Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Estimated annual effective income tax rate	25.60%	33.70%
Increase in income tax expense		$ 1.8
Increase in deferred state income taxes		2.0
Decrease in other estimated tax liabilities		0.2
Increase (decrease) in deferred tax liabilities	6.3
Effective income tax rates	26.00%	36.30%
DP&L [Member]
Estimated annual effective income tax rate	31.10%	33.70%
Increase (decrease) in deferred tax liabilities	$ 3.7
Effective income tax rates	31.30%	34.00%