Derivative Instruments and Hedging Activities (Narrative) (Details) $ in Millions	Jun. 30, 2016 USD ($)
Additional Collateral, Aggregate Fair Value, OTC Commodity Derivatives	$ 0.9
Fair value of commodity derivative instruments	45.2
Collateral Already Posted, Aggregate Fair Value	11.1
Liability position offset by the asset position of counterparties with master netting agreements	29.6
Collateral if debt were to fall below investment grade	4.5
THE DAYTON POWER AND LIGHT COMPANY [Member]
Additional Collateral, Aggregate Fair Value, OTC Commodity Derivatives	0.9
Fair value of commodity derivative instruments	45.3
Collateral Already Posted, Aggregate Fair Value	11.1
Liability position offset by the asset position of counterparties with master netting agreements	29.6
Collateral if debt were to fall below investment grade	$ 4.6