Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments Designated as Hedging Instruments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
DP&L [Member]
Derivative Liability, Fair Value	$ (22.9)
Not Designated as Hedging Instrument [Member]
Derivative Liability, Fair Value	(23.3)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	4.5	2.7
Derivative, Fair Value, Net	0.7	1.2
Derivative, Fair Value, Offset, Net	(3.8)	(1.5)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | DP&L [Member]
Derivative Asset, Fair Value	5.3	2.8
Derivative, Fair Value, Net	1.5	1.3
Derivative, Fair Value, Offset, Net	(3.8)	(1.5)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	(9.9)	(4.1)
Derivative, Fair Value, Net	0.6	0.6
Derivative, Collateral, net	(5.5)	(2.0)
Derivative, Fair Value, Offset, Net	(3.8)	(1.5)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative Liability, Fair Value	(9.5)	(2.7)
Derivative, Fair Value, Net	0.6	0.7
Derivative, Collateral, net	(5.1)	(0.5)
Derivative, Fair Value, Offset, Net	(3.8)	(1.5)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value	(2.6)	(0.8)
Derivative, Fair Value, Net	0.5	0.1
Derivative, Collateral, net	(1.5)	(0.1)
Derivative, Fair Value, Offset, Net	(0.6)	(0.6)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member] | DP&L [Member]
Derivative Liability, Fair Value	(2.6)	(0.7)
Derivative, Fair Value, Net	0.5	0.1
Derivative, Collateral, net	(1.5)
Derivative, Fair Value, Offset, Net	(0.6)	(0.6)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	1.1	3.6
Derivative, Fair Value, Net	0.5	3.0
Derivative, Fair Value, Offset, Net	(0.6)	(0.6)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member] | DP&L [Member]
Derivative Asset, Fair Value	1.1	3.6
Derivative, Fair Value, Net	0.5	3.0
Derivative, Fair Value, Offset, Net	$ (0.6)	$ (0.6)