Benefit Plans (Pension and Postretirement Benefit Plans' Obligations and Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Noncurrent liabilities	$ (101.0)	$ (101.6)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Noncurrent liabilities	(91.1)	(93.4)
Postretirement [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	12.7	15.8
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	12.7	15.8
Pension [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	419.6	410.8
Service cost	5.7	5.7	$ 7.1
Interest cost	14.2	14.7	17.3
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	3.0	2.5
Actuarial (gain) / loss	28.1	9.0
Benefit obligation at December 31	436.9	419.6	410.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	341.0	345.4
Actual return / (loss) on plan assets	44.8	13.3
Contributions to plan assets	5.4	5.4
Fair value of plan assets at December 31	357.5	341.0	345.4
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(79.4)	(78.6)
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(79.0)	(78.2)
Net asset / (liability) at December 31	(79.4)	(78.6)
Prior service cost	4.9	8.8
Net actuarial loss	111.4	108.9
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	116.3	117.7
Defined Benefit Plan, Plan Assets, Benefits Paid	33.7	23.1
Defined Benefit Plan, Benefit Obligation, Benefits Paid	33.7	23.1
Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	419.6	410.8
Service cost	5.7	5.7	7.1
Interest cost	14.2	14.7	17.3
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	3.0	2.5
Actuarial (gain) / loss	28.1	9.0
Benefit obligation at December 31	436.9	419.6	410.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	341.0	345.4
Actual return / (loss) on plan assets	44.8	13.3
Contributions to plan assets	5.4	5.4
Fair value of plan assets at December 31	357.5	341.0	$ 345.4
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(79.4)	(78.6)
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(79.0)	(78.2)
Net asset / (liability) at December 31	(79.4)	(78.6)
Prior service cost	6.7	8.8
Net actuarial loss	148.3	108.9
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	155.0	117.7
Defined Benefit Plan, Plan Assets, Benefits Paid	33.7	23.1
Defined Benefit Plan, Benefit Obligation, Benefits Paid	33.7	23.1
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	92.1	97.1
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	92.2	97.1
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	24.2	20.6
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	$ 62.8	$ 20.6