Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - Pension [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Service cost	$ 6.1	$ 5.7	$ 5.7
Interest cost	13.8	14.2	14.7
Expected return on assets	(21.2)	(22.8)	(22.8)
Defined Benefit Plan, Curtailments	0.0	4.1	3.8
Actuarial gain / (loss)	6.4	5.3	4.3
Prior service cost	0.9	1.1	1.8
Net Periodic benefit cost / (income) before adjustments	$ 6.0	$ 7.6	$ 7.5
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	3.66%	4.28%	4.49%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	6.25%	6.50%	6.50%
THE DAYTON POWER AND LIGHT COMPANY [Member]
Service cost	$ 6.1	$ 5.7	$ 5.7
Interest cost	13.8	14.2	14.7
Expected return on assets	(21.2)	(22.8)	(22.8)
Defined Benefit Plan, Curtailments	0.0	5.6	5.7
Actuarial gain / (loss)	9.4	8.7	7.2
Prior service cost	1.4	1.5	3.0
Net Periodic benefit cost / (income) before adjustments	$ 9.5	$ 12.9	$ 13.5
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	3.66%	4.28%	4.49%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	6.25%	6.50%	6.50%