Fair Value (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Long-term debt, earliest maturities		2020
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