Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Tax Cuts and Jobs Act of 2017, change in income tax expense (benefit)		$ 13.7
Tax rate before change due to Tax Cuts and Jobs Act of 2017		35.00%
Non-cash capital contribution		$ 97.1	$ 0.0	$ 0.0
Change in deferred tax regulatory asset/liability due to TCJA		135.2
Unrecognized tax benefits due to uncertainty in timing of deductibility		$ 0.9
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%
Change in deferred tax regulatory asset/liability due to TCJA		$ 135.2
Unrecognized tax benefits due to uncertainty in timing of deductibility		$ 0.9
Scenario, Forecast [Member]
Income Taxes [Line Items]
Tax rate after Tax Cuts and Jobs Act of 2017	21.00%
Scenario, Forecast [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Income Taxes [Line Items]
Tax rate after Tax Cuts and Jobs Act of 2017	21.00%