Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	$ 0.3	$ 0.4
Total Derivative Assets	0.2	0.0
Total Assets	0.5	0.4
Total Derivative Liabilities	0.1	0.0
Total Liabilities	0.1	0.0
Fair Value, Inputs, Level 1 [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities		0.0
Fair Value, Inputs, Level 1 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		0.0
Total Derivative Liabilities	0.0
Fair Value, Inputs, Level 1 [Member] | Natural Gas [Member]
Total Derivative Assets	0.2
Total Derivative Liabilities	0.1
Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3
Total Derivative Assets	0.0
Total Assets	0.3	0.4
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
Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	8.8	8.3
Total Derivative Assets	12.6	20.7
Total Assets	21.4	29.0
Total Derivative Liabilities	14.9	26.7
Total Liabilities	1,816.4	1,916.4
Fair Value, Inputs, Level 2 [Member] | Forward Contract Power [Member]
Total Derivative Assets	10.8	19.5
Total Derivative Liabilities	14.9	26.0
Fair Value, Inputs, Level 2 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.8	1.2
Total Derivative Liabilities		0.7
Fair Value, Inputs, Level 2 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		0.0
Total Derivative Liabilities	0.0
Fair Value, Inputs, Level 2 [Member] | Natural Gas [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0
Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.8
Total Derivative Assets	1.8
Total Assets	10.6	8.3
Debt Instrument, Fair Value Disclosure		732.1
Total Liabilities	640.6	732.1
Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.8
Fair Value, Inputs, 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	0.3	2.5
Total Liabilities	18.1	20.5
Fair Value, Inputs, Level 3 [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	2.5
Fair Value, Inputs, Level 3 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities		0.0
Fair Value, Inputs, Level 3 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		0.1
Total Derivative Liabilities	0.3
Fair Value, Inputs, Level 3 [Member] | Natural Gas [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Assets		0.0
Debt Instrument, Fair Value Disclosure		18.0
Total Liabilities	17.8	18.0
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	4.2	3.4
Equity Securities [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.2	3.4
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	4.3	4.4
Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.3	4.4
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	0.4
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	0.4
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]
Total Master Trust Assets, Fair Value		0.0
Hedge Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	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	0.2	0.1
Hedge Funds [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.2	0.1
Hedge Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Real Estate Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Real Estate 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
Real Estate Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	0.0	0.3
Real Estate Funds [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.3
Real Estate 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	0.1	0.1
Tangible Assets [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	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	1,801.5	1,889.7
Debt [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Fair Value Disclosure	640.6
Debt [Member] | Fair Value, Inputs, Level 3 [Member]
Debt Instrument, Fair Value Disclosure	17.8	18.0
Debt [Member] | Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Fair Value Disclosure	17.8
Fair Value [Member]
Total Master Trust Assets, Fair Value	9.1	8.7
Total Derivative Assets	12.8	20.8
Total Assets	21.9	29.5
Total Derivative Liabilities	15.3	29.2
Total Liabilities	1,834.6	1,936.9
Fair Value [Member] | Forward Contract Power [Member]
Total Derivative Assets	10.8	19.5
Total Derivative Liabilities	14.9	28.5
Fair Value [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.8	1.2
Total Derivative Liabilities		0.7
Fair Value [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets		0.1
Total Derivative Liabilities	0.3
Fair Value [Member] | Natural Gas [Member]
Total Derivative Assets	0.2
Total Derivative Liabilities	0.1
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	9.1
Total Derivative Assets	1.8
Total Assets	10.9	8.7
Total Liabilities	658.4	750.1
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.8
Fair Value [Member] | Equity Securities [Member]
Total Master Trust Assets, Fair Value	4.2	3.4
Fair Value [Member] | Equity Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.2	3.4
Fair Value [Member] | Debt Securities [Member]
Total Master Trust Assets, Fair Value	4.3	4.4
Fair Value [Member] | Debt Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.3	4.4
Fair Value [Member] | Money Market Funds [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Fair Value [Member] | Money Market Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.4
Fair Value [Member] | Hedge Funds [Member]
Total Master Trust Assets, Fair Value	0.2	0.1
Fair Value [Member] | Hedge Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.2	0.1
Fair Value [Member] | Real Estate Funds [Member]
Total Master Trust Assets, Fair Value	0.0	0.3
Fair Value [Member] | Real Estate Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.0	0.3
Fair Value [Member] | Tangible Assets [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Fair Value [Member] | Tangible Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.1
Fair Value [Member] | Debt [Member]
Debt Instrument, Fair Value Disclosure	1,819.3	1,907.7
Fair Value [Member] | Debt [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Fair Value Disclosure	$ 658.4	$ 750.1