Supplemental Financial Information and Comprehensive Income (Supplemental Financial Information) (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Unbilled revenue	$ 70.6	$ 84.5
Customer receivables	116.7	113.9
Amounts due from partners in jointly-owned plants	8.7	7.0
Coal sales	1.2	4.0
Other	5.8	7.0
Provision for uncollectible accounts	(1.2)	(0.9)
Total accounts receivable, net	201.8	215.5
Fuel, limestone and emission allowances	70.9	73.2
Plant materials and supplies	39.4	38.8
Other	5.3	3.3
Total inventories, at average cost	115.6	115.3
DP&L [Member]
Unbilled revenue	46.0	64.3
Customer receivables	90.8	95.6
Amounts due from partners in jointly-owned plants	8.7	7.0
Coal sales	1.2	4.0
Other	4.7	7.9
Provision for uncollectible accounts	(1.0)	(0.8)
Total accounts receivable, net	150.4	178.0
Fuel, limestone and emission allowances	70.6	73.2
Plant materials and supplies	38.2	37.7
Other	5.3	3.3
Total inventories, at average cost	$ 114.1	$ 114.2