Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Pension [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment				$ (4.1)
Contributions by employer			$ 7.7	5.0
Service cost	$ 1.5	$ 1.4	3.0	2.8
Interest cost	3.5	3.5	6.9	7.1
Expected return on assets	(5.4)	(5.7)	(10.6)	(11.4)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0.0	0.0	0.0	4.1
Prior service cost	0.3	0.2	0.5	0.6
Actuarial loss / (gain)	1.6	1.4	3.2	2.7
Net periodic benefit cost	1.5	0.8	3.0	5.9
Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	12.7		12.7		$ 12.7
THE DAYTON POWER AND LIGHT COMPANY [Member] | Pension [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment				(5.6)
Contributions by employer			7.7	5.0
Service cost	1.5	1.4	3.0	2.8
Interest cost	3.5	3.5	6.9	7.1
Expected return on assets	(5.4)	(5.7)	(10.6)	(11.4)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0.0	0.0	0.0	5.6
Prior service cost	0.3	0.3	0.7	0.8
Actuarial loss / (gain)	2.4	2.3	4.7	4.5
Net periodic benefit cost	2.3	$ 1.8	4.7	$ 9.4
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 12.7		$ 12.7		$ 12.7