivative Assets
Total Derivative Liabilities
DP&L [Member] | Forward Contract Power [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets	0.8
Total Derivative Liabilities	(1.4)		2.0
Forward Contract Power [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Assets	12.8		0.2	[2]
Total Derivative Liabilities	(6.6)		3.1	[2]
Forward Contract Power [Member] | Level 1 [Member]
Total Derivative Assets
Total Derivative Liabilities
Forward Contract Power [Member] | Level 2 [Member]
Total Derivative Assets	12.8		0.2
Total Derivative Liabilities	(6.6)		3.1
Forward Contract Power [Member] | Level 3 [Member]
Total Derivative Assets
Total Derivative Liabilities
Forward Contract Power [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets	10.8
Total Derivative Liabilities	(4.0)		2.0
Short-term Investments - Bonds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Short-term Investments			15.1	[2]
Short-term Investments - Bonds [Member] | Level 1 [Member]
Total Short-term Investments
Short-term Investments - Bonds [Member] | Level 2 [Member]
Total Short-term Investments			15.1
Short-term Investments - Bonds [Member] | Level 3 [Member]
Total Short-term Investments
Short-term Investments - Bonds [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Short-term Investments			15.1
Short-term Investments - VRDNs [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Derivative Assets			54.2	[2]
Short-term Investments - VRDNs [Member] | Level 1 [Member]
Total Derivative Assets
Short-term Investments - VRDNs [Member] | Level 2 [Member]
Total Derivative Assets			54.2
Short-term Investments - VRDNs [Member] | Level 3 [Member]
Total Derivative Assets
Short-term Investments - VRDNs [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Derivative Assets			54.2
DP&L [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	39.5		37.6	[2]
Total Derivative Assets	7.3		39.6	[2]
Total Assets	46.8		77.2	[2]
Total Derivative Liabilities	(9.1)		3.1	[2]
Total Liabilities	(9.1)		3.1	[2]
Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	10.0		11.8	[2]
Total Derivative Assets	16.9		60.3	[2]
Total Assets	26.9		141.4	[2]
Total Short-term Investments			69.3	[2]
Total Derivative Liabilities	(41.4)		9.7	[2]
Total Liabilities	(41.4)		9.7	[2]
DP&L [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	29.5		25.8
Total Derivative Assets	1.7		1.6
Total Assets	31.2		27.4
Total Derivative Liabilities
Total Liabilities
Level 1 [Member]
Total Master Trust Assets, Fair Value
Total Derivative Assets	1.7		1.6
Total Assets	1.7		1.6
Total Short-term Investments
Total Derivative Liabilities
Total Liabilities
DP&L [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	10.0		11.8
Total Derivative Assets	5.6		38.0
Total Assets	15.6		49.8
Total Derivative Liabilities	(9.1)		3.1
Total Liabilities	(9.1)		3.1
Level 2 [Member]
Total Master Trust Assets, Fair Value	10.0		11.8
Total Derivative Assets	15.2		58.7
Total Assets	25.2		139.8
Total Short-term Investments			69.3
Total Derivative Liabilities	(41.4)		9.7
Total Liabilities	(41.4)		9.7
DP&L [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value
Total Derivative Assets
Total Assets
Total Derivative Liabilities
Total Liabilities
Level 3 [Member]
Total Master Trust Assets, Fair Value
Total Derivative Assets
Total Assets
Total Short-term Investments
Total Derivative Liabilities
Total Liabilities
DP&L [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	39.5		37.6
Total Derivative Assets	2.0		15.9
Total Assets	41.5		53.5
Total Derivative Liabilities	(1.4)		2.0
Total Liabilities	(1.4)		2.0
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	10.0		11.8
Total Derivative Assets	12.0		36.6
Total Assets	22.0		117.7
Total Short-term Investments			69.3
Total Derivative Liabilities	(32.5)		8.6
Total Liabilities	$ (32.5)		$ 8.6

[1] DPL stock held in the DP&L Master Trust is eliminated in consolidation.

[2] *Includes credit valuation adjustments for counterparty risk.