Pension and Postretirement Benefits (Pension and Postretirement Benefit Plans' Obligations and Assets) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.6	$ 1.5	$ 3.1	$ 2.9
Interest cost	4.3	4.3	8.6	8.6
Pension [Member] | DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.6	1.5	3.1	2.9
Interest cost	4.3	4.3	8.6	8.6
Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.1	0.1	0.1
Interest cost	0.1	0.2	0.4	0.5
Postretirement [Member] | DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		0.1	0.1	0.1
Interest cost	$ 0.1	$ 0.2	$ 0.4	$ 0.5