Benefit Plans (Pension and Postretirement Benefit Plans' Obligations and Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Noncurrent liabilities	$ (87.1)	$ (95.9)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	371.7
Fair value of plan assets at December 31	345.4	371.7
Noncurrent liabilities	(87.1)	(95.9)
Postretirement [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	19.6	19.7
Service cost	0.2	0.2	$ 0.2
Interest cost	0.6	0.8	0.8
Actuarial (gain) / loss	(1.1)	0.2
Benefits paid	(1.5)	(1.3)
Benefit obligation at December 31	17.8	19.6	19.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	3.3	3.7
Contributions to plan assets	1.0	0.9
Fair value of plan assets at December 31	2.8	3.3	3.7
Funded Status of Plan	(15.0)	(16.3)
Current liabilities	(0.4)	(0.5)
Noncurrent liabilities	(14.6)	(15.8)
Net asset / (liability) at December 31	(15.0)	(16.3)
Prior service cost	0.3	0.4
Net actuarial loss	(5.5)	(5.0)
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(5.2)	(4.6)
Postretirement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	19.6	19.7
Service cost	0.2	0.2	0.2
Interest cost	0.6	0.8	0.8
Actuarial (gain) / loss	(1.1)	0.2
Benefits paid	(1.5)	(1.3)
Benefit obligation at December 31	17.8	19.6	19.7
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	3.3	3.7
Contributions to plan assets	1.0	0.9
Fair value of plan assets at December 31	2.8	3.3	3.7
Funded Status of Plan	(15.0)	(16.3)
Current liabilities	(0.4)	(0.5)
Noncurrent liabilities	(14.6)	(15.8)
Net asset / (liability) at December 31	(15.0)	(16.3)
Prior service cost	0.5	0.6
Net actuarial loss	(6.2)	(5.8)
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(5.7)	(5.2)
Pension [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	443.8	370.5
Service cost	7.1	5.9	7.2
Interest cost	17.3	17.5	15.6
Plan amendments	0.0	6.8
Actuarial (gain) / loss	(34.5)	67.3
Benefits paid	(22.9)	(24.2)
Benefit obligation at December 31	410.8	443.8	370.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	371.7	349.1
Actual return / (loss) on plan assets	(8.8)	46.4
Contributions to plan assets	5.4	0.4
Fair value of plan assets at December 31	345.4	371.7	349.1
Funded Status of Plan	(65.4)	(72.1)
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(65.0)	(71.7)
Net asset / (liability) at December 31	(65.4)	(72.1)
Prior service cost	12.0	14.1
Net actuarial loss	94.7	103.4
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	106.7	117.5
Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	443.8	370.5
Service cost	7.1	5.9	7.2
Interest cost	17.3	17.5	15.6
Plan amendments	0.0	6.8
Actuarial (gain) / loss	(34.5)	67.3
Benefits paid	(22.9)	(24.2)
Benefit obligation at December 31	410.8	443.8	370.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	371.7	349.1
Actual return / (loss) on plan assets	(8.8)	46.4
Contributions to plan assets	5.4	0.4
Fair value of plan assets at December 31	345.4	371.7	$ 349.1
Funded Status of Plan	(65.4)	(72.1)
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(65.0)	(71.7)
Net asset / (liability) at December 31	(65.4)	(72.1)
Prior service cost	17.0	20.3
Net actuarial loss	139.7	152.5
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	156.7	172.8
Regulatory Asset [Member] | Postretirement [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	0.3	0.4
Regulatory Asset [Member] | Postretirement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	0.3	0.0
Regulatory Asset [Member] | Pension [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	91.1	99.0
Regulatory Asset [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	91.1	99.0
Regulatory Liability [Member] | Postretirement [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(5.1)	(4.8)
Regulatory Liability [Member] | Postretirement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(5.1)	(4.5)
Regulatory Liability [Member] | Pension [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	0.0	0.0
Regulatory Liability [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	0.0	0.0
Accumulated Other Comprehensive Income/(Loss) [Member] | Postretirement [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(0.4)	(0.2)
Accumulated Other Comprehensive Income/(Loss) [Member] | Postretirement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	(0.9)	(0.7)
Accumulated Other Comprehensive Income/(Loss) [Member] | Pension [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	15.6	18.5
Accumulated Other Comprehensive Income/(Loss) [Member] | Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	$ 65.6	$ 73.8