Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - Pension [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Service cost	$ 5.7	$ 7.1	$ 5.9
Interest cost	14.7	17.3	17.5
Expected return on assets	(22.8)	(22.6)	(22.9)
Defined Benefit Plan, Curtailments	3.8	0.0	0.0
Actuarial gain / (loss)	4.3	5.8	3.4
Prior service cost	1.8	2.0	1.5
Net Periodic benefit cost / (income) before adjustments	$ 7.5	$ 9.6	$ 5.4
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.49%	4.02%	4.86%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	6.50%	6.50%	6.75%
THE DAYTON POWER AND LIGHT COMPANY [Member]
Service cost	$ 5.7	$ 7.1	$ 5.9
Interest cost	14.7	17.3	17.5
Expected return on assets	(22.8)	(22.6)	(22.9)
Defined Benefit Plan, Curtailments	5.7	0.0	0.0
Actuarial gain / (loss)	7.2	9.8	6.4
Prior service cost	3.0	3.3	2.8
Net Periodic benefit cost / (income) before adjustments	$ 13.5	$ 14.9	$ 9.7
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate	4.49%	4.02%	4.86%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	6.50%	6.50%	6.75%