Supplemental Financial Information (Supplemental Financial Information) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Unbilled revenue	$ 62.0	$ 72.4
Customer receivables	131.8	113.2
Amounts due from partners in jointly-owned plants	16.5	29.2
Coal sales	4.5	1.0
Other	19.4	4.4
Provision for uncollectible accounts	(1.2)	(1.1)
Total accounts receivable, net	233.0	219.1
Fuel, limestone and emission allowances	53.6	84.2
Plant materials and supplies	40.7	39.8
Other	2.0	1.8
Total inventories, at average cost	96.3	125.8
DP&L [Member]
Unbilled revenue	34.2	49.5
Customer receivables	89.3	85.8
Amounts due from partners in jointly-owned plants	16.5	29.2
Coal sales	4.5	1.0
Other	28.4	13.9
Provision for uncollectible accounts	(1.1)	(0.9)
Total accounts receivable, net	171.8	178.5
Fuel, limestone and emission allowances	53.7	82.8
Plant materials and supplies	39.5	38.6
Other	1.9	1.7
Total inventories, at average cost	$ 95.1	$ 123.1