Supplemental Financial Information (Supplemental Financial Information) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Unbilled revenue	$ 58.3	$ 75.2
Customer receivables	107.8	98.2
Amounts due from partners in jointly owned stations	10.2	19.7
Coal sales		1.6
Other	5.1	14.6
Provision for uncollectible accounts	(1.2)	(1.1)
Total accounts receivable, net	180.2	208.2
Fuel and Limestone	51.3	67.3
Plant materials and supplies	38.3	41.0
Other	2.0	1.8
Total inventories, at average cost	91.6	110.1
DP&L [Member]
Unbilled revenue	33.2	48.1
Customer receivables	62.1	62.0
Amounts due from partners in jointly owned stations	10.2	19.7
Coal sales		1.6
Other	27.7	29.5
Provision for uncollectible accounts	(1.0)	(0.9)
Total accounts receivable, net	132.2	160.0
Fuel and Limestone	51.3	67.3
Plant materials and supplies	37.1	39.8
Other	2.0	1.8
Total inventories, at average cost	$ 90.4	$ 108.9