Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Aug. 01, 2018
Supplemental Financial Information [Line Items]
Unbilled revenue	$ 16.8	$ 18.0
Customer receivables	55.8	45.2
Due from PJM transmission settlement	16.5	0.0		$ 41.6
Other	2.3	2.5
Provision for uncollectible accounts	(0.9)	(1.1)
Total accounts receivable, net	90.5	64.6
Fuel and Limestone	1.9	4.1
Plant materials and supplies	8.3	8.1
Other	0.5	0.5
Total inventories, at average cost	10.7	12.7
Assets held for sale - current	8.7	315.6
Gain (Loss) on Disposition of Business	11.7	0.0	$ 0.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges	(2.0)	16.1	(0.1)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Unbilled revenue	16.8	18.0
Customer receivables	53.3	44.2
Amounts due from partners in jointly owned stations	0.0	5.0
Due from PJM transmission settlement	16.5	0.0		$ 41.6
Amounts due from affiliates	2.3	0.6
Other	2.4	4.1
Provision for uncollectible accounts	(0.9)	(1.1)
Total accounts receivable, net	90.4	70.8
Plant materials and supplies	7.1	6.9
Other	0.6	0.4
Total inventories, at average cost	7.7	7.3
Gain (Loss) on Disposition of Business	12.4	0.0	0.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges	$ 0.2	$ 15.7	$ (0.1)