Property, Plant and Equipment (Changes in the Liability for Transmission and Distribution Asset Removal Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Changes in Liability for Transmission and Distribution Asset Removal Costs [Roll Forward]
Balance at January 1	$ 126.5	$ 121.8
Additions	12.0	11.7
Settlements	(5.7)	(7.0)
Balance at December 31	132.8	126.5
THE DAYTON POWER AND LIGHT COMPANY [Member]
Changes in Liability for Transmission and Distribution Asset Removal Costs [Roll Forward]
Balance at January 1	126.5	121.8
Additions	12.0	11.7
Settlements	(5.7)	(7.0)
Balance at December 31	$ 132.8	$ 126.5