Discontinued Operations (Components Of Discontinued Operations In Condensed Consolidated Statements Of Income) (Details) (USD $) In Millions	1 Months Ended		3 Months Ended		9 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Discontinued Operations
Net proceeds	$ 12	$ 5
Operating revenues			1.9		1.1		4.9
Operating expenses			5.9	[1]	0.6	[1]	7.6	[1]
Gain on sale of IEA					(2.5)
Interest expense and other							0.2
Income (loss) before income taxes			(4.0)		3.0		(2.9)
Income tax expense (benefit)			(2.2)		1.7		(0.9)
Income (loss) from discontinued operations, net of tax			(1.8)		1.3		(2.0)
Non cash valuation charge			$ 4.9

[1] In the third quarter of 2010, Alliant Energy recorded a pre-tax, non-cash valuation charge of $4.9 million as a result of declines in the fair value of IEA during the third quarter of 2010. The fair value was estimated using updated market information from bids received from potential buyers for IEA.