Business Segments (Narrative) (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($) mi² customer	Jun. 30, 2019 USD ($) segment	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 1,817.7	$ 1,817.7	$ 1,883.1
THE DAYTON POWER AND LIGHT COMPANY [Member]
Segment Reporting Information [Line Items]
Total assets	$ 1,775.1	$ 1,775.1	1,819.6
Number of Operating Segments | segment		1
Approximate number of retail customers | customer	525,000
Service area, square miles | mi²	6,000
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Total assets	$ 1,775.1	$ 1,775.1	1,819.6
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	$ 42.6	$ 42.6	$ 63.5