Segments Of Business (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Operating revenues	$ 1,021.6	$ 951.7	$ 2,786.1	$ 2,583.5
Operating income (loss)	185.1	246.6	384.9	456.3
Income (loss) from continuing operations, net of tax	122.0	150.9	245.3	242.3
Income (loss) from discontinued operations, net of tax		(1.8)	1.3	(2.0)
Net income (loss) attributable to Alliant Energy common shareowners	122.0	149.1	246.6	240.3
RMT [Member] | Non-Regulated Business [Member]
Operating revenues	150.7	35.9	325.5	97.9
Operating income (loss)	(24.3)	(1.0)	(22.4)	(4.7)
Income (loss) from continuing operations, net of tax		(0.6)	(13.9)	(2.9)
Net income (loss) attributable to Alliant Energy common shareowners	(14.9)	(0.6)	(13.9)	(2.9)
Non-Regulated Business [Member] | Other Segments [Member]
Operating revenues	11.8	11.4	34.6	31.6
Operating income (loss)	6.0	5.2	18.0	13.1
Income (loss) from continuing operations, net of tax		5.5	23.2	10.5
Income (loss) from discontinued operations, net of tax		(1.8)	1.3	(2.0)
Net income (loss) attributable to Alliant Energy common shareowners	(12.0)	3.7	24.5	8.5
Utility Business [Member]
Operating revenues	859.1	904.4	2,426.0	2,454.0
Operating income (loss)	203.4	242.4	389.3	447.9
Income (loss) from continuing operations, net of tax		146.0	236.0	234.7
Net income (loss) attributable to Alliant Energy common shareowners	148.9	146.0	236.0	234.7
IPL [Member] | Electric [Member]
Operating revenues	443.2	492.4	1,097.3	1,146.8
Operating income (loss)	117.5	151.9	162.3	213.7
WPL [Member] | Electric [Member]
Operating revenues	353.7	349.6	940.4	927.4
Operating income (loss)	88.6	89.2	195.3	194.6
Electric [Member]
Operating revenues	796.9	842.0	2,037.7	2,074.2
Operating income (loss)	206.1	241.1	357.6	408.3
IPL [Member] | Gas [Member]
Operating revenues	27.5	28.1	198.1	187.9
Operating income (loss)	(1.3)	(0.7)	15.1	18.1
WPL [Member] | Gas [Member]
Operating revenues	18.9	18.4	144.4	143.0
Operating income (loss)	(1.5)	(2.2)	18.9	15.7
Gas [Member]
Operating revenues	46.4	46.5	342.5	330.9
Operating income (loss)	(2.8)	(2.9)	34.0	33.8
Other Utility [Member]
Operating revenues	15.8	15.9	45.8	48.9
Operating income (loss)	0.1	4.2	(2.3)	5.8
IPL [Member] | Other Segments [Member]
Operating revenues	13.8	14.3	40.7	43.9
Operating income (loss)	0.3	4.5	4.7	7.4
WPL [Member] | Other Segments [Member]
Operating revenues	2.0	1.6	5.1	5.0
Operating income (loss)	(0.2)	(0.3)	(7.0)	(1.6)
IPL [Member]
Operating revenues	484.5	534.8	1,336.1	1,378.6
Operating income (loss)	116.5	155.7	182.1	239.2
Net income (loss) attributable to Alliant Energy common shareowners	98.3	96.8	117.7	120.0
WPL [Member]
Operating revenues	374.6	369.6	1,089.9	1,075.4
Operating income (loss)	86.9	86.7	207.2	208.7
Net income (loss) attributable to Alliant Energy common shareowners	$ 50.6	$ 49.2	$ 118.3	$ 114.7