Benefit Plans (Weighted Average Assumptions Used to Determine Benefit Obligations) (Details) - Pension [Member]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Discount rate for obligations	3.66%	4.28%	4.49%
Rate of compensation increases	3.94%	3.94%	3.94%
THE DAYTON POWER AND LIGHT COMPANY [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate for obligations	3.66%	4.28%	4.49%
Rate of compensation increases	3.94%	3.94%	3.94%