Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Operating revenues	$ 765.7	$ 877.2
Operating income (loss)	95.6	127.3
Income from continuing operations, net of tax	39.3	72.0
Income (loss) from discontinued operations, net of tax	(4.4)	1.5
Net income attributable to Alliant Energy common shareowners	34.9	73.5
IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	398.7	477.5
Operating income (loss)	23.9	47.1
Net income attributable to Alliant Energy common shareowners	(4.7)	21.7
WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	354.5	388.5
Operating income (loss)	64.6	74.8
Net income attributable to Alliant Energy common shareowners	31.1	43.6
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	572.4	620.3
Operating income (loss)	59.1	89.3
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	293.1	330.2
Operating income (loss)	6.7	27.9
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	279.3	290.1
Operating income (loss)	52.4	61.4
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	167.1	229.0
Operating income (loss)	27.7	35.9
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	92.8	131.9
Operating income (loss)	14.2	15.8
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	74.3	97.1
Operating income (loss)	13.5	20.1
Other Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	13.7	16.7
Operating income (loss)	1.7	(3.3)
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	12.8	15.4
Operating income (loss)	3.0	3.4
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.9	1.3
Operating income (loss)	(1.3)	(6.7)
Utility Business [Member]
Segment Reporting Information [Line Items]
Operating revenues	753.2	866.0
Operating income (loss)	88.5	121.9
Income from continuing operations, net of tax	26.4	65.3
Net income attributable to Alliant Energy common shareowners	26.4	65.3
Non-Regulated [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	12.5	11.2
Operating income (loss)	7.1	5.4
Income from continuing operations, net of tax	12.9	6.7
Income (loss) from discontinued operations, net of tax	(4.4)	1.5
Net income attributable to Alliant Energy common shareowners	$ 8.5	$ 8.2