Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Operating revenues	$ 718.0	$ 690.3	$ 1,577.6	$ 1,456.0
Operating income (loss)	103.2	108.8	223.9	204.4
Income (loss) from continuing operations, net of tax	65.9	65.5	138.8	104.8
Income (loss) from discontinued operations, net of tax	(0.6)	0.4	(3.6)	(4.0)
Net income (loss) attributable to Alliant Energy common shareowners	65.3	65.9	135.2	100.8
IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	383.4	360.7	861.3	759.4
Operating income (loss)	34.7	35.4	75.8	59.3
Net income (loss) attributable to Alliant Energy common shareowners	22.2	16.6	45.1	11.9
WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	319.9	315.7	689.7	670.2
Operating income (loss)	59.7	61.5	132.4	126.1
Net income (loss) attributable to Alliant Energy common shareowners	34.4	35.2	76.4	66.3
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues	612.1	612.6	1,245.3	1,185.0
Operating income (loss)	85.9	91.8	159.5	150.9
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	329.6	321.0	679.8	614.1
Operating income (loss)	29.4	33.1	46.6	39.8
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	282.5	291.6	565.5	570.9
Operating income (loss)	56.5	58.7	112.9	111.1
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues	73.4	50.0	270.7	217.1
Operating income (loss)	5.6	3.6	42.9	31.3
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	42.0	26.8	156.3	119.6
Operating income (loss)	2.6	0.8	23.3	15.0
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	31.4	23.2	114.4	97.5
Operating income (loss)	3.0	2.8	19.6	16.3
Other Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	17.8	13.8	35.0	27.5
Operating income (loss)	2.9	1.5	5.8	3.2
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	11.8	12.9	25.2	25.7
Operating income (loss)	2.7	1.5	5.9	4.5
Other Utility [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	6.0	0.9	9.8	1.8
Operating income (loss)	0.2	0	(0.1)	(1.3)
Utility Business [Member]
Segment Reporting Information [Line Items]
Operating revenues	703.3	676.4	1,551.0	1,429.6
Operating income (loss)	94.4	96.9	208.2	185.4
Income (loss) from continuing operations, net of tax	56.6	51.8	121.5	78.2
Income (loss) from discontinued operations, net of tax	0	0	0	0
Net income (loss) attributable to Alliant Energy common shareowners	56.6	51.8	121.5	78.2
Non-Regulated [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	14.7	13.9	26.6	26.4
Operating income (loss)	8.8	11.9	15.7	19.0
Income (loss) from continuing operations, net of tax	9.3	13.7	17.3	26.6
Income (loss) from discontinued operations, net of tax	(0.6)	0.4	(3.6)	(4.0)
Net income (loss) attributable to Alliant Energy common shareowners	$ 8.7	$ 14.1	$ 13.7	$ 22.6