Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Level 1 [Member]
Total Master Trust Assets, Fair Value	$ 0.2	$ 0.1
Total Assets	0.2	0.1
Total Derivative Liabilities	0.1	0.5
Total Liabilities	0.1	0.5
Level 1 [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Liabilities	0.1	0.4
Level 1 [Member] | Natural Gas [Member]
Total Derivative Liabilities		0.1
Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.2	0.1
Total Assets	0.2	0.1
Total Derivative Liabilities	0.1	0.5
Total Liabilities	0.1	0.5
Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Liabilities	0.1	0.4
Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Total Derivative Liabilities		0.1
Level 2 [Member]
Total Master Trust Assets, Fair Value	9.2	9.6
Total Derivative Assets	18.8	13.7
Total Assets	28.0	23.3
Total Derivative Liabilities	17.7	11.1
Total Liabilities	2,235.0	2,197.7
Level 2 [Member] | Forward Contract Power [Member]
Total Derivative Assets	18.8	13.7
Total Derivative Liabilities	17.7	11.1
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	9.2	9.6
Total Derivative Assets	19.0	13.9
Total Assets	28.2	23.5
Total Derivative Liabilities	17.9	11.2
Debt Instrument, Fair Value Disclosure	865.1	864.3
Total Liabilities	883.0	875.5
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	19.0	13.9
Total Derivative Liabilities	17.9	11.2
Level 3 [Member]
Total Derivative Assets	0.9	1.2
Total Assets	0.9	1.2
Total Derivative Liabilities	0.9	0.6
Total Liabilities	18.9	18.8
Level 3 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.4
Total Derivative Liabilities	0.9	0.6
Level 3 [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.5	1.2
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Derivative Assets	0.9	1.2
Total Assets	0.9	1.2
Total Derivative Liabilities	0.9	0.6
Debt Instrument, Fair Value Disclosure	18.0	18.2
Total Liabilities	18.9	18.8
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.4
Total Derivative Liabilities	0.9	0.6
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.5	1.2
Fair Value [Member]
Total Master Trust Assets, Fair Value	9.4	9.7
Total Derivative Assets	19.7	14.9
Total Assets	29.1	24.6
Total Derivative Liabilities	18.7	12.2
Total Liabilities	2,254.0	2,217.0
Fair Value [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.4
Total Derivative Liabilities	0.9	0.6
Fair Value [Member] | Forward Contract Power [Member]
Total Derivative Assets	19.3	14.9
Total Derivative Liabilities	17.7	11.1
Fair Value [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Liabilities	0.1	0.4
Fair Value [Member] | Natural Gas [Member]
Total Derivative Liabilities		0.1
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	9.4	9.7
Total Derivative Assets	19.9	15.1
Total Assets	29.3	24.8
Total Derivative Liabilities	18.9	12.3
Debt Instrument, Fair Value Disclosure	883.1	882.5
Total Liabilities	902.0	894.8
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.4
Total Derivative Liabilities	0.9	0.6
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	19.5	15.1
Total Derivative Liabilities	17.9	11.2
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Liabilities	0.1	0.4
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Total Derivative Liabilities		0.1
Money Market Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.2	0.1
Money Market Funds [Member] | Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.2	0.1
Money Market Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.2	0.1
Money Market Funds [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.2	0.1
Equity Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	3.7	3.7
Equity Securities [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.7	3.7
Equity Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	3.7	3.7
Equity Securities [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.7	3.7
Debt Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	4.5	4.7
Debt Securities [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.5	4.7
Debt Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	4.5	4.7
Debt Securities [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.5	4.7
Hedge Funds [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.7	0.8
Hedge Funds [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.7	0.8
Hedge Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.7	0.8
Hedge Funds [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.7	0.8
Real Estate Funds [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Real Estate Funds [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Real Estate Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Real Estate Funds [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Debt [Member] | Level 2 [Member]
Debt Instrument, Fair Value Disclosure	2,217.3	2,186.6
Debt [Member] | Level 3 [Member]
Debt Instrument, Fair Value Disclosure	18.0	18.2
Debt [Member] | Fair Value [Member]
Debt Instrument, Fair Value Disclosure	$ 2,235.3	$ 2,204.8