Benefit Plans (Other Changes in Plan Assets and Benefit Obligation Recognized in Accumulated Other Comprehensive Income, Regulatory Assets And Regulatory Liabilities) (Details) - Pension [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) / loss	$ 9.1	$ 20.9	$ (3.0)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax	0.0	0.0	0.0
Defined Benefit Plan, Accumulated Other Comprehensive Income, Plan Curtailments	(4.1)	(3.8)	0.0
Reversal of amortization item, Net actuarial (gain) / loss	(5.3)	(4.3)	(5.8)
Reversal of amortization item, Prior service cost / (credit)	(1.1)	(1.8)	(2.0)
Total recognized in Accumulated other comprehensive income, Regulatory assets and Regulatory liabilities	(1.4)	11.0	(10.8)
Total recognized in net periodic benefit cost and Accumulated other comprehensive income, Regulatory assets and Regulatory liabilities	6.2	18.5	(1.2)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) / loss	9.1	20.9	(3.0)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax	0.0	0.0	0.0
Defined Benefit Plan, Accumulated Other Comprehensive Income, Plan Curtailments	(5.6)	(5.7)	0.0
Reversal of amortization item, Net actuarial (gain) / loss	(8.7)	(7.2)	(9.8)
Reversal of amortization item, Prior service cost / (credit)	(1.5)	(3.0)	(3.3)
Total recognized in Accumulated other comprehensive income, Regulatory assets and Regulatory liabilities	(6.7)	5.0	(16.1)
Total recognized in net periodic benefit cost and Accumulated other comprehensive income, Regulatory assets and Regulatory liabilities	$ 6.2	$ 18.5	$ (1.2)