Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Supplemental Financial Information [Line Items]
Gain (Loss) on Disposition of Business			$ (13.5)	$ 0.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges			(1.1)	(16.2)
Unbilled revenue	$ 12.9		12.9		$ 18.0
Customer receivables	49.5		49.5		45.2
Due from PJM transmission settlement	30.1		30.1		0.0
Other	2.9		2.9		2.5
Provision for uncollectible accounts	(1.1)		(1.1)		(1.1)
Total accounts receivable, net	94.3		94.3		64.6
Fuel and limestone	2.7		2.7		4.1
Plant materials and supplies	8.2		8.2		8.1
Other	0.7		0.7		0.5
Total inventories, at average cost	11.6		11.6		12.7
Other Operating Income (Expense), Net	(1.2)	$ 0.0	(13.0)	0.0
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Gain (Loss) on Disposition of Business	0.0	0.0	(12.4)	0.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges			(0.1)	(16.2)
Unbilled revenue	12.9		12.9		18.0
Customer receivables	47.3		47.3		44.2
Due from PJM transmission settlement					0.0
Amounts due from partners in jointly owned stations	8.3		8.3		3.7
Other	2.7		2.7		6.0
Provision for uncollectible accounts	(1.1)		(1.1)		(1.1)
Total accounts receivable, net	100.2		100.2		70.8
Plant materials and supplies	7.0		7.0		6.9
Other	0.7		0.7		0.4
Total inventories, at average cost	7.7		7.7		$ 7.3
Other Operating Income (Expense) [Member]
Supplemental Financial Information [Line Items]
Insurance Recoveries	1.2	0.0	1.2	0.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges	0.0	0.0	11.7	0.0
Other Cost and Expense, Operating	0.0	0.0	0.1	0.0
Other Operating Income (Expense), Net	$ 1.2	$ 0.0	$ 13.0	$ 0.0