Income Taxes (Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, deferred tax adjustment percent	0.00%	11.00%	(75.10%)
Depreciation / property basis	$ (112.0)	$ (113.4)
Income taxes recoverable	25.0	11.0
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(15.4)	(23.1)
Investment tax credit	0.5	0.7
Compensation and employee benefits	1.4	19.0
Intangibles	(0.3)	(0.4)
Long-term debt	(2.1)	(0.2)
Other	(13.2)	(7.1)
Net non-current liabilities	$ (116.1)	$ (113.5)
Estimated Annual Effective Income Tax Rate, Continuing Operations	21.00%	35.00%
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense		$ 3.5	$ (0.9)
Deferred tax assets related to state and local tax net operating loss carryforwards, net of related valuation allowances	$ 10.9	9.3
Deferred tax assets related to state and local net operating loss carryforwards, valuation allowances	10.9	9.3
THE DAYTON POWER AND LIGHT COMPANY [Member]
Income Taxes [Line Items]
Depreciation / property basis	(130.6)	(126.5)
Income taxes recoverable	25.0	11.0
Deferred Tax Liabilities, Regulatory Assets and Liabilities	(16.2)	(23.9)
Investment tax credit	0.5	0.4
Compensation and employee benefits	0.3	17.6
Other	(10.7)	(9.6)
Net non-current liabilities	$ (131.7)	$ (131.0)
Estimated Annual Effective Income Tax Rate, Continuing Operations	21.00%	35.00%
Increase (Decrease) in Income Taxes		$ (0.7)	$ 0.4