Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments Designated as Hedging Instruments) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivative Liability, Fair Value	$ (35.5)
DP&L [Member]
Derivative Liability, Fair Value	(35.6)
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	12.9	4.9
Derivative, Fair Value, Net	3.8	0.7
Derivative, Fair Value, Offset, Net	(9.1)	(4.2)
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	13.1	4.9
Derivative, Fair Value, Net	4.1	0.7
Derivative, Fair Value, Offset, Net	(9.0)	(4.2)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative Liability, Fair Value	(14.4)
Derivative, Fair Value, Net	0.3
Derivative, Collateral, net	(5.0)
Derivative, Fair Value, Offset, Net	(9.1)
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	(14.4)	(6.6)
Derivative, Fair Value, Net	0.3	0.1
Derivative, Collateral, net	(5.0)	(2.3)
Derivative, Fair Value, Offset, Net	(9.1)	(4.2)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value		(6.6)
Derivative, Fair Value, Net		0.1
Derivative, Collateral, net		(2.3)
Derivative, Fair Value, Offset, Net		(4.2)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative Liability, Fair Value	(2.0)	(1.3)
Derivative, Fair Value, Net	0.1
Derivative, Collateral, net	(0.1)	(1.0)
Derivative, Fair Value, Offset, Net	(1.8)	(0.3)
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	(2.1)	(1.3)
Derivative, Fair Value, Net	0.2
Derivative, Collateral, net	(0.1)	(1.0)
Derivative, Fair Value, Offset, Net	(1.8)	(0.3)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	1.9	5.0
Derivative, Fair Value, Net	0.1	4.7
Derivative, Fair Value, Offset, Net	(1.8)	(0.3)
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	2.3	5.0
Derivative, Fair Value, Net	0.5	4.7
Derivative, Fair Value, Offset, Net	$ (1.8)	$ (0.3)