Benefit Plans (Pension and Postretirement Benefit Plans' Obligations and Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Noncurrent liabilities	$ (62.3)	$ (93.9)
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	(8.9)
Noncurrent liabilities	(62.3)	(93.9)
Postretirement [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Funded (Unfunded) Status of Plan		9.0
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	8.9	9.0
Pension [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	449.5	421.5
Service cost	4.5	3.7	$ 3.7
Interest cost	8.2	11.8	14.9
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	(12.7)	52.7
Benefit obligation at December 31	416.2	449.5	421.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	365.1	352.0
Actual return / (loss) on plan assets	24.0	45.6
Contributions to plan assets	10.0	7.7
Fair value of plan assets at December 31	363.5	365.1	352.0
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(52.7)	(84.4)
Current liabilities	(0.2)	(0.2)
Noncurrent liabilities	(52.5)	(84.2)
Net asset / (liability) at December 31	(52.7)	(84.4)
Prior service cost	8.3	6.9
Net actuarial loss	93.3	124.0
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	101.6	130.9
Defined Benefit Plan, Plan Assets, Benefits Paid	35.6	40.2
Defined Benefit Plan, Benefit Obligation, Benefits Paid	35.6	40.2
Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	449.5	421.5
Service cost	4.5	3.7	3.7
Interest cost	8.2	11.8	14.9
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	(12.7)	52.7
Benefit obligation at December 31	416.2	449.5	421.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	365.1	352.0
Actual return / (loss) on plan assets	24.0	45.6
Contributions to plan assets	10.0	7.7
Fair value of plan assets at December 31	363.5	365.1	$ 352.0
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(52.7)	(84.4)
Current liabilities	(0.2)	(0.2)
Noncurrent liabilities	(52.5)	(84.2)
Net asset / (liability) at December 31	(52.7)	(84.4)
Prior service cost	8.5	7.3
Net actuarial loss	119.5	152.6
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	128.0	159.9
Defined Benefit Plan, Plan Assets, Benefits Paid	35.6	40.2
Defined Benefit Plan, Benefit Obligation, Benefits Paid	35.6	40.2
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	74.1	92.7
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	74.0	92.7
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	27.5	38.2
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	$ 54.0	$ 67.2