Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Supplemental Financial Information [Line Items]
Gain (Loss) on Disposition of Business	$ 0.0	$ (11.7)
Gain (Loss) on Sale of Assets and Asset Impairment Charges	0.9	0.6
Unbilled Revenue	15.7		$ 16.8
Customer receivables	60.5		55.8
Due from PJM transmission settlement	9.8		16.5
Amounts due from partners in jointly owned stations	0.7		0.0
Other	1.6		2.3
Provision for uncollectible accounts	(0.6)		(0.9)
Total accounts receivable, net	87.7		90.5
Fuel and limestone	3.5		1.9
Plant materials and supplies	8.4		8.3
Other	0.0		0.5
Total inventories, at average cost	11.9		10.7
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Gain (Loss) on Disposition of Business	0.0	$ (12.4)
Unbilled Revenue	15.7		16.8
Customer receivables	58.0		53.3
Due from PJM transmission settlement	9.8		16.5
Amounts due from partners in jointly owned stations	3.9		2.3
Other	1.6		2.4
Provision for uncollectible accounts	(0.6)		(0.9)
Total accounts receivable, net	88.4		90.4
Plant materials and supplies	8.1		7.1
Other	0.0		0.6
Total inventories, at average cost	$ 8.1		$ 7.7