Derivative Instruments and Hedging Activities (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Mar. 01, 2011	Mar. 31, 2013 Not Designated as Hedging Instrument [Member]	Mar. 31, 2013 DP&L [Member]	Mar. 31, 2013 Short-term Derivative Positions [Member]	Dec. 31, 2012 Short-term Derivative Positions [Member]	Mar. 31, 2013 Long-term Derivative Positions [Member]	Dec. 31, 2012 Long-term Derivative Positions [Member]
Amount No Longer Considered Derivative	$ 5.8					$ 7.2		$ 1.0
Debt instrument interest percentage		4.20%
Fair value of commodity derivative instruments			23.3	22.9
Liability position offset by the asset position of counterparties with master netting agreements	4.5			4.5
Collateral Already Posted, Aggregate Fair Value	14.3			13.9
Collateral if debt were to fall below investment grade	4.5			4.5
Derivative Assets, Current					4.7	2.7
Derivative Assets, Noncurrent							$ 1.1	$ 3.6