Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
DP&L [Member]
Total Master Trust Assets, Fair Value	$ 11.4	$ 10.6	$ 10.6
Total Derivative Assets	22.5	7.5
Total Assets	33.9	18.1
Debt Instrument, Fair Value Disclosure	905.0	926.9
Total Derivative Liabilities	14.0	11.7
Total Liabilities	919.0	938.6
DP&L [Member] | Forward Contract Power [Member]
Total Derivative Assets	22.4	7.3
Total Derivative Liabilities	12.9	11.6
DP&L [Member] | Commodity Contract - FTR [Member]
Total Derivative Liabilities	1.0	0.1
DP&L [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets	0.1	0.2
Total Derivative Liabilities	0.1
Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	11.4	10.6
Total Derivative Assets	22.3	6.5
Total Assets	33.7	17.1
Total Derivative Liabilities	23.9	42.7
Total Liabilities	2,427.2	2,749.8
Estimate of Fair Value, Fair Value Disclosure [Member] | Forward Contract Power [Member]
Total Derivative Assets	22.2	6.3
Total Derivative Liabilities	13.3	13.1
Estimate of Fair Value, Fair Value Disclosure [Member] | Commodity Contract - FTR [Member]
Total Derivative Liabilities	1.0	0.1
Estimate of Fair Value, Fair Value Disclosure [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets	0.1	0.2
Total Derivative Liabilities	0.1
Estimate of Fair Value, Fair Value Disclosure [Member] | Interest Rate Contract [Member]
Total Derivative Liabilities	9.5	29.5
Level 1 [Member]
Total Master Trust Assets, Fair Value	0.6	0.2
Total Derivative Assets		0.2
Total Assets	0.6	0.4
Total Derivative Liabilities	0.1
Total Liabilities	0.1
Level 1 [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets		0.2
Total Derivative Liabilities	0.1
Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.6	0.2
Total Derivative Assets		0.2
Total Assets	0.6	0.4
Total Derivative Liabilities	0.1
Total Liabilities	0.1
Level 1 [Member] | DP&L [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets		0.2
Total Derivative Liabilities	0.1
Level 2 [Member]
Total Master Trust Assets, Fair Value	10.8	10.4
Total Derivative Assets	22.2	6.3
Total Assets	33.0	16.7
Total Derivative Liabilities	22.8	42.6
Total Liabilities	2,407.5	2,730.8
Level 2 [Member] | Forward Contract Power [Member]
Total Derivative Assets	22.2	6.3
Total Derivative Liabilities	13.3	13.1
Level 2 [Member] | Interest Rate Contract [Member]
Total Derivative Liabilities	9.5	29.5
Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	10.8	10.4
Total Derivative Assets	22.4	7.3
Total Assets	33.2	17.7
Debt Instrument, Fair Value Disclosure	886.4	908.0
Total Derivative Liabilities	12.9	11.6
Total Liabilities	899.3	919.6
Level 2 [Member] | DP&L [Member] | Forward Contract Power [Member]
Total Derivative Assets	22.4	7.3
Total Derivative Liabilities	12.9	11.6
Level 3 [Member]
Total Derivative Assets	0.1
Total Assets	0.1
Total Derivative Liabilities	1.0	0.1
Total Liabilities	19.6	19.0
Level 3 [Member] | Commodity Contract - FTR [Member]
Total Derivative Liabilities	1.0	0.1
Level 3 [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets	0.1
Level 3 [Member] | DP&L [Member]
Total Derivative Assets	0.1
Total Assets	0.1
Debt Instrument, Fair Value Disclosure	18.6	18.9
Total Derivative Liabilities	1.0	0.1
Total Liabilities	19.6	19.0
Level 3 [Member] | DP&L [Member] | Commodity Contract - FTR [Member]
Total Derivative Liabilities	1.0	0.1
Level 3 [Member] | DP&L [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets	0.1
Equity Securities [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	3.8	5.1	5.1
Equity Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	3.8	5.1
Equity Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	3.8	5.1
Equity Securities [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	3.8	5.1
Equity Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	3.8	5.1
Debt Securities [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	5.8	5.0	5.0
Debt Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	5.8	5.0
Debt Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	5.8	5.0
Debt Securities [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	5.8	5.0
Debt Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	5.8	5.0
Money Market Funds [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.6	0.2	0.2
Money Market Funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.6	0.2
Money Market Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.6	0.2
Money Market Funds [Member] | Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.6	0.2
Money Market Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.6	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
Hedge Funds, Multi-strategy [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value		0.3
Hedge Funds, Multi-strategy [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value		0.3
Hedge Funds, Multi-strategy [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value		0.3
Debt [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Debt Instrument, Fair Value Disclosure	2,403.3	2,707.1
Debt [Member] | Level 2 [Member]
Debt Instrument, Fair Value Disclosure	2,384.7	2,688.2
Debt [Member] | Level 3 [Member]
Debt Instrument, Fair Value Disclosure	18.6	18.9
Hedge Funds [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.4
Hedge Funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.4
Hedge Funds [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.4
Hedge Funds [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.4
Hedge Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.4
Hedge Funds, Global Opportunity [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.8
Hedge Funds, Global Opportunity [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Total Master Trust Assets, Fair Value	0.8
Hedge Funds, Global Opportunity [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.8
Hedge Funds, Global Opportunity [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.8
Hedge Funds, Global Opportunity [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	$ 0.8