Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Supplemental Financial Information [Line Items]
Accounts receivable, net		$ 31.0
Unbilled revenue	$ 29.4	43.3
Customer receivables	81.6	56.4
Amounts due from partners in jointly owned stations	12.8	16.0
Other	12.3	6.0
Provision for uncollectible accounts	(1.2)	(0.8)
Total accounts receivable, net	134.9	120.9
Fuel and limestone	42.0	72.2
Plant materials and supplies	36.1	34.9
Other	1.9	2.0
Total inventories, at average cost	80.0	109.1
THE DAYTON POWER AND LIGHT COMPANY [Member]
Supplemental Financial Information [Line Items]
Unbilled revenue	29.4	43.3
Customer receivables	79.2	54.1
Amounts due from partners in jointly owned stations	12.8	16.0
Other	16.1	6.9
Provision for uncollectible accounts	(1.2)	(0.8)
Total accounts receivable, net	136.3	119.5
Fuel and limestone	42.0	72.2
Plant materials and supplies	34.7	33.7
Other	1.9	2.1
Total inventories, at average cost	$ 78.6	$ 108.0