Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 859.6	$ 765.7
Operating income (loss)	120.7	95.6
Income (loss) from continuing operations, net of tax	72.9	39.3
Income (loss) from discontinued operations, net of tax	(3.0)	(4.4)
Net income (loss) attributable to Alliant Energy common shareowners	69.9	34.9
IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	477.9	398.7
Operating income (loss)	41.1	23.9
Net income (loss) attributable to Alliant Energy common shareowners	22.9	(4.7)
WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	369.8	354.5
Operating income (loss)	72.7	64.6
Net income (loss) attributable to Alliant Energy common shareowners	42.0	31.1
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	633.2	572.4
Operating income (loss)	73.6	59.1
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	350.2	293.1
Operating income (loss)	17.2	6.7
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	283.0	279.3
Operating income (loss)	56.4	52.4
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	197.3	167.1
Operating income (loss)	37.3	27.7
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	114.3	92.8
Operating income (loss)	20.7	14.2
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	83.0	74.3
Operating income (loss)	16.6	13.5
Other Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	17.2	13.7
Operating income (loss)	2.9	1.7
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	13.4	12.8
Operating income (loss)	3.2	3.0
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	3.8	0.9
Operating income (loss)	(0.3)	(1.3)
Utility Business [Member]
Segment Reporting Information [Line Items]
Operating revenues	847.7	753.2
Operating income (loss)	113.8	88.5
Income (loss) from continuing operations, net of tax	64.9	26.4
Income (loss) from discontinued operations, net of tax	0	0
Net income (loss) attributable to Alliant Energy common shareowners	64.9	26.4
Non-Regulated [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	11.9	12.5
Operating income (loss)	6.9	7.1
Income (loss) from continuing operations, net of tax	8.0	12.9
Income (loss) from discontinued operations, net of tax	(3.0)	(4.4)
Net income (loss) attributable to Alliant Energy common shareowners	$ 5.0	$ 8.5