Pension and Postretirement Benefits (Pension and Postretirement Benefit Plans' Obligations and Assets) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at December 31	$ 371.7
Noncurrent liabilities	(95.9)	(51.6)
DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at December 31	371.7	349.1
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at January 1	370.5	395.6
Service cost	5.9	7.2	6.2
Interest cost	17.5	15.6	17.3
Plan amendments	6.8
Actuarial (gain) / loss	67.3	(26.5)
Benefits paid	(24.2)	(21.4)
Benefit obligation at December 31	443.8	370.5	395.6
Fair value of plan assets at January 1	349.1	361.4
Actual return / (loss) on plan assets	46.4	8.7
Contributions to plan assets	0.4	0.4
Fair value of plan assets at December 31	371.7	349.1	361.4
Funded Status of Plan	(72.1)	(21.4)
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(71.7)	(21.0)
Net asset / (liability) at December 31	(72.1)	(21.4)
Prior service cost / (credit)	14.1	8.8
Net actuarial loss / (gain)	103.4	63.0
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	117.5	71.8
Pension [Member] | DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at January 1	370.5	395.6
Service cost	5.9	7.2	6.2
Interest cost	17.5	15.6	17.3
Plan amendments	6.8
Actuarial (gain) / loss	67.3	(26.5)
Benefits paid	(24.2)	(21.4)
Benefit obligation at December 31	443.8	370.5	395.6
Fair value of plan assets at January 1	349.1	361.4
Actual return / (loss) on plan assets	46.4	8.7
Contributions to plan assets	0.4	0.4
Fair value of plan assets at December 31	371.7	349.1	361.4
Funded Status of Plan	(72.1)	(21.4)
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(71.7)	(21.0)
Net asset / (liability) at December 31	(72.1)	(21.4)
Prior service cost / (credit)	20.3	16.3
Net actuarial loss / (gain)	152.5	115.1
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	172.8	131.4
Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at January 1	19.7	22.4
Service cost	0.2	0.2	0.1
Interest cost	0.8	0.8	0.9
Actuarial (gain) / loss	0.2	(2.2)
Benefits paid	(1.3)	(1.5)
Benefit obligation at December 31	19.6	19.7	22.4
Fair value of plan assets at January 1	3.7	4.2
Contributions to plan assets	0.9	1.0
Fair value of plan assets at December 31	3.3	3.7	4.2
Funded Status of Plan	(16.3)	(16.0)
Current liabilities	(0.5)	(0.5)
Noncurrent liabilities	(15.8)	(15.5)
Net asset / (liability) at December 31	(16.3)	(16.0)
Prior service cost / (credit)	0.4	0.5
Net actuarial loss / (gain)	(5.0)	(6.0)
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(4.6)	(5.5)
Postretirement [Member] | DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at January 1	19.7	22.4
Service cost	0.2	0.2	0.1
Interest cost	0.8	0.8	0.9
Actuarial (gain) / loss	0.2	(2.2)
Benefits paid	(1.3)	(1.5)
Benefit obligation at December 31	19.6	19.7	22.4
Fair value of plan assets at January 1	3.7	4.2
Contributions to plan assets	0.9	1.0
Fair value of plan assets at December 31	3.3	3.7	4.2
Funded Status of Plan	(16.3)	(16.0)
Current liabilities	(0.5)	(0.5)
Noncurrent liabilities	(15.8)	(15.5)
Net asset / (liability) at December 31	(16.3)	(16.0)
Prior service cost / (credit)	0.6	0.7
Net actuarial loss / (gain)	(5.8)	(6.9)
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(5.2)	(6.2)
Regulatory Asset [Member] | Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	99.0	76.3
Regulatory Asset [Member] | Pension [Member] | DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	99.0	76.3
Regulatory Asset [Member] | Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	0.4	0.4
Regulatory Liability [Member] | Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(4.8)	(5.6)
Regulatory Liability [Member] | Postretirement [Member] | DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(4.5)	(5.2)
Accumulated Other Comprehensive Income/(Loss) [Member] | Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	18.5	(4.5)
Accumulated Other Comprehensive Income/(Loss) [Member] | Pension [Member] | DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	73.8	55.1
Accumulated Other Comprehensive Income/(Loss) [Member] | Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(0.2)	(0.3)
Accumulated Other Comprehensive Income/(Loss) [Member] | Postretirement [Member] | DP&L [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	$ (0.7)	$ (1.0)