Benefit Plans (Defined Benefits Plan Assets, Target Allocations) (Details)	Dec. 31, 2017	Dec. 31, 2016
Equity Securities [Member]
Target Allocation	38.00%
Percentage of plan assets	35.00%	37.00%
Debt Securities [Member]
Target Allocation	56.00%
Percentage of plan assets	55.00%	53.00%
Real Estate [Member]
Target Allocation	6.00%
Percentage of plan assets	10.00%	10.00%
THE DAYTON POWER AND LIGHT COMPANY [Member] | Equity Securities [Member]
Target Allocation	38.00%
Percentage of plan assets	35.00%	37.00%
THE DAYTON POWER AND LIGHT COMPANY [Member] | Debt Securities [Member]
Target Allocation	56.00%
Percentage of plan assets	55.00%	53.00%
THE DAYTON POWER AND LIGHT COMPANY [Member] | Real Estate [Member]
Target Allocation	6.00%
Percentage of plan assets	10.00%	10.00%