Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - Pension [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Service cost	$ 5.7	$ 5.7	$ 7.1
Interest cost	14.2	14.7	17.3
Expected return on assets	(22.8)	(22.8)	(22.6)
Defined Benefit Plan, Curtailments	4.1	3.8	0.0
Actuarial gain / (loss)	5.3	4.3	5.8
Prior service cost	1.1	1.8	2.0
Net Periodic benefit cost / (income) before adjustments	$ 7.6	$ 7.5	$ 9.6
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.28%	4.49%	4.02%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	6.50%	6.50%	6.50%
THE DAYTON POWER AND LIGHT COMPANY [Member]
Service cost	$ 5.7	$ 5.7	$ 7.1
Interest cost	14.2	14.7	17.3
Expected return on assets	(22.8)	(22.8)	(22.6)
Defined Benefit Plan, Curtailments	5.6	5.7	0.0
Actuarial gain / (loss)	8.7	7.2	9.8
Prior service cost	1.5	3.0	3.3
Net Periodic benefit cost / (income) before adjustments	$ 12.9	$ 13.5	$ 14.9
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.28%	4.49%	4.02%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	6.50%	6.50%	6.50%