Derivative Instruments and Hedging Activities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Sep. 30, 2013 DP&L [Member]	Dec. 31, 2013 Short-term Derivative Positions [Member]	Sep. 30, 2013 Short-term Derivative Positions [Member]	Dec. 31, 2013 Long-term Derivative Positions [Member]	Sep. 30, 2013 Long-term Derivative Positions [Member]
Amount No Longer Considered Derivative	$ 0.4		$ 0.9		$ 0.1
Fair value of commodity derivative instruments	33.3	33.6
Liability position offset by the asset position of counterparties with master netting agreements	13.5	13.5
Collateral Already Posted, Aggregate Fair Value	16.0	16.0
Collateral if debt were to fall below investment grade	3.8	4.1
Derivative Asset, Current			5.4	13.2
Derivative Asset, Noncurrent					$ 8.0	$ 6.5