Derivative Instruments and Hedging Activities (Narrative) (Details)	Jun. 30, 2017 USD ($) Number_of_interest_rate_swaps	Dec. 31, 2016 USD ($)
Fair value of commodity derivative instruments	$ 11,600,000
Collateral Already Posted, Aggregate Fair Value	800,000
Liability position offset by the asset position of counterparties with master netting agreements	9,400,000
Collateral if debt were to fall below investment grade	$ 1,400,000
THE DAYTON POWER AND LIGHT COMPANY [Member]
Debt Instrument, Number of Financial Covenants | Number_of_interest_rate_swaps	2
Fair value of commodity derivative instruments	$ 11,600,000
Collateral Already Posted, Aggregate Fair Value	800,000
Liability position offset by the asset position of counterparties with master netting agreements	9,400,000
Collateral if debt were to fall below investment grade	1,400,000
One Point One Three To One Point One Seven Bonds Maturing In August Two Thousand Twenty [Member]
Long-term Debt, Gross	200,000,000
One Point One Three To One Point One Seven Bonds Maturing In August Two Thousand Twenty [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Long-term Debt, Gross	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] | Designated as Hedging Instrument [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Derivative, Notional Amount, Purchase (Sales), Net	$ 200,000,000	$ 200,000,000