Fair Value Measurements (Fair Value and Cost of Non-Derivative Instruments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
DP&L [Member]
Total Master Trust Assets, Cost	$ 8.9	$ 10.3
Total Master Trust Assets, Fair Value	10.0	11.5
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost		10.3
Total Assets	8.9
Fair Value [Member]
Total Assets	10.0	11.5
Equity Securities [Member] | DP&L [Member]
Total Master Trust Assets, Cost	2.8	3.3
Total Master Trust Assets, Fair Value	3.7	4.4
Equity Securities [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	2.8	3.3
Equity Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	3.7	4.4
Debt Securities [Member] | DP&L [Member]
Total Master Trust Assets, Cost	4.8	5.4
Total Master Trust Assets, Fair Value	4.9	5.5
Hedge Funds, Multi-strategy [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value		0.4
Money Market Funds [Member] | DP&L [Member]
Total Master Trust Assets, Cost	0.1	0.3
Total Master Trust Assets, Fair Value	0.1	0.3
Money Market Funds [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	0.1	0.3
Money Market Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.1	0.3
Debt [Member] | DP&L [Member]
Debt, Cost	877.1	877.1
Debt, Fair Value	878.6	859.6
Debt [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	4.8	5.4
Debt, Cost	2,294.4	2,294.4
Debt [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	4.9	5.5
Debt, Fair Value	2,364.2	2,334.6
Hedge Funds, Global Opportunity [Member] | DP&L [Member]
Total Master Trust Assets, Cost	0.8	0.9
Total Master Trust Assets, Fair Value	0.9	0.9
Hedge Funds [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.4	0.9
Hedge Funds [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	0.8	0.9
Hedge Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.9	0.9
Real Estate Funds [Member] | DP&L [Member]
Total Master Trust Assets, Cost	0.4	0.4
Total Master Trust Assets, Fair Value	0.4	0.4
Real Estate Funds [Member] | Carrying (Reported) Amount, Fair Value Disclosure [Member]
Total Master Trust Assets, Cost	0.4	0.4
Real Estate Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	$ 0.4	$ 0.4