Derivative Instruments and Hedging Activities (Narrative) (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Fair value of commodity derivative instruments	$ 29,200,000
Collateral Already Posted, Aggregate Fair Value	6,300,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	16,800,000
Collateral if debt were to fall below investment grade	6,100,000
THE DAYTON POWER AND LIGHT COMPANY [Member]
Fair value of commodity derivative instruments	29,200,000
Collateral Already Posted, Aggregate Fair Value	6,300,000
Derivative Liability, Fair Value, Amount Offset Against Collateral	16,800,000
Collateral if debt were to fall below investment grade	6,100,000
Tax-exempt First Mortgage Bonds - rates from: 1.29% - 1.42% (a) and 1.13% - 1.17% (b)
Long-term Debt, Gross	200,000,000	$ 200,000,000
Tax-exempt First Mortgage Bonds - rates from: 1.29% - 1.42% (a) and 1.13% - 1.17% (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
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