Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Supplemental Financial Information [Line Items]
Impairment of Long-Lived Assets Held-for-use			$ 0.0	$ 1.2
Gain (Loss) on Sale of Assets and Asset Impairment Charges			(0.9)	(1.1)
Unbilled Revenue	$ 16.3		16.3		$ 16.8
Customer receivables	47.2		47.2		55.8
Due from PJM transmission settlement	3.0		3.0		16.5
Amounts due from partners in jointly owned stations	0.2		0.2		0.0
Other	1.2		1.2		2.3
Provision for uncollectible accounts	(0.4)		(0.4)		(0.9)
Total accounts receivable, net	67.5		67.5		90.5
Fuel and limestone	2.8		2.8		1.9
Plant materials and supplies	11.9		11.9		8.3
Other	0.0		0.0		0.5
Total inventories, at average cost	14.7		14.7		10.7
Other Operating Income (Expense), Net	0.0	$ (1.2)	(0.9)	(13.0)
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
Unbilled Revenue	16.3		16.3		16.8
Customer receivables	44.4		44.4		53.3
Due from PJM transmission settlement	3.0		3.0		16.5
Amounts due from partners in jointly owned stations	7.1		7.1		2.3
Other	1.2		1.2		2.4
Provision for uncollectible accounts	(0.4)		(0.4)		(0.9)
Total accounts receivable, net	71.6		71.6		90.4
Plant materials and supplies	11.2		11.2		7.1
Other	0.0		0.0		0.6
Total inventories, at average cost	11.2		11.2		$ 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	1.2	0.0	1.2
Other Cost and Expense, Operating	0.0	0.0	(0.9)	(0.1)
Other Operating Income (Expense), Net	$ 0.0	$ (1.2)	$ (0.9)	$ (13.0)