Fair Value Measurements (Fair Value and Cost of Non-Derivative Instruments) (Details) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
DP&L [Member] | Equity Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	$ 17.3	[1]	$ 17.5	[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]
Equity Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	3.9		3.8
Equity Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	4.0		4.4
DP&L [Member] | Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	5.1		5.2
DP&L [Member] | Debt Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	5.6		5.5
Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	5.1		5.2
Debt Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	5.6		5.5
DP&L [Member] | Hedge Funds, Multi-strategy [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	0.3		0.3
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
Hedge Funds, Multi-strategy [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	0.3		0.3
Hedge Funds, Multi-strategy [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2		0.3
DP&L [Member] | Money Market Funds [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	0.2		1.6
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
Money Market Funds [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	0.2		1.6
Money Market Funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2		1.6
DP&L [Member] | Debt [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Debt	903.4		884.1
DP&L [Member] | Debt [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Long-term Debt, Fair Value	884.2		850.6
Debt [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Debt	1,224.0		1,324.1
Debt [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Long-term Debt, Fair Value	1,204.6		1,307.5
DP&L [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	22.9		24.6
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	9.5		10.9
Total Short-term Investments			69.3
Total Assets	9.5		80.2
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Short-term Investments - Bonds [Member]
Total Master Trust Assets, Cost			15.1
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Short-term Investments - VRDNs [Member]
Total Master Trust Assets, Cost			54.2
DP&L [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	39.5		37.6
Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	10.0		11.8
Total Short-term Investments			69.3
Total Assets			81.1
Estimate of Fair Value, Fair Value Disclosure [Member] | Short-term Investments - Bonds [Member]
Total Short-term Investments			15.1
Estimate of Fair Value, Fair Value Disclosure [Member] | Short-term Investments - VRDNs [Member]
Total Short-term Investments			$ 54.2

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