Pension and Postretirement Benefits (Net Periodic Benefit Cost (Income)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended	3 Months Ended					12 Months Ended	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Pension [Member]	Mar. 31, 2011 Pension [Member]	Mar. 31, 2012 Postretirement [Member]	Mar. 31, 2011 Postretirement [Member]	Mar. 31, 2012 DP&L [Member]	Dec. 31, 2011 DP&L [Member]	Mar. 31, 2012 DP&L [Member] Pension [Member]	Mar. 31, 2011 DP&L [Member] Pension [Member]	Mar. 31, 2012 DP&L [Member] Postretirement [Member]	Mar. 31, 2011 DP&L [Member] Postretirement [Member]
Service cost			$ 1.5	$ 1.4	$ 0.1				$ 1.5	$ 1.4	$ 0.1
Interest cost			4.3	4.3	0.3	0.3			4.3	4.3	0.3	0.3
Expected return on assets			(5.7)	(6.1)	(0.1)	(0.1)			(5.7)	(6.1)	(0.1)	(0.1)
Actuarial (gain) / loss			1.2	2.3	(0.2)	(0.2)			0.8	2.3	(0.2)	(0.2)
Prior service cost			0.4	0.5					2.7	0.5
Net periodic benefit cost / (income) before adjustments			1.7	2.4	0.1				3.6	2.4	0.1
Period in which the difference between market related value of assets is calculated in years, max	5						5
Period in which the difference between actual and estimated asset returns in MRVA is calculated	3						3
Market related value of assets	$ 336	$ 317					$ 335	$ 316