Derivative Instruments and Hedging Activities (Outstanding Derivative Instruments) (Details) - Designated as Hedging Instrument [Member] - Interest Rate Swap [Member] $ in Millions	Dec. 31, 2020 USD ($)
Derivative, Notional Amount, Purchase (Sales), Net	$ 140.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Notional Amount, Purchase (Sales), Net	$ 140.0