Fair Value Measurements (Fair Value and Cost of Non-Derivative Instruments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Total Master Trust Assets, Cost	$ 9.1	$ 9.4
Total Master Trust Assets, Fair Value	10.6	10.3
DP&L [Member]
Total Master Trust Assets, Cost	9.1	9.4
Total Master Trust Assets, Fair Value	10.6	10.3
Equity Securities [Member]
Total Master Trust Assets, Cost	4.0	3.9
Total Master Trust Assets, Fair Value	5.1	4.4
Equity Securities [Member] | DP&L [Member]
Total Master Trust Assets, Cost	4.0	3.9
Total Master Trust Assets, Fair Value	5.1	4.4
Debt Securities [Member]
Total Master Trust Assets, Cost	4.6	5.0
Total Master Trust Assets, Fair Value	5.0	5.5
Debt Securities [Member] | DP&L [Member]
Total Master Trust Assets, Cost	4.6	5.0
Total Master Trust Assets, Fair Value	5.0	5.5
Hedge Funds, Multi-strategy [Member]
Total Master Trust Assets, Cost	0.3	0.3
Total Master Trust Assets, Fair Value	0.3	0.2
Money Market Funds [Member]
Total Master Trust Assets, Cost	0.2	0.2
Total Master Trust Assets, Fair Value	0.2	0.2
Money Market Funds [Member] | DP&L [Member]
Total Master Trust Assets, Cost	0.2	0.2
Total Master Trust Assets, Fair Value	0.2	0.2
Debt [Member]
Debt, Cost	2,609.9	2,629.3
Debt, Fair Value	2,707.1	2,710.6
Debt [Member] | DP&L [Member]
Debt, Cost	903.1	903.4
Debt, Fair Value	926.9	934.5
Multi Strategy Fund [Member] | DP&L [Member]
Total Master Trust Assets, Cost	0.3	0.3
Total Master Trust Assets, Fair Value	$ 0.3	$ 0.2