Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Estimated annual effective income tax rate	33.70%	33.50%	33.70%	33.50%
Increase in income tax expense	$ 0.1		$ 1.9
Increase in deferred state income taxes			2.0
Decrease in other estimated tax liabilities			0.1
Increase in deferred tax liabilities			32.4
Certain costs incurred associated with the merger			5.2
Additional tax expense			1.8
DP&L [Member]
Increase in income tax expense	0.1		0.2
Increase in deferred tax liabilities			$ 37.8