Goodwill And Other Intangible Asset (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2015
Goodwill [Line Items]
Goodwill, Impairment Loss			$ 0.0	$ 0.0	$ 317.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Goodwill [Line Items]
Goodwill						$ 317.0
Goodwill, Impairment Loss	$ 317.0	$ 317.0