Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Supplemental Financial Information [Line Items]
Unbilled revenue	$ 61.9	$ 79.2
Customer receivables	92.0	104.8
Amounts due from partners in jointly owned stations	20.0	14.2
Other	3.2	4.0
Provision for uncollectible accounts	(1.4)	(1.3)
Total accounts receivable, net	175.7	200.9
Fuel and Limestone	67.8	65.3
Plant materials and supplies	34.8	33.5
Other	2.1	1.4
Total inventories, at average cost	104.7	100.2
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Unbilled revenue	43.5	49.0
Customer receivables	69.5	68.7
Amounts due from partners in jointly owned stations	20.0	14.2
Other	5.6	21.7
Provision for uncollectible accounts	(1.0)	(0.9)
Total accounts receivable, net	137.6	152.7
Fuel and Limestone	67.8	65.3
Plant materials and supplies	33.7	32.3
Other	2.0	1.4
Total inventories, at average cost	$ 103.5	$ 99.0