Benefit Plans (Estimated Amounts that will be Amortized from Accumulated Other Comprehensive Income, Regulatory Assets And Regulatory Liabilities) (Details) - Pension [Member] $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) / loss	$ 5.8
Prior service cost	1.4
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) / loss	9.7
Prior service cost	$ 1.9