Supplemental Financial Information (Supplemental Financial Information) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Unbilled revenue	$ 58.6	$ 79.2
Customer receivables	109.3	104.8
Amounts due from partners in jointly owned stations	8.3	14.2
Other	5.8	4.0
Provision for uncollectible accounts	(1.4)	(1.3)
Total accounts receivable, net	180.6	200.9
Fuel and Limestone	58.9	65.3
Plant materials and supplies	34.3	33.5
Other	1.7	1.4
Total inventories, at average cost	94.9	100.2
DP&L [Member]
Unbilled revenue	40.6	49.0
Customer receivables	81.7	68.7
Amounts due from partners in jointly owned stations	8.3	14.2
Other	25.5	21.7
Provision for uncollectible accounts	(1.1)	(0.9)
Total accounts receivable, net	155.0	152.7
Fuel and Limestone	58.8	65.3
Plant materials and supplies	33.1	32.3
Other	1.8	1.4
Total inventories, at average cost	$ 93.7	$ 99.0