Property, Plant and Equipment (Narrative) (Details) $ in Millions	6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2017 generating_facility MW	Dec. 31, 2017 USD ($) generating_facility	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Jointly Owned Utility Plant Interests [Line Items]
Number of generating facilities | generating_facility		5	3
Fixed-asset impairment (Note 15)	$ 623.5		$ 175.8	$ 859.0	$ 0.0
Estimated costs of removal	126.5		$ 132.8	126.5	121.8
THE DAYTON POWER AND LIGHT COMPANY [Member]
Jointly Owned Utility Plant Interests [Line Items]
Number of generating facilities | generating_facility			5
Fixed-asset impairment (Note 15)			$ 66.3	1,353.5	0.0
Estimated costs of removal	$ 126.5		$ 132.8	$ 126.5	$ 121.8
Stuart Station Unit 1 [Member]
Jointly Owned Utility Plant Interests [Line Items]
Production Plan Capacity | MW		202