Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Supplemental Financial Information [Line Items]
Unbilled revenue	$ 15.5	$ 43.0
Customer receivables	62.1	73.9
Amounts due from partners in jointly owned stations	3.6	12.7
Other	19.4	6.7
Provision for uncollectible accounts	(1.0)	(1.2)
Total accounts receivable, net	99.6	135.1
Fuel and limestone	17.6	38.9
Plant materials and supplies	10.4	36.6
Other	1.3	1.7
Total inventories, at average cost	29.3	77.2
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Unbilled revenue	15.5	43.0
Customer receivables	55.9	71.2
Amounts due from partners in jointly owned stations	3.6	12.7
Amounts due from affiliates	2.1	2.9
Other	19.0	6.0
Provision for uncollectible accounts	(1.0)	(1.2)
Total accounts receivable, net	95.1	134.6
Fuel and limestone	17.6	38.8
Plant materials and supplies	8.8	35.3
Other	1.3	1.7
Total inventories, at average cost	$ 27.7	$ 75.8