Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Supplemental Financial Information [Line Items]
Impairment of Long-Lived Assets Held-for-use			$ 0.0	$ 2.8
Gain (Loss) on Sale of Assets and Asset Impairment Charges			(0.9)	0.6
Customer receivables	$ 51.1		51.1		$ 55.8
Unbilled Revenue	15.2		15.2		16.8
Amounts due from partners in jointly owned stations	0.2		0.2		0.0
Due from PJM transmission settlement	2.4		2.4		16.5
Other	1.3		1.3		2.3
Provision for uncollectible accounts	(0.4)		(0.4)		(0.9)
Total accounts receivable, net	69.8		69.8		90.5
Fuel and limestone	3.0		3.0		1.9
Plant materials and supplies	10.0		10.0		8.3
Other	0.0		0.0		0.5
Total inventories, at average cost	13.0		13.0		10.7
Other Operating Income (Expense), Net	0.0	$ (1.6)	(0.9)	(14.6)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Gain (Loss) on Disposition of Business	0.0	0.0	0.0	12.4
Customer receivables	50.0		50.0		53.3
Unbilled Revenue	15.2		15.2		16.8
Amounts due from partners in jointly owned stations	3.2		3.2		2.3
Due from PJM transmission settlement	2.4		2.4		16.5
Other	1.3		1.3		2.4
Provision for uncollectible accounts	(0.4)		(0.4)		(0.9)
Total accounts receivable, net	71.7		71.7		90.4
Plant materials and supplies	9.8		9.8		7.1
Other	0.0		0.0		0.6
Total inventories, at average cost	9.8		9.8		$ 7.7
Other Operating Income (Expense) [Member]
Supplemental Financial Information [Line Items]
Gain (Loss) on Disposition of Business	0.0	0.0	0.0	11.7
Impairment of Long-Lived Assets Held-for-use	0.0	1.6	0.0	2.8
Other Cost and Expense, Operating	0.0	0.0	(0.9)	(0.1)
Other Operating Income (Expense), Net	$ 0.0	$ (1.6)	$ (0.9)	$ (14.6)