Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Contributions by employer			$ 7.5	$ 7.5
Pension [Member]
Service cost	$ 1.0	$ 0.9	1.9	1.8
Interest cost	2.9	3.8	5.9	7.5
Expected return on assets	(4.7)	(5.0)	(9.4)	(10.0)
Prior service cost	0.2	0.3	0.5	0.6
Actuarial loss / (gain)	1.6	1.0	3.1	2.1
Net periodic benefit cost	1.0	1.0	2.0	2.0
Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	9.6		9.6		$ 9.6
THE DAYTON POWER AND LIGHT COMPANY [Member] | Pension [Member]
Contributions by employer			7.5	7.5
Service cost	1.0	0.9	1.9	1.8
Interest cost	2.9	3.8	5.9	7.5
Expected return on assets	(4.7)	(5.0)	(9.4)	(10.0)
Prior service cost	0.4	0.4	0.7	0.9
Actuarial loss / (gain)	2.1	1.8	4.3	3.5
Net periodic benefit cost	1.7	$ 1.9	3.4	$ 3.7
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 9.6		$ 9.6		$ 9.6