Property, Plant and Equipment (Narrative) (Details) $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) generating_facility	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Jointly Owned Utility Plant Interests [Line Items]
Number of generating facilities | generating_facility		5
Fixed-asset impairment (Note 15)	$ 623.5	$ 859.0	$ 0.0	$ 11.5
Estimated costs of removal	126.5	$ 126.5	121.8	119.3
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)	496.4	$ 1,353.5	0.0	0.0
Estimated costs of removal	$ 126.5	$ 126.5	$ 121.8	$ 119.3