Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Operating revenues	$ 690.3	$ 712.5	$ 1,456.0	$ 1,589.7
Operating income (loss)	108.8	70.6	204.4	197.9
Income from continuing operations, net of tax	65.5	50.3	104.8	122.3
Income (loss) from discontinued operations, net of tax	0.4	0.8	(4.0)	2.3
Net income attributable to Alliant Energy common shareowners	65.9	51.1	100.8	124.6
Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	690.3	712.5	1,456.0	1,589.7
Operating income (loss)	108.8	70.6	204.4	197.9
Income from continuing operations, net of tax	65.5	50.3	104.8	122.3
Income (loss) from discontinued operations, net of tax	0.4	0.8	(4.0)	2.3
Net income attributable to Alliant Energy common shareowners	65.9	51.1	100.8	124.6
IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	360.7	374.1	759.4	851.6
Operating income (loss)	35.4	18.5	59.3	65.6
Net income attributable to Alliant Energy common shareowners	16.6	(2.3)	11.9	19.4
WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	315.7	326.8	670.2	715.3
Operating income (loss)	61.5	45.5	126.1	120.3
Net income attributable to Alliant Energy common shareowners	35.2	24.1	66.3	67.7
Electric [Member] | Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	612.6	620.5	1,185.0	1,240.8
Operating income (loss)	91.8	62.2	150.9	151.5
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	321.0	323.9	614.1	654.1
Operating income (loss)	33.1	16.9	39.8	44.8
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	291.6	296.6	570.9	586.7
Operating income (loss)	58.7	45.3	111.1	106.7
Gas [Member] | Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	50.0	67.1	217.1	296.1
Operating income (loss)	3.6	0.9	31.3	36.8
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	26.8	38.7	119.6	170.6
Operating income (loss)	0.8	0.6	15.0	16.4
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	23.2	28.4	97.5	125.5
Operating income (loss)	2.8	0.3	16.3	20.4
Other Utility [Member] | Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	13.8	13.3	27.5	30.0
Operating income (loss)	1.5	0.9	3.2	(2.4)
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	12.9	11.5	25.7	26.9
Operating income (loss)	1.5	1.0	4.5	4.4
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.9	1.8	1.8	3.1
Operating income (loss)	0	(0.1)	(1.3)	(6.8)
Utility Business [Member] | Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	676.4	700.9	1,429.6	1,566.9
Operating income (loss)	96.9	64.0	185.4	185.9
Income from continuing operations, net of tax	51.8	21.8	78.2	87.1
Income (loss) from discontinued operations, net of tax	0	0	0	0
Net income attributable to Alliant Energy common shareowners	51.8	21.8	78.2	87.1
Non-Regulated [Member] | Other Segments [Member] | Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	13.9	11.6	26.4	22.8
Operating income (loss)	11.9	6.6	19.0	12.0
Income from continuing operations, net of tax	13.7	28.5	26.6	35.2
Income (loss) from discontinued operations, net of tax	0.4	0.8	(4.0)	2.3
Net income attributable to Alliant Energy common shareowners	$ 14.1	$ 29.3	$ 22.6	$ 37.5