Supplemental Financial Information (Supplemental Financial Information) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Unbilled revenue	$ 70.2	$ 75.2
Customer receivables	94.7	98.2
Amounts due from partners in jointly owned stations	8.3	19.7
Coal sales	0.8	1.6
Other	4.7	14.6
Provision for uncollectible accounts	(1.2)	(1.1)
Total accounts receivable, net	177.5	208.2
Fuel and Limestone	60.9	67.3
Plant materials and supplies	38.0	41.0
Other	2.1	1.8
Total inventories, at average cost	101.0	110.1
DP&L [Member]
Unbilled revenue	41.7	48.1
Customer receivables	55.3	62.0
Amounts due from partners in jointly owned stations	8.3	19.7
Coal sales	0.8	1.6
Other	26.5	29.5
Provision for uncollectible accounts	(0.8)	(0.9)
Total accounts receivable, net	131.8	160.0
Fuel and Limestone	60.9	67.3
Plant materials and supplies	36.8	39.8
Other	2.1	1.8
Total inventories, at average cost	$ 99.8	$ 108.9