Pension and Postretirement Benefits (Net Periodic Benefit Cost (Income)) (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
Period in which the difference between market related value of assets is calculated in years, max			5
Period in which the difference between actual and estimated asset returns in MRVA is calculated			3
Market related value of assets	$ 335.0	$ 316.0	$ 336.0	$ 316.0
Pension [Member]
Service cost	1.5	0.8	4.6	3.7
Interest cost	4.3	4.1	12.9	12.7
Expected return on assets	(5.7)	(6.2)	(17.0)	(18.4)
Actuarial (gain) / loss	1.3	1.7	3.7	6.2
Prior service cost	0.4	0.5	1.1	1.6
Net Periodic benefit cost / (income) before adjustments	1.8	0.9	5.3	5.8
Settlement Cost	0.2		0.2
Net periodic benefit cost / (income)	2.0	0.9	5.5	5.8
Postretirement [Member]
Service cost			0.1	0.1
Interest cost	0.2	0.2	0.6	0.7
Expected return on assets	(0.1)	(0.1)	(0.2)	(0.2)
Actuarial (gain) / loss	(0.1)	(0.5)	(0.5)	(0.9)
Prior service cost		0.1		0.1
Net Periodic benefit cost / (income) before adjustments		(0.3)		(0.2)
Net periodic benefit cost / (income)		(0.3)		(0.2)
DP&L [Member]
Period in which the difference between market related value of assets is calculated in years, max	2,012	2,011
Period in which the difference between actual and estimated asset returns in MRVA is calculated			2,012	2,011
Market related value of assets			335.0	316.0
DP&L [Member] | Pension [Member]
Service cost	1.5	0.8	4.6	3.7
Interest cost	4.3	4.1	12.9	12.7
Expected return on assets	(5.7)	(6.2)	(17.0)	(18.4)
Actuarial (gain) / loss	2.4	1.7	7.1	6.2
Prior service cost	0.7	0.5	2.2	1.6
Net Periodic benefit cost / (income) before adjustments	3.2	0.9	9.8	5.8
Settlement Cost	0.5		0.5
Net periodic benefit cost / (income)	3.7	0.9	10.3	5.8
DP&L [Member] | Postretirement [Member]
Service cost			0.1	0.1
Interest cost	0.2	0.2	0.6	0.7
Expected return on assets	(0.1)	(0.1)	(0.2)	(0.2)
Actuarial (gain) / loss	(0.2)	(0.5)	(0.7)	(0.9)
Prior service cost	0.1	0.1	0.1	0.1
Net Periodic benefit cost / (income) before adjustments		(0.3)	(0.1)	(0.2)
Net periodic benefit cost / (income)		$ (0.3)	$ (0.1)	$ (0.2)