Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension [Member]
Contributions by employer			$ 5.0	$ 0.0
Service cost	$ 1.4	$ 1.8	2.8	3.6
Interest cost	3.7	4.3	7.4	8.6
Expected return on assets	(5.7)	(5.6)	(11.4)	(11.3)
Prior service cost	0.5	0.5	1.0	1.0
Actuarial loss / (gain)	1.1	1.4	2.1	2.9
Net periodic benefit cost	1.0	2.4	1.9	4.8
Postretirement [Member]
Service cost	0.1	0.0	0.1	0.0
Interest cost	0.1	0.1	0.2	0.3
Expected return on assets	0.0	0.0	0.0	0.0
Prior service cost	0.0	0.0	0.0	0.0
Actuarial loss / (gain)	(0.2)	(0.1)	(0.3)	(0.2)
Net periodic benefit cost	0.0	0.0	0.0	0.1
THE DAYTON POWER AND LIGHT COMPANY [Member] | Pension [Member]
Contributions by employer			5.0	0.0
Service cost	1.4	1.7	2.8	3.5
Interest cost	3.7	4.3	7.4	8.6
Expected return on assets	(5.7)	(5.5)	(11.4)	(11.2)
Prior service cost	0.7	0.8	1.5	1.6
Actuarial loss / (gain)	1.8	2.4	3.6	4.8
Net periodic benefit cost	1.9	3.7	3.9	7.3
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement [Member]
Service cost	0.1	0.0	0.1	0.0
Interest cost	0.2	0.2	0.3	0.4
Expected return on assets	0.0	0.0	0.0	0.0
Prior service cost	0.0	0.0	0.0	0.0
Actuarial loss / (gain)	(0.3)	(0.1)	(0.4)	(0.3)
Net periodic benefit cost	$ 0.0	$ 0.1	$ 0.0	$ 0.1