Benefit Plans (Pension and Postretirement Benefit Plans' Obligations and Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Noncurrent liabilities	$ (82.3)	$ (90.3)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Noncurrent liabilities	(83.2)	(91.1)
Postretirement [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	9.2	12.7
Postretirement [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	9.2	12.7
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment	0.0	(4.1)	$ (3.8)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	436.9	419.6
Service cost	6.1	5.7	5.7
Interest cost	13.8	14.2	14.7
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	5.1	0.0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0.0	3.0
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	(34.6)	28.1
Benefit obligation at December 31	386.5	436.9	419.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	357.5	341.0
Actual return / (loss) on plan assets	(11.7)	44.8
Contributions to plan assets	7.9	5.4
Fair value of plan assets at December 31	312.9	357.5	341.0
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(73.6)	(79.4)
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(73.2)	(79.0)
Net asset / (liability) at December 31	(73.6)	(79.4)
Prior service cost	9.1	4.9
Net actuarial loss	103.3	111.4
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	112.4	116.3
Defined Benefit Plan, Plan Assets, Benefits Paid	40.8	33.7
Defined Benefit Plan, Benefit Obligation, Benefits Paid	40.8	33.7
Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment	0.0	(5.6)	(5.7)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	436.9	419.6
Service cost	6.1	5.7	5.7
Interest cost	13.8	14.2	14.7
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	5.1	0.0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0.0	3.0
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	(34.6)	28.1
Benefit obligation at December 31	386.5	436.9	419.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	357.5	341.0
Actual return / (loss) on plan assets	(11.7)	44.8
Contributions to plan assets	7.9	5.4
Fair value of plan assets at December 31	312.9	357.5	$ 341.0
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(73.6)	(79.4)
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(73.2)	(79.0)
Net asset / (liability) at December 31	(73.6)	(79.4)
Prior service cost	10.4	6.7
Net actuarial loss	137.2	148.3
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	147.6	155.0
Defined Benefit Plan, Plan Assets, Benefits Paid	40.8	33.7
Defined Benefit Plan, Benefit Obligation, Benefits Paid	40.8	33.7
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	87.2	92.1
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	87.3	92.2
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	25.2	24.2
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	$ 60.3	$ 62.8