Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - Pension [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Service cost	$ 4.5	$ 3.7	$ 3.7
Interest cost	8.2	11.8	14.9
Expected return on assets	(15.0)	(18.6)	(20.1)
Actuarial gain / (loss)	9.0	1.0	4.2
Prior service cost	0.9	6.1	1.3
Net Periodic benefit cost / (income) before adjustments	$ 7.6	$ 4.0	$ 4.0
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	2.44%	3.33%	4.35%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	4.55%	5.60%	6.25%
THE DAYTON POWER AND LIGHT COMPANY [Member]
Service cost	$ 4.5	$ 3.7	$ 3.7
Interest cost	8.2	11.8	14.9
Expected return on assets	(15.0)	(18.6)	(20.1)
Actuarial gain / (loss)	11.4	8.7	7.0
Prior service cost	1.1	1.3	1.8
Net Periodic benefit cost / (income) before adjustments	$ 10.2	$ 6.9	$ 7.3
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	2.44%	3.33%	4.35%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	4.55%	5.60%	6.25%