Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Supplemental Financial Information [Line Items]
Unbilled revenue	$ 43.0	$ 43.3
Customer receivables	73.9	56.4
Amounts due from partners in jointly owned stations	12.7	16.0
Other	6.7	6.0
Provision for uncollectible accounts	(1.2)	(0.8)
Total accounts receivable, net	135.1	120.9
Fuel and Limestone	38.9	72.2
Plant materials and supplies	36.6	34.9
Other	1.7	2.0
Total inventories, at average cost	77.2	109.1
Assets held for sale - current	0.0	62.2
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Unbilled revenue	43.0	43.3
Customer receivables	71.2	54.1
Amounts due from partners in jointly owned stations	12.7	16.0
Other	8.9	6.9
Provision for uncollectible accounts	(1.2)	(0.8)
Total accounts receivable, net	134.6	119.5
Fuel and Limestone	38.8	72.2
Plant materials and supplies	35.3	33.7
Other	1.7	2.1
Total inventories, at average cost	$ 75.8	108.0
Accounts Receivable [Member]
Supplemental Financial Information [Line Items]
Assets held for sale - current		$ 31.0