Derivative Instruments and Hedging Activities (Fair Value and Balance Sheet Location (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Total Assets [Member]
Derivative Asset, Fair Value	$ 14.9	$ 13.8
Derivative, Fair Value, Net	8.3	5.9
Derivative, Collateral, net		(3.2)
Derivative, Fair Value, Offset, Net	(6.6)	(4.7)
Total Assets [Member] | DP&L [Member]
Derivative Asset, Fair Value	15.1	13.8
Derivative, Fair Value, Net	8.5	5.9
Derivative, Collateral, net		(3.2)
Derivative, Fair Value, Offset, Net	(6.6)	(4.7)
Total Liabilities [Member]
Derivative, Fair Value, Net	0.7	0.3
Derivative, Collateral, net	(4.9)	(5.6)
Derivative, Fair Value, Offset, Net	(6.6)	(4.7)
Derivative Liability, Fair Value	12.2	10.6
Total Liabilities [Member] | DP&L [Member]
Derivative, Fair Value, Net	0.8	0.3
Derivative, Collateral, net	(4.9)	(5.6)
Derivative, Fair Value, Offset, Net	(6.6)	(4.7)
Derivative Liability, Fair Value	12.3	10.6
Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member] | DP&L [Member]
Derivative, Collateral, net	(0.3)
Derivative, Fair Value, Offset, Net	(0.3)
Derivative Liability, Fair Value	0.6
Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative, Fair Value, Net	0.6
Derivative Liability, Fair Value	0.6
Commodity Contract - Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative, Collateral, net	(0.4)
Derivative Liability, Fair Value	0.4
Natural Gas Contract [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative, Collateral, net	(0.1)
Derivative Liability, Fair Value	0.1
Not Designated as Hedging Instrument [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member] | DP&L [Member]
Derivative, Collateral, net		(1.0)
Derivative, Fair Value, Offset, Net		(0.3)
Derivative Liability, Fair Value		1.3
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	5.5	4.9
Derivative, Fair Value, Net	2.1	0.7
Derivative, Fair Value, Offset, Net	(3.4)	(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	5.6	4.9
Derivative, Fair Value, Net	2.2	0.7
Derivative, Fair Value, Offset, Net	(3.4)	(4.2)
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative, Fair Value, Net		0.1
Derivative, Collateral, net	(4.1)	(2.3)
Derivative, Fair Value, Offset, Net	(3.4)	(4.2)
Derivative Liability, Fair Value	7.5	6.6
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative Asset, Fair Value		6.6
Derivative, Fair Value, Net		0.1
Derivative, Collateral, net	(4.1)	(2.3)
Derivative, Fair Value, Offset, Net	(3.4)	(4.2)
Derivative Liability, Fair Value	7.5
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative, Collateral, net		(1.0)
Derivative, Fair Value, Offset, Net	(0.9)	(0.3)
Derivative Liability, Fair Value	0.9	1.3
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member] | DP&L [Member]
Derivative, Fair Value, Net	0.1
Derivative, Fair Value, Offset, Net	(0.9)
Derivative Liability, Fair Value	1.0
Not Designated as Hedging Instrument [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	3.5	5.0
Derivative, Fair Value, Net	2.6	4.7
Derivative, Fair Value, Offset, Net	(0.9)	(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	2.7	4.7
Derivative, Fair Value, Offset, Net	(0.9)	(0.3)
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value		0.2
Derivative, Fair Value, Net		0.2
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | DP&L [Member]
Derivative Asset, Fair Value		0.2
Derivative, Fair Value, Net		0.2
Not Designated as Hedging Instrument [Member] | Commodity Contract - FTR [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative, Fair Value, Net	0.6
Derivative Liability, Fair Value	0.6
Not Designated as Hedging Instrument [Member] | Commodity Contract - Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value		0.2
Derivative, Collateral, net		(0.2)
Not Designated as Hedging Instrument [Member] | Commodity Contract - Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | DP&L [Member]
Derivative Asset, Fair Value		0.2
Derivative, Collateral, net		(0.2)
Not Designated as Hedging Instrument [Member] | Commodity Contract - Heating Oil [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative, Collateral, net	(0.4)
Derivative Liability, Fair Value	0.4
Not Designated as Hedging Instrument [Member] | Natural Gas Contract [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative, Collateral, net	(0.1)
Derivative Liability, Fair Value	0.1
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member]
Derivative Asset, Fair Value	5.6	0.5
Derivative, Fair Value, Net	3.6	0.3
Derivative, Fair Value, Offset, Net	(2.0)	(0.2)
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Prepayments and Current Assets [Member] | DP&L [Member]
Derivative Asset, Fair Value	5.6	0.5
Derivative, Fair Value, Net	3.6	0.3
Derivative, Fair Value, Offset, Net	(2.0)	(0.2)
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member]
Derivative, Fair Value, Net	0.1	0.2
Derivative, Collateral, net		(2.3)
Derivative, Fair Value, Offset, Net	(2.0)	(0.2)
Derivative Liability, Fair Value	2.1	2.7
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Short-term Derivative Positions [Member] | Other Current Liabilities [Member] | DP&L [Member]
Derivative, Fair Value, Net	0.1	0.2
Derivative, Collateral, net		(2.3)
Derivative, Fair Value, Offset, Net	(2.0)	(0.2)
Derivative Liability, Fair Value	2.1	2.7
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Credit [Member]
Derivative, Collateral, net	(0.3)
Derivative, Fair Value, Offset, Net	(0.3)
Derivative Liability, Fair Value	0.6
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member]
Derivative Asset, Fair Value	0.3	3.0
Derivative, Collateral, net		(3.0)
Derivative, Fair Value, Offset, Net	(0.3)
Cash Flow Hedging [Member] | Forward Contract Power [Member] | Long-term Derivative Positions [Member] | Other Deferred Asset [Member] | DP&L [Member]
Derivative Asset, Fair Value	0.3	3.0
Derivative, Collateral, net		(3.0)
Derivative, Fair Value, Offset, Net	$ (0.3)