Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	$ (34.2)	$ 23.4
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	17.1	5.1	(0.6)
Other comprehensive income / (loss)	(17.1)	28.5
Accumulated Other Comprehensive Income (Loss), Net of Tax	7.5	24.6	(3.9)
Financial Instruments Reclassification	0.2	1.4	(0.1)
DP&L [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(31.2)	4.2
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	15.6	7.8	(0.8)
Other comprehensive income / (loss)	(15.6)	12.0
Accumulated Other Comprehensive Income (Loss), Net of Tax	(42.3)	(26.7)	(38.7)
Financial Instruments Reclassification	0.2	1.4	(0.1)
Available-for-sale Securities [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(0.3)	(1.2)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.2	1.4
Other comprehensive income / (loss)	(0.1)	0.2
Accumulated Other Comprehensive Income (Loss), Net of Tax	0.5	0.6	0.4
Available-for-sale Securities [Member] | DP&L [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(0.3)	(1.6)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.2	1.4
Other comprehensive income / (loss)	(0.1)	(0.2)
Accumulated Other Comprehensive Income (Loss), Net of Tax	0.7	0.8	1.0
Derivative [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(19.0)	19.7
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	16.9	3.4
Other comprehensive income / (loss)	(2.1)	23.1
Accumulated Other Comprehensive Income (Loss), Net of Tax	18.5	20.6	(2.5)
Derivative [Member] | DP&L [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(18.8)	1.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	15.4	2.6
Other comprehensive income / (loss)	(3.4)	3.6
Accumulated Other Comprehensive Income (Loss), Net of Tax	2.8	6.2	2.6
Other Pension Plan, Defined Benefit [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(14.9)	4.9
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0.3
Other comprehensive income / (loss)	(14.9)	5.2
Accumulated Other Comprehensive Income (Loss), Net of Tax	(11.5)	3.4	(1.8)
Other Pension Plan, Defined Benefit [Member] | DP&L [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(12.1)	4.8
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		3.8
Other comprehensive income / (loss)	(12.1)	8.6
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (45.8)	$ (33.7)	$ (42.3)