Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Level 1 [Member]
Total Master Trust Assets, Fair Value	$ 0.2	$ 0.1
Total Derivative Assets	0.0	0.0
Total Assets	0.2	0.1
Total Derivative Liabilities	0.0	0.5
Total Liabilities	0.0	0.5
Level 1 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0	0.0
Level 1 [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	0.0
Level 1 [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Liabilities		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 Derivative Assets	0.0	0.0
Total Assets	0.2	0.1
Total Derivative Liabilities	0.0	0.5
Total Liabilities	0.0	0.5
Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0	0.0
Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	0.0
Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Liabilities		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	8.8	9.6
Total Derivative Assets	30.0	13.7
Total Assets	38.8	23.3
Total Derivative Liabilities	22.2	11.1
Total Liabilities	2,037.8	2,197.7
Level 2 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0	0.0
Level 2 [Member] | Forward Contract Power [Member]
Total Derivative Assets	30.0	13.7
Total Derivative Liabilities	22.2	11.1
Level 2 [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Liabilities		0.0
Level 2 [Member] | Natural Gas [Member]
Total Derivative Liabilities		0.0
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.8	9.6
Total Derivative Assets	30.2	13.9
Total Assets	39.0	23.5
Total Derivative Liabilities	22.3	11.2
Total Liabilities	768.5	875.5
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0	0.0
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	30.2	13.9
Total Derivative Liabilities	22.3	11.2
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Liabilities		0.0
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Total Derivative Liabilities		0.0
Level 3 [Member]
Total Derivative Assets	0.4	1.2
Total Assets	0.4	1.2
Total Derivative Liabilities	2.9	0.6
Total Liabilities	21.0	18.8
Level 3 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.4
Total Derivative Liabilities	0.7	0.6
Level 3 [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.0	1.2
Total Derivative Liabilities	2.2
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Derivative Assets	0.4	1.2
Total Assets	0.4	1.2
Total Derivative Liabilities	2.9	0.6
Total Liabilities	21.0	18.8
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.4
Total Derivative Liabilities	0.7	0.6
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.0	1.2
Total Derivative Liabilities	2.2
Fair Value [Member]
Total Master Trust Assets, Fair Value	9.0	9.7
Total Derivative Assets	30.4	14.9
Total Assets	39.4	24.6
Total Derivative Liabilities	25.1	12.2
Total Liabilities	2,058.8	2,217.0
Fair Value [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.4
Total Derivative Liabilities	0.7	0.6
Fair Value [Member] | Forward Contract Power [Member]
Total Derivative Assets	30.0	14.9
Total Derivative Liabilities	24.4	11.1
Fair Value [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Liabilities		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	9.0	9.7
Total Derivative Assets	30.6	15.1
Total Assets	39.6	24.8
Total Derivative Liabilities	25.2	12.3
Total Liabilities	789.5	894.8
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.4
Total Derivative Liabilities	0.7	0.6
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	30.2	15.1
Total Derivative Liabilities	24.5	11.2
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - Heating Oil [Member]
Total Derivative Liabilities		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] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
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 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] | Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	3.4	3.7
Equity Securities [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.4	3.7
Equity Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	3.4	3.7
Equity Securities [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.4	3.7
Debt Securities [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	4.4	4.7
Debt Securities [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.4	4.7
Debt Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	4.4	4.7
Debt Securities [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.4	4.7
Hedge Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] | Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
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 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Real Estate Funds [Member] | Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
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 1 [Member]
Debt	0.0	0.0
Debt [Member] | Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt	0.0	0.0
Debt [Member] | Level 2 [Member]
Debt	2,015.6	2,186.6
Debt [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt	746.2	864.3
Debt [Member] | Level 3 [Member]
Debt	18.1	18.2
Debt [Member] | Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt	18.1	18.2
Debt [Member] | Fair Value [Member]
Debt	2,033.7	2,204.8
Debt [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt	$ 764.3	$ 882.5