Fixed-asset Impairment Fixed-asset Impairment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Impairment of Long-Lived Assets Held-for-use	$ 0.0	$ 66.4
Stuart [Member]
Impairment of Long-Lived Assets Held-for-use		39.1
Impaired Assets to be Disposed of by Method Other than Sale, Amount of Impairment Loss		9.8
Killen [Member]
Impairment of Long-Lived Assets Held-for-use		27.3
Impaired Assets to be Disposed of by Method Other than Sale, Amount of Impairment Loss		6.4
THE DAYTON POWER AND LIGHT COMPANY [Member]
Impairment of Long-Lived Assets Held-for-use	$ 0.0	66.3
Level 3 [Member] | Stuart [Member]
Property, Plant, and Equipment, Fair Value Disclosure		3.3
Level 3 [Member] | Killen [Member]
Property, Plant, and Equipment, Fair Value Disclosure		$ 7.9