Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Total Reclassifications out of AOCI	$ 0.3	$ 5.7
Total Adjustments to AOCI	0.6
Other Comprehensive Income (Loss), Net of Tax	0.9	(7.5)
Accumulated Other Comprehensive Income (Loss), Net of Tax	8.4		7.5
DP&L [Member]
Total Reclassifications out of AOCI	1.0	6.5
Total Adjustments to AOCI	0.6
Other Comprehensive Income (Loss), Net of Tax	1.6
Accumulated Other Comprehensive Income (Loss), Net of Tax	(40.7)		(42.3)
Available-for-sale Securities [Member]
Total Reclassifications out of AOCI	(0.4)
Total Adjustments to AOCI	0.5
Other Comprehensive Income (Loss), Net of Tax	0.1
Accumulated Other Comprehensive Income (Loss), Net of Tax	0.6		0.5
Available-for-sale Securities [Member] | DP&L [Member]
Total Reclassifications out of AOCI	(0.4)
Total Adjustments to AOCI	0.5
Other Comprehensive Income (Loss), Net of Tax	0.1
Accumulated Other Comprehensive Income (Loss), Net of Tax	0.8		0.7
Derivative [Member]
Total Reclassifications out of AOCI	0.6
Total Adjustments to AOCI	0.1
Other Comprehensive Income (Loss), Net of Tax	0.7
Accumulated Other Comprehensive Income (Loss), Net of Tax	19.2		18.5
Derivative [Member] | DP&L [Member]
Total Reclassifications out of AOCI	0.5
Total Adjustments to AOCI	0.1
Other Comprehensive Income (Loss), Net of Tax	0.6
Accumulated Other Comprehensive Income (Loss), Net of Tax	3.4		2.8
Other Pension Plan, Defined Benefit [Member]
Total Reclassifications out of AOCI	0.1
Other Comprehensive Income (Loss), Net of Tax	0.1
Accumulated Other Comprehensive Income (Loss), Net of Tax	(11.4)		(11.5)
Other Pension Plan, Defined Benefit [Member] | DP&L [Member]
Total Reclassifications out of AOCI	0.9
Other Comprehensive Income (Loss), Net of Tax	0.9
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (44.9)		$ (45.8)