Supplemental Financial Information and Comprehensive Income (Supplemental Financial Information) (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Unbilled revenue	$ 60.0	$ 84.5
Customer receivables	124.7	113.9
Amounts due from partners in jointly-owned plants	10.8	7.0
Coal sales	3.8	4.0
Other	5.2	7.0
Provision for uncollectible accounts	(1.3)	(0.9)
Total accounts receivable, net	203.2	215.5
Fuel, limestone and emission allowances	80.4	73.2
Plant materials and supplies	39.7	38.8
Other	6.7	3.3
Total inventories, at average cost	126.8	115.3
DP&L [Member]
Unbilled revenue	38.0	64.3
Customer receivables	97.4	95.6
Amounts due from partners in jointly-owned plants	10.8	7.0
Coal sales	3.8	4.0
Other	3.5	7.9
Provision for uncollectible accounts	(1.0)	(0.8)
Total accounts receivable, net	152.5	178.0
Fuel, limestone and emission allowances	80.4	73.2
Plant materials and supplies	38.5	37.7
Other	6.7	3.3
Total inventories, at average cost	$ 125.6	$ 114.2