Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
DP&L [Member]
Total Master Trust Assets, Fair Value	$ 11.1	$ 10.6
Total Derivative Assets	6.8	7.5
Total Assets	17.9	18.1
Debt Instrument, Fair Value Disclosure	921.8	926.9
Total Derivative Liabilities	22.9	11.7
Total Liabilities	944.7	938.6
DP&L [Member] | Forward Contract Power [Member]
Total Derivative Assets	6.6	7.3
Total Derivative Liabilities	22.9	11.6
DP&L [Member] | Commodity Contract - FTR [Member]
Total Derivative Liabilities		0.1
DP&L [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets	0.2	0.2
Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	11.1	10.6
Total Derivative Assets	6.0	6.5
Total Assets	17.1	17.1
Total Derivative Liabilities	46.5	42.7
Total Liabilities	2,751.9	2,749.8
Estimate of Fair Value, Fair Value Disclosure [Member] | Forward Contract Power [Member]
Total Derivative Assets	5.8	6.3
Total Derivative Liabilities	23.3	13.1
Estimate of Fair Value, Fair Value Disclosure [Member] | Commodity Contract - FTR [Member]
Total Derivative Liabilities		0.1
Estimate of Fair Value, Fair Value Disclosure [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets	0.2	0.2
Estimate of Fair Value, Fair Value Disclosure [Member] | Interest Rate Contract [Member]
Total Derivative Liabilities	23.2	29.5
Level 1 [Member]
Total Master Trust Assets, Fair Value	0.2	0.2
Total Derivative Assets	0.2	0.2
Total Assets	0.4	0.4
Level 1 [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets	0.2	0.2
Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.2	0.2
Total Derivative Assets	0.2	0.2
Total Assets	0.4	0.4
Level 1 [Member] | DP&L [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets	0.2	0.2
Level 2 [Member]
Total Master Trust Assets, Fair Value	10.9	10.4
Total Derivative Assets	5.8	6.3
Total Assets	16.7	16.7
Total Derivative Liabilities	46.5	42.6
Total Liabilities	2,733.1	2,730.8
Level 2 [Member] | Forward Contract Power [Member]
Total Derivative Assets	5.8	6.3
Total Derivative Liabilities	23.3	13.1
Level 2 [Member] | Interest Rate Contract [Member]
Total Derivative Liabilities	23.2	29.5
Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	10.9	10.4
Total Derivative Assets	6.6	7.3
Total Assets	17.5	17.7
Debt Instrument, Fair Value Disclosure	903.0	908.0
Total Derivative Liabilities	22.9	11.6
Total Liabilities	925.9	919.6
Level 2 [Member] | DP&L [Member] | Forward Contract Power [Member]
Total Derivative Assets	6.6	7.3
Total Derivative Liabilities	22.9	11.6
Level 3 [Member]
Total Derivative Liabilities		0.1
Total Liabilities	18.8	19.0
Level 3 [Member] | Commodity Contract - FTR [Member]
Total Derivative Liabilities		0.1
Level 3 [Member] | DP&L [Member]
Debt Instrument, Fair Value Disclosure	18.8	18.9
Total Derivative Liabilities		0.1
Total Liabilities	18.8	19.0
Level 3 [Member] | DP&L [Member] | Commodity Contract - FTR [Member]
Total Derivative Liabilities		0.1
Equity Securities [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	5.7	5.1
Equity Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	5.7	5.1
Equity Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	5.7	5.1
Equity Securities [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	5.7	5.1
Equity Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	5.7	5.1
Debt Securities [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	4.9	5.0
Debt Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	4.9	5.0
Debt Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	4.9	5.0
Debt Securities [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	4.9	5.0
Debt Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	4.9	5.0
Money Market Funds [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.2	0.2
Money Market Funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.2	0.2
Money Market Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.2	0.2
Money Market Funds [Member] | Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.2	0.2
Money Market Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.2	0.2
Hedge Funds, Multi-strategy [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Hedge Funds, Multi-strategy [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Hedge Funds, Multi-strategy [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Hedge Funds, Multi-strategy [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Hedge Funds, Multi-strategy [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Debt [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Debt Instrument, Fair Value Disclosure	2,705.4	2,707.1
Debt [Member] | Level 2 [Member]
Debt Instrument, Fair Value Disclosure	2,686.6	2,688.2
Debt [Member] | Level 3 [Member]
Debt Instrument, Fair Value Disclosure	$ 18.8	$ 18.9