Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Unrealized gains and immaterial losses on Master Trust assets in AOCI, after tax		$ 0.0	$ 0.0	$ 0.4
Asset Retirement Obligation, Revision of Estimate		(3.4)	2.6
Proceeds from Sale of Debt Securities, Available-for-sale		0.3
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax		0.2
AOCI reclassed to Retained Earnings before tax	$ 1.6
AOCI reclassed to Retained Earnings, net of tax	1.0		(1.0)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Asset Retirement Obligations, Noncurrent
Unrealized gains and immaterial losses on Master Trust assets in AOCI, after tax		0.0	0.0	$ 0.4
Proceeds from Sale of Debt Securities, Available-for-sale		0.3
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax		$ 0.2
AOCI reclassed to Retained Earnings before tax	1.7
AOCI reclassed to Retained Earnings, net of tax	$ 1.1		$ (1.1)