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, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) / loss	$ 20.9	$ (3.0)	$ 43.8
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	0.0	0.0	6.8
Defined Benefit Plan, Accumulated Other Comprehensive Income, Plan Curtailments	(3.8)	0.0	0.0
Reversal of amortization item, Net actuarial (gain) / loss	(4.3)	(5.8)	(3.4)
Reversal of amortization item, Prior service cost / (credit)	(1.8)	(2.0)	(1.5)
Total recognized in Accumulated other comprehensive income, Regulatory assets and Regulatory liabilities	11.0	(10.8)	45.7
Total recognized in net periodic benefit cost and Accumulated other comprehensive income, Regulatory assets and Regulatory liabilities	18.5	(1.2)	51.1
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) / loss	20.9	(3.0)	43.8
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	0.0	0.0	6.8
Defined Benefit Plan, Accumulated Other Comprehensive Income, Plan Curtailments	(5.7)	0.0	0.0
Reversal of amortization item, Net actuarial (gain) / loss	(7.2)	(9.8)	(6.4)
Reversal of amortization item, Prior service cost / (credit)	(3.0)	(3.3)	(2.8)
Total recognized in Accumulated other comprehensive income, Regulatory assets and Regulatory liabilities	5.0	(16.1)	41.4
Total recognized in net periodic benefit cost and Accumulated other comprehensive income, Regulatory assets and Regulatory liabilities	$ 18.5	$ (1.2)	$ 51.1