Pension and Postretirement Benefits (Net Periodic Benefit Cost (Income)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Period in which the difference between market related value of assets is calculated in years, max	five
Period in which the difference between actual and estimated asset returns in MRVA is calculated	three
Market related value of assets	$ 346.0		$ 336.0
Pension [Member]
Service cost	1.8		1.5
Interest cost	3.9		4.3
Expected return on assets	(5.9)	[1]	(5.7)	[1]
Actuarial (gain) / loss	1.2		1.2
Prior service cost	0.4		0.4
Net periodic benefit cost / (income)	1.4		1.7
Postretirement [Member]
Service cost	0.1		0.1
Interest cost	0.2		0.3
Expected return on assets	(0.1)	[1]	(0.1)	[1]
Actuarial (gain) / loss	(0.1)		(0.2)
Net periodic benefit cost / (income)	0.1		0.1
DP&L [Member]
Market related value of assets	346.0		335.0
DP&L [Member] | Pension [Member]
Service cost	1.8		1.5
Interest cost	3.9		4.3
Expected return on assets	(5.9)	[1]	(5.7)	[1]
Actuarial (gain) / loss	2.3		2.7
Prior service cost	0.7		0.8
Net periodic benefit cost / (income)	2.8		3.6
DP&L [Member] | Postretirement [Member]
Service cost	0.1		0.1
Interest cost	0.2		0.3
Expected return on assets	(0.1)	[1]	(0.1)	[1]
Actuarial (gain) / loss	(0.1)		(0.2)
Net periodic benefit cost / (income)	$ 0.1		$ 0.1

[1] (a) For purposes of calculating the expected return on pension plan assets, under GAAP, the market-related value of assets (MRVA) is used. GAAP requires that the difference between actual plan asset returns and estimated plan asset returns be included in the MRVA equally over a period not to exceed five years.  We use a methodology under which we include the difference between actual and estimated asset returns in the MRVA equally over a three year period.  The MRVA used in the calculation of expected return on pension plan assets for the 2013 and 2012 net periodic benefit cost was approximately $346.0 million and $336.0 million, respectively. Benefit payments, which reflect future service, are expected to be paid as follows:Estimated Future Benefit Payments and Medicare Part D Reimbursements$ in millionsPensionPostretirement2013$ 16.6$ 1.72014 22.5 2.22015 23.0 2.02016 23.3 1.92017 23.7 1.72018 - 2022 124.4 6.8