Supplemental Financial Information (Reclassification out of ACOI) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Reclassification to earnings of available-for-sale securities, net of income tax expense/(benefit)	$ 0.1
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	(0.1)	0.6
Reclassification of earnings of Cash Flow Hedge	0.2	(0.9)
Other Comprehensive Income (Loss),Net Gain (Loss) Reclassification	0.3
Income tax (expense)/benefit on reclassification of earnings related to pension and postretirement activity	(0.3)
Reclassification to earnings of Pension	0.3
Total Reclassifications out of AOCI	0.6
Available-for-sale Securities [Member]
Other Comprehensive Income (Loss) - Financial Instruments Reclassfication	0.1
Reclassification to earnings of available-for-sale securities, net of income tax expense/(benefit)	0.1
Total Reclassifications out of AOCI	0.1
Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	0.3
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	(0.1)
Reclassification of earnings of Cash Flow Hedge	0.2
Total Reclassifications out of AOCI	0.2
Other Pension Plans, Defined Benefit [Member]
Other Comprehensive Income (Loss),Net Gain (Loss) Reclassification	0.3
Income tax (expense)/benefit on reclassification of earnings related to pension and postretirement activity	0.3
Reclassification to earnings of Pension	0.3
Total Reclassifications out of AOCI	0.3
Other Income [Member] | Available-for-sale Securities [Member]
Other Comprehensive Income (Loss) - Financial Instruments Reclassfication	0.1
Revenue [Member] | Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	(0.5)
Purchased Power [Member] | Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	0.8
DP&L [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment for Sale of Securities Included in Net Income, Tax	(0.1)
Reclassification to earnings of available-for-sale securities, net of income tax expense/(benefit)	0.1
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	(0.2)
Reclassification of earnings of Cash Flow Hedge	(0.2)
Total Reclassifications out of AOCI	(0.1)
DP&L [Member] | Available-for-sale Securities [Member]
Other Comprehensive Income (Loss) - Financial Instruments Reclassfication	0.2
Total Reclassifications out of AOCI	0.1
DP&L [Member] | Derivative [Member]
Total Reclassifications out of AOCI	(0.2)
DP&L [Member] | Other Income [Member] | Available-for-sale Securities [Member]
Other Comprehensive Income (Loss) - Financial Instruments Reclassfication	0.2
DP&L [Member] | Interest Expense [Member] | Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	(0.6)
DP&L [Member] | Revenue [Member] | Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	(0.5)
DP&L [Member] | Purchased Power [Member] | Derivative [Member]
Other Comprehensive Income (Loss) - Cash Flow Hedge Reclassfications	$ 1.1