Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	$ 0.4	$ 0.2
Total Derivative Assets	0.0	0.0
Total Assets	0.4	0.2
Total Derivative Liabilities	0.0	0.0
Total Liabilities	0.0	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
Total Derivative Liabilities	0.0
Fair Value, Inputs, Level 1 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.4	0.2
Total Derivative Assets	0.0	0.0
Total Assets	0.4	0.2
Total Derivative Liabilities	0.0	0.0
Debt Instrument, Fair Value Disclosure	0.0	0.0
Total Liabilities	0.0	0.0
Fair Value, Inputs, 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
Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0
Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	8.3	8.8
Total Derivative Assets	20.7	30.5
Total Assets	29.0	39.3
Total Derivative Liabilities	26.7	23.9
Total Liabilities	1,916.4	1,981.1
Fair Value, Inputs, Level 2 [Member] | Forward Contract Power [Member]
Total Derivative Assets	19.5	30.5
Total Derivative Liabilities	26.0	23.9
Fair Value, Inputs, Level 2 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.2
Total Derivative Liabilities	0.7
Fair Value, Inputs, Level 2 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.3	8.8
Total Derivative Assets	20.7	30.6
Total Assets	29.0	39.4
Total Derivative Liabilities	26.7	23.9
Debt Instrument, Fair Value Disclosure	745.5	746.1
Total Liabilities	772.2	770.0
Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	19.5	30.6
Total Derivative Liabilities	26.0	23.9
Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.2
Total Derivative Liabilities	0.7
Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	0.0	0.0
Fair Value, Inputs, Level 3 [Member]
Total Derivative Assets	0.1	0.2
Total Assets	0.1	0.2
Total Derivative Liabilities	2.5	3.6
Total Liabilities	20.5	21.7
Fair Value, Inputs, Level 3 [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	2.5	3.1
Fair Value, Inputs, Level 3 [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0
Fair Value, Inputs, Level 3 [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.1	0.2
Total Derivative Liabilities	0.0	0.5
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Derivative Assets	0.1	0.2
Total Assets	0.1	0.2
Total Derivative Liabilities	2.5	3.6
Debt Instrument, Fair Value Disclosure	18.0	18.1
Total Liabilities	20.5	21.7
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	0.0	0.0
Total Derivative Liabilities	2.5	3.1
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	0.0
Total Derivative Liabilities	0.0
Fair Value, Inputs, Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.1	0.2
Total Derivative Liabilities	0.0	0.5
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	3.4	3.8
Equity Securities [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.4	3.8
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	4.4	4.3
Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.4	4.3
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	0.4	0.2
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.4	0.2
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
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	0.1	0.4
Hedge Funds [Member] | Fair Value, Inputs, Level 2 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.4
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	0.3	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	0.3	0.3
Tangible Assets [Member] | Fair Value, Inputs, Level 1 [Member]
Total Master Trust Assets, Fair Value	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
Tangible Assets [Member] | Fair Value, Inputs, Level 2 [Member]
Total Master Trust Assets, Fair Value	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
Debt [Member] | Fair Value, Inputs, Level 1 [Member]
Debt Instrument, Fair Value Disclosure	0.0	0.0
Debt [Member] | Fair Value, Inputs, Level 2 [Member]
Debt Instrument, Fair Value Disclosure	1,889.7	1,957.2
Debt [Member] | Fair Value, Inputs, Level 3 [Member]
Debt Instrument, Fair Value Disclosure	18.0	18.1
Fair Value [Member]
Total Master Trust Assets, Fair Value	8.7	9.0
Total Derivative Assets	20.8	30.7
Total Assets	29.5	39.7
Total Derivative Liabilities	29.2	27.5
Total Liabilities	1,936.9	2,002.8
Fair Value [Member] | Forward Contract Power [Member]
Total Derivative Assets	19.5	30.5
Total Derivative Liabilities	28.5	27.0
Fair Value [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.2
Total Derivative Liabilities	0.7
Fair Value [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.1	0.2
Total Derivative Liabilities	0.0	0.5
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	8.7	9.0
Total Derivative Assets	20.8	30.8
Total Assets	29.5	39.8
Total Derivative Liabilities	29.2	27.5
Debt Instrument, Fair Value Disclosure	763.5
Total Liabilities	792.7	791.7
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Forward Contract Power [Member]
Total Derivative Assets	19.5	30.6
Total Derivative Liabilities	28.5	27.0
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Interest Rate Contract [Member]
Total Derivative Assets	1.2
Total Derivative Liabilities	0.7
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Commodity Contract - FTR [Member]
Total Derivative Assets	0.1	0.2
Total Derivative Liabilities	0.0	0.5
Fair Value [Member] | Equity Securities [Member]
Total Master Trust Assets, Fair Value	3.4	3.8
Fair Value [Member] | Equity Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	3.4	3.8
Fair Value [Member] | Debt Securities [Member]
Total Master Trust Assets, Fair Value	4.4	4.3
Fair Value [Member] | Debt Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	4.4	4.3
Fair Value [Member] | Money Market Funds [Member]
Total Master Trust Assets, Fair Value	0.4	0.2
Fair Value [Member] | Money Market Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.4	0.2
Fair Value [Member] | Hedge Funds [Member]
Total Master Trust Assets, Fair Value	0.1	0.4
Fair Value [Member] | Hedge Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.4
Fair Value [Member] | Real Estate Funds [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Fair Value [Member] | Real Estate Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.3	0.3
Fair Value [Member] | Tangible Assets [Member]
Total Master Trust Assets, Fair Value	0.1	0.0
Fair Value [Member] | Tangible Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	0.1	0.0
Fair Value [Member] | Debt [Member]
Debt Instrument, Fair Value Disclosure	$ 1,907.7	1,975.3
Fair Value [Member] | Debt [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Fair Value Disclosure		$ 764.2