Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Pension [Member]
Contributions by employer	$ 7.5	$ 7.5
Service cost	0.9	1.5
Interest cost	3.7	3.4
Expected return on assets	(5.0)	(5.2)
Prior service cost	0.3	0.2
Actuarial loss / (gain)	1.1	1.6
Net periodic benefit cost	1.0	1.5
Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	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
Interest cost	3.7	3.4
Expected return on assets	(5.0)	(5.2)
Prior service cost	0.5	0.4
Actuarial loss / (gain)	1.7	2.3
Net periodic benefit cost	1.8	$ 2.4
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 9.0		$ 9.2