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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

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