Benefit Plans (Pension and Postretirement Benefit Plans' Obligations and Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Noncurrent liabilities	$ (101.6)	$ (87.1)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Noncurrent liabilities	(101.6)	(87.1)
Postretirement [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Defined Benefit Plan, Funded Status of Plan	15.8	15.0
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 Status of Plan	15.8	15.0
Pension [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	410.8	443.8
Service cost	5.7	7.1	$ 5.9
Interest cost	14.7	17.3	17.5
Defined Benefit Plan, Curtailments	2.5	0.0
Actuarial (gain) / loss	9.0	(34.5)
Benefits paid	(23.1)	(22.9)
Benefit obligation at December 31	419.6	410.8	443.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	345.4	371.7
Actual return / (loss) on plan assets	13.3	(8.8)
Contributions to plan assets	5.4	5.4
Fair value of plan assets at December 31	341.0	345.4	371.7
Defined Benefit Plan, Funded Status of Plan	(78.6)	(65.4)
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(78.2)	(65.0)
Net asset / (liability) at December 31	(78.6)	(65.4)
Prior service cost	8.8	12.0
Net actuarial loss	108.9	94.7
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	117.7	106.7
Pension [Member] | THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at January 1	410.8	443.8
Service cost	5.7	7.1	5.9
Interest cost	14.7	17.3	17.5
Defined Benefit Plan, Curtailments	2.5	0.0
Actuarial (gain) / loss	9.0	(34.5)
Benefits paid	(23.1)	(22.9)
Benefit obligation at December 31	419.6	410.8	443.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at January 1	345.4	371.7
Actual return / (loss) on plan assets	13.3	(8.8)
Contributions to plan assets	5.4	5.4
Fair value of plan assets at December 31	341.0	345.4	$ 371.7
Defined Benefit Plan, Funded Status of Plan	(78.6)	(65.4)
Current liabilities	(0.4)	(0.4)
Noncurrent liabilities	(78.2)	(65.0)
Net asset / (liability) at December 31	(78.6)	(65.4)
Prior service cost	10.8	17.0
Net actuarial loss	150.9	139.7
Accumulated other comprehensive income, regulatory assets and regulatory liabilities, pre-tax	161.7	156.7
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	97.1	91.1
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	97.1	91.1
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] | 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	20.6	15.6
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	$ 64.6	$ 65.6