Supplemental Financial Information and Comprehensive Income (Comprehensive Income) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Net income	$ 67.1	$ 86.4	$ 142.3	$ 218.8
Net change in unrealized gains (losses) on financial instruments, net of income tax expense	(0.3)	0.3	(0.3)	0.2	0.6
Net change in deferred gains (losses) on cash flow hedges, net of income tax expense	(46.6)	(9.1)	(55.4)	(14.7)
Net change in unrealized gains (losses) on pension and postretirement benefits, net of income tax expense	0.9	0.8	2.5	3.4
Comprehensive income	21.1	78.4	89.1	207.7
Income tax benefit (expense) on unrealized gains (losses) on financial instruments	0.2	0.1	(0.2)	0.1
Income tax benefit (expense) on unrealized gains (losses) on cash flow hedges	(24.9)	(3.6)	(28.8)	(6.0)
Income tax benefit (expense) on unrealized gains (losses) on pension and postretirement benefits	0.1	0.4	0.7	0.3
DP&L [Member]
Net income	63.9	83.2	147.4	214.7
Net change in unrealized gains (losses) on financial instruments, net of income tax expense	(0.4)	1.8	2.3	(0.9)	8.5
Net change in deferred gains (losses) on cash flow hedges, net of income tax expense	1.3	(0.2)	0.2	(0.1)
Net change in unrealized gains (losses) on pension and postretirement benefits, net of income tax expense	1.0	0.8	2.6	3.4
Comprehensive income	65.8	85.6	152.5	217.1
Income tax benefit (expense) on unrealized gains (losses) on financial instruments	0.1	(1.0)	1.3	(0.5)
Income tax benefit (expense) on unrealized gains (losses) on cash flow hedges	0.9	(0.2)	1.1	1.0
Income tax benefit (expense) on unrealized gains (losses) on pension and postretirement benefits	$ 0.1	$ 0.4	$ 0.7	$ 0.3