Income Taxes (Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, deferred tax adjustment percent	0.00%	0.00%	15.50%
Depreciation / property basis	$ (160.5)	$ (118.3)
Income taxes recoverable	14.4	17.1
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(21.0)	(24.8)
Investment tax credit	0.6	0.6
Compensation and employee benefits	(1.7)	2.2
Intangibles	(0.4)	(0.4)
Long-term debt	(1.3)	(2.1)
Other	(7.3)	(8.0)
Net non-current liabilities	(177.2)	(133.7)
Deferred Tax Liabilities, net Non-current Including Discontinued Operations		133.7
THE DAYTON POWER AND LIGHT COMPANY [Member]
Income Taxes [Line Items]
Depreciation / property basis	(161.7)	(141.5)
Income taxes recoverable	14.4	17.1
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(21.7)	(25.6)
Investment tax credit	0.5	0.5
Compensation and employee benefits	(0.1)	3.1
Other	(3.5)	(11.7)
Net non-current liabilities	$ (172.1)	$ (158.1)