Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Pension [Member]
Contributions by employer			$ 7.5	$ 7.5
Service cost	$ 0.9	$ 1.5	2.7	4.5
Interest cost	3.7	3.4	11.2	10.3
Expected return on assets	(5.0)	(5.3)	(15.0)	(15.9)
Prior service cost	0.3	0.3	0.9	0.8
Actuarial loss / (gain)	1.1	1.6	3.2	4.8
Net periodic benefit cost	1.0	1.5	3.0	4.5
Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	9.0		9.0		$ 9.2
THE DAYTON POWER AND LIGHT COMPANY [Member] | Pension [Member]
Contributions by employer			7.5	7.5
Service cost	0.9	1.5	2.7	4.5
Interest cost	3.7	3.4	11.2	10.3
Expected return on assets	(5.0)	(5.3)	(15.0)	(15.9)
Prior service cost	0.4	0.4	1.3	1.1
Actuarial loss / (gain)	1.8	2.4	5.3	7.1
Net periodic benefit cost	1.8	$ 2.4	5.5	$ 7.1
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 9.0		$ 9.0		$ 9.2