Fixed-asset Impairment Fixed-asset Impairment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Impairment of Long-Lived Assets Held-for-use	$ 66.4	$ 0.0
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	66.3	$ 0.0
THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member]
Impairment of Long-Lived Assets Held-for-use	39.0
Impaired Assets to be Disposed of by Method Other than Sale, Amount of Impairment Loss	9.8
THE DAYTON POWER AND LIGHT COMPANY [Member] | 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
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
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Stuart [Member]
Property, Plant, and Equipment, Fair Value Disclosure	3.3
Level 3 [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Killen [Member]
Property, Plant, and Equipment, Fair Value Disclosure	$ 7.9