Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Supplemental Financial Information [Line Items]
Unbilled revenue	$ 14.7	$ 43.0
Customer receivables	68.4	73.9
Amounts due from partners in jointly owned stations	6.6	12.7
Other	12.7	6.7
Provision for uncollectible accounts	(1.0)	(1.2)
Total accounts receivable, net	101.4	135.1
Fuel and limestone	38.8	38.9
Plant materials and supplies	20.6	36.6
Other	1.5	1.7
Total inventories, at average cost	60.9	77.2
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Unbilled revenue	14.7	43.0
Customer receivables	65.2	71.2
Amounts due from partners in jointly owned stations	6.6	12.7
Amounts due from affiliates	14.8	2.9
Other	10.3	6.0
Provision for uncollectible accounts	(1.0)	(1.2)
Total accounts receivable, net	110.6	134.6
Fuel and limestone	38.8	38.8
Plant materials and supplies	19.2	35.3
Other	1.5	1.7
Total inventories, at average cost	$ 59.5	$ 75.8