Derivative Instruments and Hedging Activities (Fair Values of Derivative Instruments Designated as Hedging Instruments) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivative Liability, Fair Value	$ (28.0)
DP&L [Member]
Derivative Liability, Fair Value	(28.0)
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	6.3	4.9
Derivative, Fair Value, Net	0.9	0.7
Derivative, Fair Value, Offset, Net	(5.4)	(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	6.5	4.9
Derivative, Fair Value, Net	1.1	0.7
Derivative, Fair Value, Offset, Net	(5.4)	(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	(10.2)
Derivative, Fair Value, Net	2.0
Derivative, Collateral, net	(2.8)
Derivative, Fair Value, Offset, Net	(5.4)
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	(10.2)	(6.6)
Derivative, Fair Value, Net	2.0	0.1
Derivative, Collateral, net	(2.8)	(2.3)
Derivative, Fair Value, Offset, Net	(5.4)	(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	(3.4)	(1.3)
Derivative, Fair Value, Net	0.1
Derivative, Collateral, net	(0.7)	(1.0)
Derivative, Fair Value, Offset, Net	(2.6)	(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	(3.4)	(1.3)
Derivative, Fair Value, Net	0.1
Derivative, Collateral, net	(0.7)	(1.0)
Derivative, Fair Value, Offset, Net	(2.6)	(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	3.3	5.0
Derivative, Fair Value, Net	0.8	4.7
Derivative, Fair Value, Offset, Net	(2.5)	(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	3.6	5.0
Derivative, Fair Value, Net	1.0	4.7
Derivative, Fair Value, Offset, Net	$ (2.6)	$ (0.3)