Pension and Postretirement Benefits (Net Periodic Benefit Cost (Income)) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Contributions by employer			$ 5,000,000	$ 0
Pension [Member]
Service cost	$ 1,700,000	$ 1,500,000	5,300,000	4,500,000
Interest cost	4,400,000	4,300,000	13,000,000	13,100,000
Expected return on assets	(5,700,000)	(5,800,000)	(17,000,000)	(17,200,000)
Prior service cost	500,000	400,000	1,500,000	1,100,000
Actuarial loss / (gain)	1,500,000	900,000	4,400,000	2,600,000
Net periodic benefit cost	2,400,000	1,300,000	7,200,000	4,100,000
Postretirement [Member]
Service cost	100,000	0	100,000	100,000
Interest cost	200,000	200,000	500,000	600,000
Expected return on assets	(100,000)	0	(100,000)	(100,000)
Prior service cost	0	0	0	0
Actuarial loss / (gain)	(100,000)	(100,000)	(300,000)	(400,000)
Net periodic benefit cost	100,000	100,000	200,000	200,000
THE DAYTON POWER AND LIGHT COMPANY [Member]
Contributions by employer			5,000,000.0	0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Pension [Member]
Service cost	1,800,000	1,500,000	5,300,000	4,400,000
Interest cost	4,200,000	4,300,000	12,800,000	13,000,000
Expected return on assets	(5,600,000)	(5,700,000)	(16,800,000)	(17,000,000)
Prior service cost	900,000	700,000	2,500,000	2,100,000
Actuarial loss / (gain)	2,400,000	1,600,000	7,200,000	4,800,000
Net periodic benefit cost	3,700,000	2,400,000	11,000,000	7,300,000
THE DAYTON POWER AND LIGHT COMPANY [Member] | Postretirement [Member]
Service cost	100,000	0	100,000	100,000
Interest cost	100,000	200,000	500,000	600,000
Expected return on assets	(100,000)	(100,000)	(100,000)	(200,000)
Prior service cost	100,000	100,000	100,000	100,000
Actuarial loss / (gain)	(200,000)	(200,000)	(500,000)	(500,000)
Net periodic benefit cost	$ 0	$ 0	$ 100,000	$ 100,000