Property, Plant and Equipment (Changes in the Liability for Transmission and Distribution Asset Removal Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Changes in Liability for Transmission and Distribution Asset Removal Costs [Roll Forward]
Balance at January 1	$ 121.8	$ 119.3
Additions	11.7	24.3
Settlements	(7.0)	(21.8)
Balance at December 31	126.5	121.8
THE DAYTON POWER AND LIGHT COMPANY [Member]
Changes in Liability for Transmission and Distribution Asset Removal Costs [Roll Forward]
Balance at January 1	121.8	119.3
Additions	11.7	24.3
Settlements	(7.0)	(21.8)
Balance at December 31	$ 126.5	$ 121.8