Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments Designated as Hedging Instruments) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative Liability, Fair Value	$ (10.3)
DP&L [Member]
Derivative Liability, Fair Value	(10.1)
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	5.1	2.7
Derivative, Fair Value, Net	2.5	1.2
Derivative, Fair Value, Offset, Net	(2.6)	(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	5.2	2.8
Derivative, Fair Value, Net	2.6	1.3
Derivative, Fair Value, Offset, Net	(2.6)	(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	(5.0)	(4.1)
Derivative, Fair Value, Net	0.3	0.6
Derivative, Collateral, net	(2.1)	(2.0)
Derivative, Fair Value, Offset, Net	(2.6)	(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	(4.7)	(2.7)
Derivative, Fair Value, Net	0.3	0.7
Derivative, Collateral, net	(1.8)	(0.5)
Derivative, Fair Value, Offset, Net	(2.6)	(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	(1.1)	(0.8)
Derivative, Fair Value, Net	0.4	0.1
Derivative, Collateral, net		(0.1)
Derivative, Fair Value, Offset, Net	(0.7)	(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	(1.1)	(0.7)
Derivative, Fair Value, Net	0.3	0.1
Derivative, Collateral, net	(0.1)
Derivative, Fair Value, Offset, Net	(0.7)	(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	12.7	3.6
Derivative, Fair Value, Net	12.0	3.0
Derivative, Fair Value, Offset, Net	(0.7)	(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	12.7	3.6
Derivative, Fair Value, Net	12.0	3.0
Derivative, Fair Value, Offset, Net	$ (0.7)	$ (0.6)