Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	$ 0.3	$ 0.3
Total Derivative Assets	0.0	0.0
Total Assets	0.3	0.3
Total Liabilities	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Total Derivative Assets	0.0	0.0
Total Assets	0.3	0.3
Debt Instrument, Fair Value Disclosure		0.0
Total Liabilities	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	8.6	8.5
Total Derivative Assets	0.0	0.1
Total Assets	8.6	8.6
Total Liabilities	1,554.2	1,386.5
Fair Value, Inputs, Level 2 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.1
Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.6	8.5
Total Derivative Assets	0.0	0.1
Total Assets	8.6	8.6
Debt Instrument, Fair Value Disclosure		583.0
Total Liabilities	638.6	583.0
Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.1
Fair Value, Inputs, 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	17.4	17.5
Fair Value, Inputs, Level 3 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Assets		0.0
Debt Instrument, Fair Value Disclosure		17.5
Total Liabilities	17.4	17.5
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Equity Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	4.5	4.2
Equity Securities [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.5	4.2
Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	4.1	4.1
Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.1	4.1
Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] | Fair Value, Inputs, 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, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Hedge Funds [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Hedge Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] | Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] | Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Tangible Assets [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Tangible Assets [Member] | Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt [Member] | Fair Value, Inputs, Level 1 [Member]
Debt Instrument, Fair Value Disclosure	0.0	0.0
Debt [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Fair Value Disclosure	0.0
Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Debt Instrument, Fair Value Disclosure	1,554.2	1,386.5
Debt [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Fair Value Disclosure	638.6
Debt [Member] | Fair Value, Inputs, Level 3 [Member]
Debt Instrument, Fair Value Disclosure	17.4	17.5
Debt [Member] | Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Fair Value Disclosure	17.4
Fair Value [Member]
Total Master Trust Assets, Fair Value	8.9	8.8
Total Derivative Assets	0.0	0.1
Total Assets	8.9	8.9
Total Liabilities	1,571.6	1,404.0
Fair Value [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.1
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.9	8.8
Total Derivative Assets	0.0	0.1
Total Assets	8.9	8.9
Total Liabilities	656.0	600.5
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.1
Fair Value [Member] | Equity Securities [Member]
Total Master Trust Assets, Fair Value	4.5	4.2
Fair Value [Member] | Equity Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.5	4.2
Fair Value [Member] | Debt Securities [Member]
Total Master Trust Assets, Fair Value	4.1	4.1
Fair Value [Member] | Debt Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.1	4.1
Fair Value [Member] | Money Market Funds [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Fair Value [Member] | Money Market Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Fair Value [Member] | Hedge Funds [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Fair Value [Member] | Hedge Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Fair Value [Member] | Tangible Assets [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Fair Value [Member] | Tangible Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.1
Fair Value [Member] | Debt [Member]
Debt Instrument, Fair Value Disclosure	1,571.6	1,404.0
Fair Value [Member] | Debt [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Fair Value Disclosure	$ 656.0	$ 600.5