Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Total Reclassifications out of AOCI	$ 0.6
Total Adjustments to AOCI	1.4
Other Comprehensive Income (Loss), Net of Tax	2.0	7.1
Accumulated Other Comprehensive Income (Loss), Net of Tax	(1.9)		(3.9)
DP&L [Member]
Total Reclassifications out of AOCI	(0.1)
Total Adjustments to AOCI	(1.5)
Other Comprehensive Income (Loss), Net of Tax	(1.6)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(40.3)		(38.7)
Available-for-sale Securities [Member]
Total Reclassifications out of AOCI	0.1
Total Adjustments to AOCI	0.2
Other Comprehensive Income (Loss), Net of Tax	0.3
Accumulated Other Comprehensive Income (Loss), Net of Tax	0.7		0.4
Available-for-sale Securities [Member] | DP&L [Member]
Total Reclassifications out of AOCI	0.1
Total Adjustments to AOCI	0.2
Other Comprehensive Income (Loss), Net of Tax	0.3
Accumulated Other Comprehensive Income (Loss), Net of Tax	1.3		1.0
Derivative [Member]
Total Reclassifications out of AOCI	0.2
Total Adjustments to AOCI	1.2
Other Comprehensive Income (Loss), Net of Tax	1.4
Accumulated Other Comprehensive Income (Loss), Net of Tax	(1.1)		(2.5)
Derivative [Member] | DP&L [Member]
Total Reclassifications out of AOCI	(0.2)
Total Adjustments to AOCI	(2.6)
Other Comprehensive Income (Loss), Net of Tax	(2.8)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(0.2)		2.6
Other Pension Plans, Defined Benefit [Member]
Total Reclassifications out of AOCI	0.3
Other Comprehensive Income (Loss), Net of Tax	0.3
Accumulated Other Comprehensive Income (Loss), Net of Tax	(1.5)		(1.8)
Other Pension Plans, Defined Benefit [Member] | DP&L [Member]
Total Adjustments to AOCI	0.9
Other Comprehensive Income (Loss), Net of Tax	0.9
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (41.4)		$ (42.3)