Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Supplemental Financial Information [Line Items]
Gain (Loss) on Disposition of Business	$ (13.6)	$ 0.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges	(0.6)	(19.4)
Unbilled revenue	12.1		$ 18.0
Customer receivables	65.7		57.8
Amounts due from partners in jointly owned stations	13.6		19.1
Other	10.9		4.9
Provision for uncollectible accounts	(1.1)		(1.1)
Total accounts receivable, net	101.2		98.7
Fuel and limestone	13.3		15.5
Plant materials and supplies	8.4		8.5
Other	0.6		0.5
Total inventories, at average cost	22.3		24.5
Other Operating Income (Expense), Net	(14.9)	(18.2)
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Gain (Loss) on Disposition of Business	(12.4)	0.0
Gain (Loss) on Sale of Assets and Asset Impairment Charges	0.0	(19.4)
Unbilled revenue	12.1		18.0
Customer receivables	49.8		44.2
Amounts due from partners in jointly owned stations	3.8		0.0
Amounts due from affiliates	2.1		5.0
Other	6.3		4.7
Provision for uncollectible accounts	(1.1)		(1.1)
Total accounts receivable, net	73.0		70.8
Plant materials and supplies	7.0		6.9
Other	0.6		0.4
Total inventories, at average cost	7.6		$ 7.3
Other Operating Income (Expense), Net	(12.4)	0.0
Other Operating Income (Expense) [Member]
Supplemental Financial Information [Line Items]
Gain (Loss) on Disposition of Business	13.6	0.0
Insurance Recoveries	0.0	(1.2)
Gain (Loss) on Sale of Assets and Asset Impairment Charges	0.6	19.4
Other Cost and Expense, Operating	0.7	0.0
Other Operating Income (Expense), Net	14.9	$ 18.2
AES Ohio Generation peakers [Member]
Supplemental Financial Information [Line Items]
Gain (Loss) on Disposition of Business	$ 1.9