Fair Value (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
AOCI reclassed to Retained Earnings before tax	$ 1.6
AOCI reclassed to Retained Earnings, net of tax	1.0	$ 0.0	$ 0.0	$ (1.0)	$ 0.0
Long-term debt, earliest maturities				2019
Long-term debt, latest maturities				2061
Unrealized Gains and Immaterial Unrealized Losses in AOCI, Before Tax						$ 1.6
Unrealized gains and immaterial unrealized losses in AOCI, after tax						1.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI reclassed to Retained Earnings before tax	1.7
AOCI reclassed to Retained Earnings, net of tax	$ 1.1	$ 0.0	$ 0.0	$ (1.1)	$ 0.0
Long-term debt, earliest maturities				2020
Long-term debt, latest maturities				2061
Unrealized Gains and Immaterial Unrealized Losses in AOCI, Before Tax						1.7
Unrealized gains and immaterial unrealized losses in AOCI, after tax						$ 1.1