Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Supplemental Financial Information [Line Items]
Unbilled revenue	$ 14.4	$ 43.0
Customer receivables	74.2	73.9
Amounts due from partners in jointly owned stations	17.8	12.7
Other	6.1	6.7
Provision for uncollectible accounts	(1.1)	(1.2)
Total accounts receivable, net	111.4	135.1
Fuel and limestone	16.8	38.9
Plant materials and supplies	10.9	36.6
Other	1.0	1.7
Total inventories, at average cost	28.7	77.2
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Unbilled revenue	14.4	43.0
Customer receivables	68.7	71.2
Amounts due from partners in jointly owned stations	2.3	2.9
Amounts due from affiliates	17.8	12.7
Other	5.8	6.0
Provision for uncollectible accounts	(1.1)	(1.2)
Total accounts receivable, net	107.9	134.6
Fuel and limestone	16.8	38.8
Plant materials and supplies	9.1	35.3
Other	1.0	1.7
Total inventories, at average cost	$ 26.9	$ 75.8