Fair Value Measurements (Fair Value and Cost of Non-Derivative Instruments) (Details) (USD $) In Millions	Jun. 30, 2011		Dec. 31, 2010
DP&L [Member] | Equity Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	$ 16.9	[1]	$ 17.5	[1]
DP&L [Member] | Equity Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	33.7	[1]	30.2	[1]
Equity Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	3.5		3.8
Equity Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	4.2		4.4
DP&L [Member] | Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	5.2		5.2
DP&L [Member] | Debt Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	5.5		5.5
Debt Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	5.2		5.2
Debt Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	5.5		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] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.3		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] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.3		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] | Fair Value [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] | Fair Value [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] | Fair Value [Member]
Long-term Debt, Fair Value	877.3		850.6
Debt [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Debt	1,221.4		1,324.1
Debt [Member] | Fair Value [Member]
Long-term Debt, Fair Value	1,198.7		1,307.5
DP&L [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	22.6		24.6
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	9.2		10.9
Total Short-term Investments			69.3
Total Assets	9.2		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] | Fair Value [Member]
Total Master Trust Assets, Fair Value	39.7		37.6
Fair Value [Member]
Total Master Trust Assets, Fair Value	10.2		11.8
Total Assets			81.1
Estimate of Fair Value, Fair Value Disclosure [Member]
Total Short-term Investments			69.3
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.