Pension and Postretirement Benefits (Pension and Postretirement Benefit Plans' Obligations and Assets) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.5	$ 0.8	$ 4.6	$ 3.7
Interest cost	4.3	4.1	12.9	12.7
Pension [Member] | DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.5	0.8	4.6	3.7
Interest cost	4.3	4.1	12.9	12.7
Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.1	0.1
Interest cost	0.2	0.2	0.6	0.7
Postretirement [Member] | DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			0.1	0.1
Interest cost	$ 0.2	$ 0.2	$ 0.6	$ 0.7