Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Pension [Member]
Contributions by employer	$ 7.5	$ 7.5
Service cost	0.9	0.9
Interest cost	3.0	3.7
Expected return on assets	(4.7)	(5.0)
Prior service cost	0.3	0.3
Actuarial loss / (gain)	1.5	1.1
Net periodic benefit cost	1.0	1.0
Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	9.7		$ 9.6
THE DAYTON POWER AND LIGHT COMPANY [Member] | Pension [Member]
Contributions by employer	7.5	7.5
Service cost	0.9	0.9
Interest cost	3.0	3.7
Expected return on assets	(4.7)	(5.0)
Prior service cost	0.3	0.5
Actuarial loss / (gain)	2.2	1.7
Net periodic benefit cost	1.7	$ 1.8
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 9.7		$ 9.6