Derivative Instruments and Hedging Activities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2014 DP&L [Member]	Dec. 31, 2013 Short-term Derivative Positions [Member]	Dec. 31, 2013 Long-term Derivative Positions [Member]
Amount No Longer Considered Derivative		$ 0.1		$ 0.9	$ 0.1
Fair value of commodity derivative instruments	28.0		28.0
Liability position offset by the asset position of counterparties with master netting agreements	9.5		9.5
Collateral Already Posted, Aggregate Fair Value	14.1		14.1
Collateral if debt were to fall below investment grade	$ 4.4		$ 4.4