Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Operating revenues	$ 887.6	$ 870.9	$ 2,343.6	$ 2,460.6
Operating income (loss)	213.7	209.4	418.1	407.3
Income (loss) from continuing operations, net of tax	149.0	136.9	253.8	259.2
Income (loss) from discontinued operations, net of tax	1.7	(14.9)	(2.3)	(12.6)
Net income (loss) attributable to Alliant Energy common shareowners	150.7	122.0	251.5	246.6
Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	887.6	870.9	2,343.6	2,460.6
Operating income (loss)	213.7	209.4	418.1	407.3
Income (loss) from continuing operations, net of tax	149.0	136.9	253.8	259.2
Income (loss) from discontinued operations, net of tax	1.7	(14.9)	(2.3)	(12.6)
Net income (loss) attributable to Alliant Energy common shareowners	150.7	122.0	251.5	246.6
IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	497.7	484.5	1,257.1	1,336.1
Operating income (loss)	112.0	116.5	171.3	182.1
Net income (loss) attributable to Alliant Energy common shareowners	103.3	98.3	115.2	117.7
WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	376.6	374.6	1,046.8	1,089.9
Operating income (loss)	93.4	86.9	219.5	207.2
Net income (loss) attributable to Alliant Energy common shareowners	55.9	50.6	122.2	118.3
Electric [Member] | Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	815.3	796.9	2,000.3	2,037.7
Operating income (loss)	203.1	206.1	354.0	357.6
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	456.6	443.2	1,070.7	1,097.3
Operating income (loss)	109.5	117.5	149.3	162.3
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	358.7	353.7	929.6	940.4
Operating income (loss)	93.6	88.6	204.7	195.3
Gas [Member] | Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	46.8	46.4	263.9	342.5
Operating income (loss)	0.8	(2.8)	32.1	34.0
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	29.6	27.5	149.2	198.1
Operating income (loss)	0.6	(1.3)	15.6	15.1
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	17.2	18.9	114.7	144.4
Operating income (loss)	0.2	(1.5)	16.5	18.9
Other Utility [Member] | Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	12.2	15.8	39.7	45.8
Operating income (loss)	1.5	0.1	4.7	(2.3)
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	11.5	13.8	37.2	40.7
Operating income (loss)	1.9	0.3	6.4	4.7
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.7	2.0	2.5	5.1
Operating income (loss)	(0.4)	(0.2)	(1.7)	(7.0)
Utility Business [Member] | Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	874.3	859.1	2,303.9	2,426.0
Operating income (loss)	205.4	203.4	390.8	389.3
Income (loss) from continuing operations, net of tax	159.2	148.9	237.4	236.0
Income (loss) from discontinued operations, net of tax	0	0	0	0
Net income (loss) attributable to Alliant Energy common shareowners	159.2	148.9	237.4	236.0
Non-Regulated [Member] | Other Segments [Member] | Alliant Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	13.3	11.8	39.7	34.6
Operating income (loss)	8.3	6.0	27.3	18.0
Income (loss) from continuing operations, net of tax	(10.2)	(12.0)	16.4	23.2
Income (loss) from discontinued operations, net of tax	1.7	(14.9)	(2.3)	(12.6)
Net income (loss) attributable to Alliant Energy common shareowners	$ (8.5)	$ (26.9)	$ 14.1	$ 10.6