Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension [Member]
Service cost	$ 7.1	$ 5.9	$ 7.2
Interest cost	17.3	17.5	15.6
Expected return on assets	(22.6)	(22.9)	(23.3)
Actuarial gain / (loss)	5.8	3.4	4.9
Prior service cost	2.0	1.5	1.5
Net Periodic benefit cost / (income) before adjustments	9.6	5.4	5.9
Postretirement [Member]
Service cost	0.2	0.2	0.2
Interest cost	0.6	0.8	0.8
Expected return on assets	(0.1)	(0.2)	(0.1)
Actuarial gain / (loss)	(0.6)	(0.6)	(0.5)
Prior service cost	0.1	0.0	0.0
Net Periodic benefit cost / (income) before adjustments	0.2	0.2	0.4
THE DAYTON POWER AND LIGHT COMPANY [Member] | Pension [Member]
Service cost	7.1	5.9	7.2
Interest cost	17.3	17.5	15.6
Expected return on assets	(22.6)	(22.9)	(23.6)
Actuarial gain / (loss)	9.8	6.4	9.3
Prior service cost	3.3	2.8	2.8
Net Periodic benefit cost / (income) before adjustments	14.9	9.7	11.3
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement [Member]
Service cost	0.2	0.2	0.2
Interest cost	0.6	0.8	0.8
Expected return on assets	(0.1)	(0.2)	(0.2)
Actuarial gain / (loss)	(0.6)	(0.8)	(0.7)
Prior service cost	0.1	0.1	0.1
Net Periodic benefit cost / (income) before adjustments	$ 0.2	$ 0.1	$ 0.2