Fair Value (Narrative) (Details) - Adjustments for New Accounting Pronouncement [Member] $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
AOCI reclassed to Retained Earnings before tax	$ 1.6
AOCI reclassed to Retained Earnings, net of 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