Supplemental Financial Information (Supplemental Financial Information) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Unbilled revenue	$ 71.5	$ 72.4
Customer receivables	108.2	113.2
Amounts due from partners in jointly-owned plants	20.4	29.2
Coal sales	3.1	1.0
Other	5.9	4.4
Provision for uncollectible accounts	(1.0)	(1.1)
Total accounts receivable, net	208.1	219.1
Fuel, limestone and emission allowances	85.0	84.2
Plant materials and supplies	40.0	39.8
Other	2.0	1.8
Total inventories, at average cost	127.0	125.8
DP&L [Member]
Unbilled revenue	48.0	49.5
Customer receivables	75.8	85.8
Amounts due from partners in jointly-owned plants	20.4	29.2
Coal sales	3.1	1.0
Other	10.6	13.9
Provision for uncollectible accounts	(0.9)	(0.9)
Total accounts receivable, net	157.0	178.5
Fuel, limestone and emission allowances	84.9	82.8
Plant materials and supplies	38.8	38.6
Other	1.9	1.7
Total inventories, at average cost	$ 125.6	$ 123.1