Supplemental Financial Information (Supplemental Financial Information) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Unbilled revenue	$ 65.3	$ 75.2
Customer receivables	107.5	98.2
Amounts due from partners in jointly owned stations	15.3	19.7
Coal sales		1.6
Other	3.1	14.6
Provision for uncollectible accounts	(1.1)	(1.1)
Total accounts receivable, net	190.1	208.2
Fuel and Limestone	72.0	67.3
Plant materials and supplies	40.6	41.0
Other	1.9	1.8
Total inventories, at average cost	114.5	110.1
DP&L [Member]
Unbilled revenue	41.6	48.1
Customer receivables	68.3	62.0
Amounts due from partners in jointly owned stations	15.3	19.7
Coal sales		1.6
Other	24.0	29.5
Provision for uncollectible accounts	(1.0)	(0.9)
Total accounts receivable, net	148.2	160.0
Fuel and Limestone	71.9	67.3
Plant materials and supplies	39.4	39.8
Other	1.9	1.8
Total inventories, at average cost	$ 113.2	$ 108.9