Income Taxes (Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Depreciation / property basis	$ (103.6)	$ (234.8)
Income taxes recoverable	11.0	(11.9)
Regulatory assets	(23.1)	(7.8)
Investment tax credit	0.5	0.5
Compensation and employee benefits	11.3	5.5
Intangibles	(0.4)	(1.5)
Long-term debt	(0.2)	(0.7)
Other	(6.7)	(1.7)
Net non-current liabilities	$ (111.2)	(252.4)
Estimated Annual Effective Income Tax Rate, Continuing Operations	35.00%
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	$ 3.5	(0.3)	$ 0.2
Deferred tax assets related to state and local tax net operating loss carryforwards, net of related valuation allowances	36.3	38.3
Deferred tax assets related to state and local net operating loss carryforwards, valuation allowances	36.3	38.3
THE DAYTON POWER AND LIGHT COMPANY [Member]
Income Taxes [Line Items]
Depreciation / property basis	(126.5)	(238.0)
Income taxes recoverable	11.0	(12.2)
Regulatory assets	(23.9)	(9.1)
Investment tax credit	0.4	0.4
Compensation and employee benefits	17.6	(0.3)
Other	(9.6)	(7.7)
Net non-current liabilities	$ (131.0)	(266.9)
Estimated Annual Effective Income Tax Rate, Continuing Operations	35.00%
Increase (Decrease) in Income Taxes	$ 0.0	$ (0.4)	$ (0.1)