Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Tax Cuts and Jobs Act of 2017, change in income tax expense (benefit)	$ 15.5	$ 13.7
Tax rate before change due to Tax Cuts and Jobs Act of 2017			35.00%
Tax rate after Tax Cuts and Jobs Act of 2017			21.00%
Non-cash capital contribution			$ 40.0	$ 97.1	$ 0.0
Change in deferred tax regulatory asset/liability due to TCJA	17.0	135.2	17.0	135.2
Unrecognized tax benefits due to uncertainty in timing of deductibility			$ 3.5
THE DAYTON POWER AND LIGHT COMPANY [Member]
Income Taxes [Line Items]
Tax rate before change due to Tax Cuts and Jobs Act of 2017			35.00%
Tax rate after Tax Cuts and Jobs Act of 2017			21.00%
Change in deferred tax regulatory asset/liability due to TCJA	$ 17.0	$ 135.2	$ 17.0	$ 135.2
Unrecognized tax benefits due to uncertainty in timing of deductibility			$ 4.8