Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2011	Sep. 30, 2010
Estimated annual effective income tax rate		33.20%	32.60%
Increase in income tax expense	$ 2.6	$ 0.7
Increase in deferred state income taxes		2.0
Increase in deferred tax liabilities		38.8
Certain costs incurred associated with the merger		8.9
Additional tax expense		3.1
DP&L [Member]
Increase in income tax expense	2.6	2.2
Increase in deferred tax liabilities		$ 56.0