Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Supplemental Financial Information [Line Items]
Unbilled revenue	$ 49.3	$ 79.2
Customer receivables	86.8	104.8
Amounts due from partners in jointly owned stations	12.0	14.2
Other	5.6	4.0
Provision for uncollectible accounts	(1.0)	(1.3)
Total accounts receivable, net	152.7	200.9
Fuel and Limestone	60.3	65.3
Plant materials and supplies	34.7	33.5
Other	2.0	1.4
Total inventories, at average cost	97.0	100.2
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Unbilled revenue	34.8	49.0
Customer receivables	63.6	68.7
Amounts due from partners in jointly owned stations	12.0	14.2
Other	7.6	21.7
Provision for uncollectible accounts	(0.9)	(0.9)
Total accounts receivable, net	117.1	152.7
Fuel and Limestone	60.4	65.3
Plant materials and supplies	33.6	32.3
Other	1.9	1.4
Total inventories, at average cost	$ 95.9	$ 99.0