Supplemental Financial Information (Supplemental Financial Information) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Unbilled revenue	$ 75.2	$ 72.4
Customer receivables	98.2	113.2
Amounts due from partners in jointly owned stations	19.7	29.2
Coal sales	1.6	1.0
Other	14.6	4.4
Provision for uncollectible accounts	(1.1)	(1.1)
Total accounts receivable, net	208.2	219.1
Fuel and Limestone	67.3	84.2
Plant materials and supplies	41.0	39.8
Other	1.8	1.8
Total inventories, at average cost	110.1	125.8
DP&L [Member]
Unbilled revenue	48.1	49.5
Customer receivables	62.0	85.8
Amounts due from partners in jointly owned stations	19.7	29.2
Coal sales	1.6	1.0
Other	29.5	13.9
Provision for uncollectible accounts	(0.9)	(0.9)
Total accounts receivable, net	160.0	178.5
Fuel and Limestone	67.3	82.8
Plant materials and supplies	39.8	38.6
Other	1.8	1.7
Total inventories, at average cost	$ 108.9	$ 123.1