Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Total Master Trust Assets, Fair Value	$ 9.7	[1]	$ 11.5	[1]
Total Derivative Assets	14.9	[1]	13.8	[1]
Total Assets	24.6	[1]	25.3	[1]
Total Derivative Liabilities	12.2	[1]	10.6	[1]
Total Liabilities	2,217.0	[1]	2,345.2	[1]
Long Term Debt, Noncurrent	2,139.6		2,284.2
Forward Contract Power [Member]
Total Derivative Assets	14.9	[1]	13.4	[1]
Total Derivative Liabilities	11.1	[1]	10.6	[1]
Commodity Contract - FTR [Member]
Total Derivative Assets			0.2	[1]
Total Derivative Liabilities	0.6	[1]
Commodity Contract - Heating Oil [Member]
Total Derivative Assets			0.2	[1]
Total Derivative Liabilities	0.4	[1]
Natural Gas [Member]
Total Derivative Liabilities	0.1	[1]
DP&L [Member]
Total Master Trust Assets, Fair Value	9.7		11.5
Estimate of Fair Value, Fair Value Disclosure [Member]
Long Term Debt, Noncurrent	2,204.8	[1]	2,334.6	[1]
Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	9.7	[1]	11.5	[1]
Total Derivative Assets	15.1	[1]	13.8	[1]
Total Assets	24.8	[1]	25.3	[1]
Total Derivative Liabilities	12.3	[1]	10.6	[1]
Total Liabilities	894.8	[1]	870.2	[1]
Long Term Debt, Noncurrent	882.5	[1]	859.6	[1]
Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member] | Forward Contract Power [Member]
Total Derivative Assets	15.1	[1]	13.4	[1]
Total Derivative Liabilities	11.2	[1]	10.6	[1]
Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets			0.2	[1]
Total Derivative Liabilities	0.6	[1]
Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets			0.2	[1]
Total Derivative Liabilities	0.4	[1]
Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member] | Natural Gas [Member]
Total Derivative Liabilities	0.1	[1]
Level 1 [Member]
Total Master Trust Assets, Fair Value	8.9		10.6
Total Derivative Assets			0.2
Total Assets	8.9		10.8
Total Derivative Liabilities	0.5
Total Liabilities	0.5
Level 1 [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets			0.2
Total Derivative Liabilities	0.4
Level 1 [Member] | Natural Gas [Member]
Total Derivative Liabilities	0.1
Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	8.9		10.6
Total Derivative Assets			0.2
Total Assets	8.9		10.8
Total Derivative Liabilities	0.5
Total Liabilities	0.5
Level 1 [Member] | DP&L [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Assets			0.2
Total Derivative Liabilities	0.4
Level 1 [Member] | DP&L [Member] | Natural Gas [Member]
Total Derivative Liabilities	0.1
Level 2 [Member]
Total Master Trust Assets, Fair Value	0.8		0.9
Total Derivative Assets	13.7		13.4
Total Assets	14.5		14.3
Total Derivative Liabilities	11.1		10.6
Total Liabilities	2,197.7		2,326.7
Long Term Debt, Noncurrent	2,186.6		2,316.1
Level 2 [Member] | Forward Contract Power [Member]
Total Derivative Assets	13.7		13.4
Total Derivative Liabilities	11.1		10.6
Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.8		0.9
Total Derivative Assets	13.9		13.4
Total Assets	14.7		14.3
Total Derivative Liabilities	11.2		10.6
Total Liabilities	875.5		851.7
Long Term Debt, Noncurrent	864.3		841.1
Level 2 [Member] | DP&L [Member] | Forward Contract Power [Member]
Total Derivative Assets	13.9		13.4
Total Derivative Liabilities	11.2		10.6
Level 3 [Member]
Total Derivative Assets	1.2		0.2
Total Assets	1.2		0.2
Total Derivative Liabilities	0.6
Total Liabilities	18.8		18.5
Long Term Debt, Noncurrent	18.2		18.5
Level 3 [Member] | Forward Contract Power [Member]
Total Derivative Assets	1.2
Level 3 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets			0.2
Total Derivative Liabilities	0.6
Level 3 [Member] | DP&L [Member]
Total Derivative Assets	1.2		0.2
Total Assets	1.2		0.2
Total Derivative Liabilities	0.6
Total Liabilities	18.8		18.5
Long Term Debt, Noncurrent	18.2		18.5
Level 3 [Member] | DP&L [Member] | Forward Contract Power [Member]
Total Derivative Assets	1.2
Level 3 [Member] | DP&L [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets			0.2
Total Derivative Liabilities	0.6
Equity Securities [Member]
Total Master Trust Assets, Fair Value	3.7	[1]	4.4	[1]
Equity Securities [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	3.7		4.4
Equity Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	3.7	[1]	4.4	[1]
Equity Securities [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	3.7		4.4
Equity Securities [Member] | Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	3.7		4.4
Debt Securities [Member]
Total Master Trust Assets, Fair Value	4.7	[1]	5.5	[1]
Debt Securities [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	4.7		5.5
Debt Securities [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	4.7	[1]	5.5	[1]
Debt Securities [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	4.7		5.5
Debt Securities [Member] | Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	4.7		5.5
Money Market Funds [Member]
Total Master Trust Assets, Fair Value	0.1	[1]	0.3	[1]
Money Market Funds [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.1		0.3
Money Market Funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.1	[1]	0.3	[1]
Money Market Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.1		0.3
Money Market Funds [Member] | Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.1		0.3
Hedge Funds, Multi-strategy [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value			0.9	[1]
Hedge Funds, Multi-strategy [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value			0.9
Hedge Funds [Member]
Total Master Trust Assets, Fair Value	0.8	[1]	0.9	[1]
Hedge Funds [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.8		0.9
Hedge Funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.8	[1]
Hedge Funds [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.8		0.9
Hedge Funds [Member] | Level 2 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.8
Real Estate Funds [Member]
Total Master Trust Assets, Fair Value	0.4	[1]	0.4	[1]
Real Estate Funds [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.4		0.4
Real Estate Funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	0.4	[1]	0.4	[1]
Real Estate Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.4		0.4
Real Estate Funds [Member] | Level 1 [Member] | DP&L [Member]
Total Master Trust Assets, Fair Value	$ 0.4		$ 0.4

[1] (a)Includes credit valuation adjustment