Fair Value (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Level 1 [Member]
Total Master Trust Assets, Fair Value	$ 0.3	$ 0.4
Total Derivative Assets	0.3	0.0
Total Assets	0.6	0.4
Total Derivative Liabilities	0.5	0.0
Total Liabilities	0.5	0.0
Level 1 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		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] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	0.0
Level 1 [Member] | Natural Gas [Member]
Total Derivative Assets	0.3
Total Derivative Liabilities	0.5
Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Total Derivative Assets	0.3	0.0
Total Assets	0.6	0.4
Total Derivative Liabilities	0.5	0.0
Total Liabilities	0.5	0.0
Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		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] | Interest Rate Contract [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] | Natural Gas [Member]
Total Derivative Assets	0.3
Total Derivative Liabilities	0.5
Level 2 [Member]
Total Master Trust Assets, Fair Value	8.4	8.3
Total Derivative Assets	20.3	20.7
Total Assets	28.7	29.0
Total Derivative Liabilities	17.3	26.7
Total Liabilities	1,940.0	1,916.4
Level 2 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		0.0
Level 2 [Member] | Forward Contract Power [Member]
Total Derivative Assets	18.9	19.5
Total Derivative Liabilities	17.0	26.0
Level 2 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.4	1.2
Total Derivative Liabilities	0.3	0.7
Level 2 [Member] | Natural Gas [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.4	8.3
Total Derivative Assets	20.4	20.7
Total Assets	28.8	29.0
Total Derivative Liabilities	17.3	26.7
Total Liabilities	761.7	772.2
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		0.0
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	19.0	19.5
Total Derivative Liabilities	17.0	26.0
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.4	1.2
Total Derivative Liabilities	0.3	0.7
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0
Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Total Derivative Assets	0.0	0.1
Total Assets	0.0	0.1
Total Derivative Liabilities	1.1	2.5
Total Liabilities	19.0	20.5
Level 3 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		0.1
Level 3 [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	1.1	2.5
Level 3 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	0.0
Level 3 [Member] | Natural Gas [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Total Derivative Assets	0.0	0.1
Total Assets	0.0	0.1
Total Derivative Liabilities	1.1	2.5
Total Liabilities	19.0	20.5
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		0.1
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	1.1	2.5
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	0.0
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0
Fair Value [Member]
Total Master Trust Assets, Fair Value	8.7	8.7
Total Derivative Assets	20.6	20.8
Total Assets	29.3	29.5
Total Derivative Liabilities	18.9	29.2
Total Liabilities	1,959.5	1,936.9
Fair Value [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		0.1
Fair Value [Member] | Forward Contract Power [Member]
Total Derivative Assets	18.9	19.5
Total Derivative Liabilities	18.1	28.5
Fair Value [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.4	1.2
Total Derivative Liabilities	0.3	0.7
Fair Value [Member] | Natural Gas [Member]
Total Derivative Assets	0.3
Total Derivative Liabilities	0.5
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.7	8.7
Total Derivative Assets	20.7	20.8
Total Assets	29.4	29.5
Total Derivative Liabilities	18.9	29.2
Total Liabilities	781.2	792.7
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		0.1
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	19.0	19.5
Total Derivative Liabilities	18.1	28.5
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.4	1.2
Total Derivative Liabilities	0.3	0.7
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Natural Gas [Member]
Total Derivative Assets	0.3
Total Derivative Liabilities	0.5
Money Market Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Money Market Funds [Member] | Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
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] | Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] | Level 3 [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	0.3	0.4
Money Market Funds [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
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	3.9	3.4
Equity Securities [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.9	3.4
Equity Securities [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] | Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	3.9	3.4
Equity Securities [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.9	3.4
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	4.3	4.4
Debt Securities [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.3	4.4
Debt Securities [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	4.3	4.4
Debt Securities [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.3	4.4
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	0.1	0.1
Hedge Funds [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Hedge Funds [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] | Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Hedge Funds [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Real Estate Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value		0.0
Real Estate Funds [Member] | Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value		0.0
Real Estate Funds [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value		0.3
Real Estate Funds [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value		0.3
Real Estate Funds [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value		0.0
Real Estate Funds [Member] | Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value		0.0
Real Estate Funds [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.0	0.3
Real Estate Funds [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.3
Tangible Assets [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] | Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0
Tangible Assets [Member] | Level 2 [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Tangible Assets [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Tangible Assets [Member] | Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] | Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] | Fair Value [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Tangible Assets [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
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	1,922.7	1,889.7
Debt [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt	744.4	745.5
Debt [Member] | Level 3 [Member]
Debt	17.9	18.0
Debt [Member] | Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt	17.9	18.0
Debt [Member] | Fair Value [Member]
Debt	1,940.6	1,907.7
Debt [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt	$ 762.3	$ 763.5