Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Pension [Member]
Contributions by employer			$ 5.0	$ 5.0
Service cost	$ 1.4	$ 1.4	2.8	2.8
Interest cost	3.5	3.7	7.1	7.4
Expected return on assets	(5.7)	(5.7)	(11.4)	(11.4)
Defined Benefit Plan, Curtailments	0.0	0.0	(4.1)	0.0
Prior service cost	0.2	0.5	0.6	1.0
Actuarial loss / (gain)	1.4	1.1	2.7	2.1
Net periodic benefit cost	0.8	1.0	5.9	1.9
Postretirement [Member]
Defined Benefit Plan, Funded Status of Plan	15.7		15.7		$ 15.8
THE DAYTON POWER AND LIGHT COMPANY [Member] | Pension [Member]
Contributions by employer			5.0	5.0
Service cost	1.4	1.4	2.8	2.8
Interest cost	3.5	3.7	7.1	7.4
Expected return on assets	(5.7)	(5.7)	(11.4)	(11.4)
Defined Benefit Plan, Curtailments	0.0	0.0	(5.6)	0.0
Prior service cost	0.3	0.7	0.8	1.5
Actuarial loss / (gain)	2.3	1.8	4.5	3.6
Net periodic benefit cost	1.8	$ 1.9	9.4	$ 3.9
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement [Member]
Defined Benefit Plan, Funded Status of Plan	$ 15.7		$ 15.7		$ 15.8