Fair Value (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Long-term debt, earliest maturities	2019
Long-term debt, latest maturities	2061
THE DAYTON POWER AND LIGHT COMPANY [Member]
Long-term debt, earliest maturities	2020
Long-term debt, latest maturities	2061
Adjustments for New Accounting Pronouncement [Member]
AOCI reclassed to Retained Earnings before tax		$ 1.6
AOCI reclassed to Retained Earnings, net of tax		1.0
Adjustments for New Accounting Pronouncement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
AOCI reclassed to Retained Earnings before tax	$ 1.7	1.7
AOCI reclassed to Retained Earnings, net of tax	$ 1.1	$ 1.1