Regulatory Assets and Liabilities (Narrative) (Details) - USD ($) $ in Millions			12 Months Ended
	Oct. 20, 2017	Jan. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Regulatory assets earning a return			$ 12.5
Distribution Modernization Rider	$ 105.0
Return on Equity SEET Threshold			12.00%
Distribution Modernization Rider Period	3 years
Regulatory Assets			$ 187.1	$ 204.0
Regulatory Liabilities			236.0	164.1
Regulatory assets, non-current			$ 163.2	203.9
Tax rate before change due to Tax Cuts and Jobs Act of 2017			35.00%
Regulatory liabilities, non-current			$ 221.2	130.4
Deferred Income Tax Charge [Member]
Regulatory assets, non-current			0.0	35.9
THE DAYTON POWER AND LIGHT COMPANY [Member]
Regulatory assets earning a return			$ 12.5
Distribution Modernization Rider		$ 105.0
Return on Equity SEET Threshold			12.00%
Distribution Modernization Rider Period		3 years
Regulatory Assets			$ 187.1	204.0
Regulatory Liabilities			236.0	164.1
Regulatory assets, non-current			$ 163.2	203.9
Tax rate before change due to Tax Cuts and Jobs Act of 2017			35.00%
Regulatory liabilities, non-current			$ 221.2	130.4
THE DAYTON POWER AND LIGHT COMPANY [Member] | Deferred Income Tax Charge [Member]
Regulatory assets, non-current			0.0	35.9
Deferred Income Tax Charge [Member]
Regulatory liabilities, non-current			83.4	0.0
Deferred Income Tax Charge [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Regulatory liabilities, non-current			$ 83.4	$ 0.0