Fair Value Measurements (Fair Value and Cost of Non-Derivative Instruments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Hedge Funds [Member]
Total Master Trust Assets, Fair Value		$ 0.0
Carrying Value [Member]
Total Assets	$ 7.3	7.6
Carrying Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Assets	7.3	7.6
Carrying Value [Member] | Money Market Funds [Member]
Total Master Trust Assets, Cost	0.3	0.4
Carrying Value [Member] | Money Market Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.3	0.4
Carrying Value [Member] | Equity Securities [Member]
Total Master Trust Assets, Cost	2.5	2.4
Carrying Value [Member] | Equity Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	2.5	2.4
Carrying Value [Member] | Debt Securities [Member]
Total Master Trust Assets, Cost	4.3	4.4
Carrying Value [Member] | Debt Securities [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	4.3	4.4
Carrying Value [Member] | Hedge Funds [Member]
Total Master Trust Assets, Cost	0.1	0.0
Carrying Value [Member] | Hedge Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.1	0.0
Carrying Value [Member] | Real Estate Funds [Member]
Total Master Trust Assets, Cost	0.0	0.3
Carrying Value [Member] | Real Estate Funds [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.0	0.3
Carrying Value [Member] | Tangible Assets [Member]
Total Master Trust Assets, Cost	0.1	0.1
Carrying Value [Member] | Tangible Assets [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Cost	0.1	0.1
Carrying Value [Member] | Debt [Member]
Long-term Debt	1,704.8	1,858.4
Carrying Value [Member] | Debt [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Long-term Debt	646.6	735.7
Fair Value [Member]
Total Master Trust Assets, Fair Value	9.1	8.7
Total Assets	9.1	8.7
Fair Value [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Total Master Trust Assets, Fair Value	9.1
Total Assets	9.1	8.7
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] | 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] | 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, Fair Value	1,819.3	1,907.7
Fair Value [Member] | Debt [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt, Fair Value	$ 658.4	$ 750.1