Supplemental Financial Information (Supplemental Financial Information) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Unbilled revenue	$ 61.0	$ 72.4
Customer receivables	115.8	113.2
Amounts due from partners in jointly-owned plants	31.2	29.2
Coal sales	9.2	1.0
Other	8.3	4.4
Provision for uncollectible accounts	(1.1)	(1.1)
Total accounts receivable, net	224.4	219.1
Fuel, limestone and emission allowances	84.0	84.2
Plant materials and supplies	40.6	39.8
Other	1.8	1.8
Total inventories, at average cost	126.4	125.8
DP&L [Member]
Unbilled revenue	39.3	49.5
Customer receivables	89.2	85.8
Amounts due from partners in jointly-owned plants	31.2	29.2
Coal sales	9.2	1.0
Other	13.2	13.9
Provision for uncollectible accounts	(1.0)	(0.9)
Total accounts receivable, net	181.1	178.5
Fuel, limestone and emission allowances	83.9	82.8
Plant materials and supplies	39.4	38.6
Other	1.8	1.7
Total inventories, at average cost	$ 125.1	$ 123.1