Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - Pension [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Service cost	$ 3.7	$ 3.7	$ 6.1
Interest cost	11.8	14.9	13.8
Expected return on assets	(18.6)	(20.1)	(21.2)
Actuarial gain / (loss)	1.0	4.2	6.4
Prior service cost	6.1	1.3	0.9
Net Periodic benefit cost / (income) before adjustments	$ 4.0	$ 4.0	$ 6.0
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	3.33%	4.35%	3.66%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	5.60%	6.25%	6.25%
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax		$ 0.0	$ 0.0	$ 0.0
Service cost	$ 3.7	3.7	6.1
Interest cost	11.8	14.9	13.8
Expected return on assets	(18.6)	(20.1)	(21.2)
Actuarial gain / (loss)	8.7	7.0	9.4
Prior service cost	1.3	1.8	1.4
Net Periodic benefit cost / (income) before adjustments	$ 6.9	$ 7.3	$ 9.5
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	3.33%	4.35%	3.66%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	5.60%	6.25%	6.25%