Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Pension [Member]
Contributions by employer	$ 7.6	$ 5.0
Service cost	1.5	1.4
Interest cost	3.4	3.6
Expected return on assets	(5.2)	(5.7)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0.0	4.1
Prior service cost	0.2	0.4
Actuarial loss / (gain)	1.6	1.3
Net periodic benefit cost	1.5	5.1
Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan			$ 12.7
THE DAYTON POWER AND LIGHT COMPANY [Member] | Pension [Member]
Contributions by employer	7.6	5.0
Service cost	1.5	1.4
Interest cost	3.4	3.6
Expected return on assets	(5.2)	(5.7)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0.0	5.6
Prior service cost	0.4	0.5
Actuarial loss / (gain)	2.3	2.2
Net periodic benefit cost	$ 2.4	$ 7.6
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan			$ 12.7