Fair Value (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
	Jan. 01, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
AOCI reclassed to Retained Earnings before tax		$ 1.6
AOCI reclassed to Retained Earnings, net of tax		$ 1.0
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
Percent of inputs to the fair value of derivative instruments from quoted market prices (percent)		94.00%
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)	$ 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
Percent of inputs to the fair value of derivative instruments from quoted market prices (percent)		100.00%