Supplemental Financial Information (Reclassification out of ACOI) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Tax	$ 0.2	$ (0.1)
Reclassification to earnings of available-for-sale securities, net of income tax expense/(benefit)	0.4	(0.2)
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	(0.3)	(3.1)
Reclassification of earnings of Cash Flow Hedge	(0.6)	(5.5)
Other Comprehensive Income (Loss), Net of Tax	0.9	(7.5)
Other Comprehensive Income (Loss) - Pension Reclassifications	0.1
Other Comprehensive Income (Loss),Net Gain (Loss) Reclassification	(0.1)
Reclassification to earnings of Pension	(0.1)
Total Reclassifications out of AOCI	0.3	5.7
Available-for-sale Securities [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Tax	0.2	(0.1)
Reclassification to earnings of available-for-sale securities, net of income tax expense/(benefit)	(0.4)	0.2
Other Comprehensive Income (Loss), Net of Tax	0.1
Total Reclassifications out of AOCI	(0.4)
Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	0.9	8.6
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	(0.3)	(3.1)
Reclassification of earnings of Cash Flow Hedge	0.6	5.5
Other Comprehensive Income (Loss), Net of Tax	0.7
Total Reclassifications out of AOCI	0.6
Other Pension Plan, Defined Benefit [Member]
Other Comprehensive Income (Loss), Net of Tax	0.1
Other Comprehensive Income (Loss),Net Gain (Loss) Reclassification	0.1
Reclassification to earnings of Pension	0.1
Total Reclassifications out of AOCI	0.1
Other Income [Member] | Available-for-sale Securities [Member]
Other Comprehensive Income (Loss) - Financial Instruments Reclassfication	(0.6)	0.3
Interest Expense [Member] | Derivative [Member]
Other Comprehensive Income (Loss), Net of Tax	(0.2)	(0.5)
Revenue [Member] | Derivative [Member]
Other Comprehensive Income (Loss), Net of Tax	(0.3)	10.2
Purchased Power [Member] | Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	1.4	(1.1)
DP&L [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Tax	0.2	(0.1)
Reclassification to earnings of available-for-sale securities, net of income tax expense/(benefit)	(0.4)	0.2
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	(0.3)	(3.2)
Reclassification of earnings of Cash Flow Hedge	0.5	5.7
Other Comprehensive Income (Loss), Net of Tax	1.6
Other Comprehensive Income (Loss) - Pension Reclassifications	1.4	1.0
Other Comprehensive Income (Loss),Net Gain (Loss) Reclassification	0.9	0.6
Income tax (expense)/benefit on reclassification of earnings related to pension and postretirement activity	(0.5)	(0.4)
Reclassification to earnings of Pension	0.9	0.6
Total Reclassifications out of AOCI	1.0	6.5
DP&L [Member] | Available-for-sale Securities [Member]
Other Comprehensive Income (Loss), Net of Tax	0.1
Total Reclassifications out of AOCI	(0.4)
DP&L [Member] | Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	0.8	8.9
Other Comprehensive Income (Loss), Net of Tax	0.6
Total Reclassifications out of AOCI	0.5
DP&L [Member] | Other Pension Plan, Defined Benefit [Member]
Other Comprehensive Income (Loss), Net of Tax	0.9
Total Reclassifications out of AOCI	0.9
DP&L [Member] | Other Income [Member] | Available-for-sale Securities [Member]
Other Comprehensive Income (Loss) - Financial Instruments Reclassfication	(0.6)	0.3
DP&L [Member] | Interest Expense [Member] | Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	(0.3)	(0.3)
DP&L [Member] | Revenue [Member] | Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	(0.3)	(1.0)
DP&L [Member] | Purchased Power [Member] | Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	$ 1.4	$ 10.2