Business Segments (Narrative) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) mi² customer segment	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 1,816.3	$ 1,883.1
THE DAYTON POWER AND LIGHT COMPANY [Member]
Segment Reporting Information [Line Items]
Total assets	$ 1,779.4	1,819.6
Number of Operating Segments | segment	1
Approximate number of retail customers | customer	524,000
Service area, square miles | mi²	6,000
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Total assets	$ 1,779.4	1,819.6
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	$ 36.9	$ 63.5