Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Fair value of commodity derivative instruments	$ 15,300,000
Collateral Already Posted, Aggregate Fair Value	8,700,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	1,400,000
Collateral if debt were to fall below investment grade	4,900,000
Tax-exempt First Mortgage Bonds - rates from: 1.52% - 1.92% (a) and 1.29% - 1.42% (b)
Long-term Debt, Gross	200,000,000	$ 200,000,000
Tax-exempt First Mortgage Bonds - rates from: 1.52% - 1.92% (a) and 1.29% - 1.42% (b) | THE DAYTON POWER AND LIGHT COMPANY [Member]
Long-term Debt, Gross	200,000,000	200,000,000
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]
Derivative, Notional Amount, Purchase (Sales), Net	200,000,000	$ 200,000,000
Interest Rate Swap [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member] | Designated as Hedging Instrument [Member]
Derivative, Notional Amount, Purchase (Sales), Net	$ 200,000,000