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Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Segment Reporting Information [Line Items]    
Operating revenues $ 952.8 $ 859.6
Operating income (loss) 154.2 120.7
Income (loss) from continuing operations, net of tax 108.0 72.9
Income (loss) from discontinued operations, net of tax 0 (3.0)
Net income (loss) attributable to Alliant Energy common shareowners 108.0 69.9
IPL [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 528.9 477.9
Operating income (loss) 57.5 41.1
Net income (loss) attributable to Alliant Energy common shareowners 43.4 22.9
WPL [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 410.4 369.8
Operating income (loss) 87.7 72.7
Net income (loss) attributable to Alliant Energy common shareowners 54.8 42.0
Electric [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 675.8 633.2
Operating income (loss) 94.3 73.6
Electric [Member] | IPL [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 374.2 350.2
Operating income (loss) 26.5 17.2
Electric [Member] | WPL [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 301.6 283.0
Operating income (loss) 67.8 56.4
Gas [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 240.7 197.3
Operating income (loss) 43.0 37.3
Gas [Member] | IPL [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 135.7 114.3
Operating income (loss) 23.3 20.7
Gas [Member] | WPL [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 105.0 83.0
Operating income (loss) 19.7 16.6
Other Utilities [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 22.8 17.2
Operating income (loss) 7.9 2.9
Other Utilities [Member] | IPL [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 19.0 13.4
Operating income (loss) 7.7 3.2
Other Utilities [Member] | WPL [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 3.8 3.8
Operating income (loss) 0.2 (0.3)
Utilities Business [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 939.3 847.7
Operating income (loss) 145.2 113.8
Income (loss) from continuing operations, net of tax   64.9
Income (loss) from discontinued operations, net of tax   0
Net income (loss) attributable to Alliant Energy common shareowners 98.2 64.9
Non-Regulated, Parent and Other [Member] | Other Segments [Member]
    
Segment Reporting Information [Line Items]    
Operating revenues 13.5 11.9
Operating income (loss) 9.0 6.9
Income (loss) from continuing operations, net of tax   8.0
Income (loss) from discontinued operations, net of tax   (3.0)
Net income (loss) attributable to Alliant Energy common shareowners $ 9.8 $ 5.0