Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Financial Information [Line Items]
Unbilled revenue	$ 18.0	$ 43.0
Customer receivables	57.8	73.9
Amounts due from partners in jointly owned stations	19.1	12.7
Other	4.9	6.7
Provision for uncollectible accounts	(1.1)	(1.2)
Total accounts receivable, net	98.7	135.1
Fuel and Limestone	15.5	38.9
Plant materials and supplies	8.5	36.6
Other	0.5	1.7
Total inventories, at average cost	24.5	77.2
Assets held for sale - current	250.3	0.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges	2.2	(49.2)	$ 0.4
Other Operating Income (Expense), Net	(6.6)	(0.1)	0.4
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Unbilled revenue	18.0	43.0
Customer receivables	44.2	45.9
Amounts due from partners in jointly owned stations	5.0	4.0
Other	4.7	8.1
Provision for uncollectible accounts	(1.1)	(1.2)
Total accounts receivable, net	70.8	99.8
Plant materials and supplies	6.9	6.9
Other	0.4	2.4
Total inventories, at average cost	7.3	9.3
Assets held for sale - current	0.0	324.6
Gain (Loss) on Sale of Assets and Asset Impairment Charges	15.7	0.0	0.4
Other Operating Income (Expense), Net	(0.5)	(0.4)	0.1
Other Operating Income (Expense) [Member]
Supplemental Financial Information [Line Items]
Impaired Assets to be Disposed of by Method Other than Sale, Amount of Impairment Loss	16.2	0.0	0.0
Gain (Loss) on Disposition of Business	(14.0)	0.0	0.0
Insurance Recoveries	(8.7)	(0.7)	0.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges	0.0	(0.1)	0.4
Other Cost and Expense, Operating	(0.1)	0.7	0.0
Other Operating Income (Expense), Net	$ (6.6)	$ (0.1)	$ 0.4