Derivative Instruments and Hedging Activities (Fair Value and Balance Sheet Location (Details) - Designated as Hedging Instrument [Member] - Interest Rate Swap [Member] - Other Current Assets [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivative Asset, Fair Value	$ 0.0	$ 0.1
THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative Asset, Fair Value	$ 0.0	$ 0.1