Income Taxes (Components of Income Tax Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Estimated Annual Effective Income Tax Rate, Continuing Operations	35.00%
Federal income tax	$ (42.0)	$ (277.6)	$ (81.0)
State income taxes, net of federal effect	(0.5)	(1.0)	(0.1)
Depreciation of AFUDC - Equity	0.8	2.7	(3.5)
Investment tax credit amortized	(0.3)	(0.4)	(0.5)
Section 199 - domestic production deduction	0.0	(4.5)	(4.1)
Non-deductible goodwill impairment	0.0	0.0	111.0
Accrual (settlement) for open tax years	(0.4)	2.2	0.0
Other, net	17.1	(0.2)	(1.8)
Tax expense / (benefit)	(25.3)	(278.8)	20.0
Federal - Current	(2.9)	14.7	30.1
State and Local - Current	0.0	0.6	0.8
Total Current	(2.9)	15.3	30.9
Federal - Deferred	(22.0)	(290.2)	(9.9)
State and Local - Deferred	(0.4)	(3.9)	(1.0)
Total Deferred	(22.4)	(294.1)	(10.9)
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	$ 3.5	(0.3)	0.2
THE DAYTON POWER AND LIGHT COMPANY [Member]
Income Taxes [Line Items]
Estimated Annual Effective Income Tax Rate, Continuing Operations	35.00%
Federal income tax	$ 31.0	50.1	65.8
State income taxes, net of federal effect	0.4	0.4	0.4
Depreciation of AFUDC - Equity	1.2	3.0	(3.1)
Investment tax credit amortized	(0.3)	(0.4)	(0.4)
Accrual (settlement) for open tax years	(0.5)	3.4	0.0
Other, net	(0.7)	(10.5)	(3.7)
Tax expense / (benefit)	31.1	46.0	59.0
Federal - Current	13.5	37.7	68.3
State and Local - Current	0.2	0.5	0.9
Total Current	13.7	38.2	69.2
Federal - Deferred	17.0	7.7	(9.9)
State and Local - Deferred	0.4	0.1	(0.3)
Total Deferred	17.4	7.8	(10.2)
Increase (Decrease) in Income Taxes	$ 0.0	$ (0.4)	$ (0.1)