Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 866.6	$ 887.6	$ 2,444.2	$ 2,343.6
Operating income (loss)	201.4	213.7	425.3	418.1
Income (loss) from continuing operations, net of tax	158.9	149.0	297.7	253.8
Income (loss) from discontinued operations, net of tax	(1.3)	1.7	(4.9)	(2.3)
Net income (loss) attributable to Alliant Energy common shareowners	157.6	150.7	292.8	251.5
IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	494.4	497.7	1,355.7	1,257.1
Operating income (loss)	100.0	112.0	175.8	171.3
Net income (loss) attributable to Alliant Energy common shareowners	110.0	103.3	155.1	115.2
WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	360.9	376.6	1,050.6	1,046.8
Operating income (loss)	95.9	93.4	228.3	219.5
Net income (loss) attributable to Alliant Energy common shareowners	61.3	55.9	137.7	122.2
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	798.1	815.3	2,043.4	2,000.3
Operating income (loss)	199.6	203.1	359.1	354.0
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	457.6	456.6	1,137.4	1,070.7
Operating income (loss)	99.6	109.5	146.2	149.3
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	340.5	358.7	906.0	929.6
Operating income (loss)	100.0	93.6	212.9	204.7
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	39.8	46.8	310.5	263.9
Operating income (loss)	(3.4)	0.8	39.5	32.1
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	24.6	29.6	180.9	149.2
Operating income (loss)	(0.7)	0.6	22.6	15.6
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	15.2	17.2	129.6	114.7
Operating income (loss)	(2.7)	0.2	16.9	16.5
Other Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	17.4	12.2	52.4	39.7
Operating income (loss)	(0.3)	1.5	5.5	4.7
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	12.2	11.5	37.4	37.2
Operating income (loss)	1.1	1.9	7.0	6.4
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	5.2	0.7	15.0	2.5
Operating income (loss)	(1.4)	(0.4)	(1.5)	(1.7)
Utility Business [Member]
Segment Reporting Information [Line Items]
Operating revenues	855.3	874.3	2,406.3	2,303.9
Operating income (loss)	195.9	205.4	404.1	390.8
Income (loss) from continuing operations, net of tax	171.3	159.2	292.8	237.4
Income (loss) from discontinued operations, net of tax	0	0	0	0
Net income (loss) attributable to Alliant Energy common shareowners	171.3	159.2	292.8	237.4
Non-Regulated, Parent and Other [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	11.3	13.3	37.9	39.7
Operating income (loss)	5.5	8.3	21.2	27.3
Income (loss) from continuing operations, net of tax	(12.4)	(10.2)	4.9	16.4
Income (loss) from discontinued operations, net of tax	(1.3)	1.7	(4.9)	(2.3)
Net income (loss) attributable to Alliant Energy common shareowners	$ (13.7)	$ (8.5)	$ 0	$ 14.1