Business Segments (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) mi² customer segment	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 1,918.4	$ 1,935.8
THE DAYTON POWER AND LIGHT COMPANY [Member]
Segment Reporting Information [Line Items]
Total assets	$ 1,874.6	1,883.2
Number of Operating Segments | segment	1
Approximate number of retail customers | customer	527,000
Service area, square miles | mi²	6,000
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Total assets	$ 1,874.6	1,883.2
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	$ 43.8	$ 52.6