Income Taxes (Components of Income Tax Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Estimated Annual Effective Income Tax Rate, Continuing Operations	35.00%
Federal income tax	$ (277.6)	$ (81.0)	$ 25.4
State income taxes, net of federal effect	(1.0)	(0.1)	0.8
Depreciation of AFUDC - Equity	2.7	(3.5)	(3.4)
Investment tax credit amortized	(0.4)	(0.5)	(0.5)
Section 199 - domestic production deduction	(4.5)	(4.1)	(1.1)
Non-deductible goodwill impairment	0.0	111.0	0.0
Accrual (settlement) for open tax years	2.2	0.0	(6.6)
Other, net	(0.2)	(1.8)	0.8
Total tax expense	(278.8)	20.0	15.4
Federal - Current	14.7	30.1	(5.2)
State and Local - Current	0.6	0.8	0.4
Total Current	15.3	30.9	(4.8)
Federal - Deferred	(290.2)	(9.9)	19.6
State and Local - Deferred	(3.9)	(1.0)	0.6
Total Deferred	(294.1)	(10.9)	20.2
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	$ (0.3)	0.2	0.4
THE DAYTON POWER AND LIGHT COMPANY [Member]
Income Taxes [Line Items]
Estimated Annual Effective Income Tax Rate, Continuing Operations	35.00%
Federal income tax	$ (418.5)	49.3	53.8
State income taxes, net of federal effect	(5.0)	0.4	1.2
Depreciation of AFUDC - Equity	3.3	(2.8)	(2.7)
Investment tax credit amortized	(2.3)	(2.4)	(2.5)
Section 199 - domestic production deduction	(5.3)	(6.1)	(4.6)
Accrual (settlement) for open tax years	3.4	0.0	(6.6)
Other, net	2.0	(3.3)	1.1
Total tax expense	(422.4)	35.1	39.7
Federal - Current	51.6	55.8	34.1
State and Local - Current	0.6	0.8	0.5
Total Current	52.2	56.6	34.6
Federal - Deferred	(466.3)	(21.0)	4.1
State and Local - Deferred	(8.3)	(0.5)	1.0
Total Deferred	(474.6)	(21.5)	5.1
Increase (Decrease) in Income Taxes	$ (2.9)	$ 0.4	$ (0.7)