Fair Value (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Asset Retirement Obligations, Noncurrent	$ 9.4	$ 9.4
Level 1 [Member]
Total Master Trust Assets, Fair Value	0.1	0.4
Total Derivative Assets	0.0	0.0
Total Assets	0.1	0.4
Total Liabilities	0.0	0.0
Level 1 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.4
Total Derivative Assets	0.0	0.0
Total Assets	0.1	0.4
Total 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
Level 2 [Member]
Total Master Trust Assets, Fair Value	8.2	7.7
Total Derivative Assets	1.1	1.5
Total Assets	9.3	9.2
Total Liabilities	1,527.3	1,501.9
Level 2 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.1	1.5
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.2	7.7
Total Derivative Assets	1.1	1.5
Total Assets	9.3	9.2
Total Liabilities	575.0	576.1
Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.1	1.5
Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Total Derivative Assets	0.0	0.0
Total Assets	0.0	0.0
Total Liabilities	17.6	17.7
Level 3 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	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	0.0
Total Assets	0.0	0.0
Total Liabilities	17.6	17.7
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Fair Value [Member]
Total Master Trust Assets, Fair Value	8.3	8.1
Total Derivative Assets	1.1	1.5
Total Assets	9.4	9.6
Total Liabilities	1,544.9	1,519.6
Fair Value [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.1	1.5
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.3	8.1
Total Derivative Assets	1.1	1.5
Total Assets	9.4	9.6
Total Liabilities	592.6	593.8
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.1	1.5
Money Market Funds [Member] | Level 1 [Member]
Total Master Trust Assets, Fair Value	0.1	0.4
Money Market Funds [Member] | Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	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	0.1	0.4
Money Market Funds [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	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	3.5
Equity Securities [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.9	3.5
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	3.5
Equity Securities [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.9	3.5
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	4.1	4.0
Debt Securities [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.1	4.0
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	4.1	4.0
Debt Securities [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.1	4.0
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
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	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	1,527.3	1,501.9
Debt [Member] | Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt	575.0	576.1
Debt [Member] | Level 3 [Member]
Debt	17.6	17.7
Debt [Member] | Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt	17.6	17.7
Debt [Member] | Fair Value [Member]
Debt	1,544.9	1,519.6
Debt [Member] | Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt	$ 592.6	$ 593.8