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Segments Of Business (Schedule Of Segments Of Business) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Segment Reporting Information [Line Items]        
Operating revenues $ 765.3 $ 754.6 $ 1,619.2 $ 1,598.4
Operating income (loss) 149.3 128.6 292.2 274.5
Income from continuing operations, net of tax 94.3 84.4 193.3 182.0
Income (loss) from discontinued operations, net of tax 0.0 (0.5) 1.4 (1.6)
Net income attributable to common shareowners 94.3 83.9 194.7 180.4
IPL [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 420.2 411.0 870.7 869.7
Operating income (loss) 66.3 48.0 115.9 110.0
Net income attributable to common shareowners 42.8 31.9 80.0 77.5
WPL [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 334.8 334.3 727.9 709.9
Operating income (loss) 73.7 75.0 159.7 153.8
Net income attributable to common shareowners 38.1 43.2 83.6 89.7
Electric [Member]        
Segment Reporting Information [Line Items]        
Operating revenues [1] 680.9 675.9 1,358.5 1,344.8
Operating income (loss) [1] 135.6 119.3 242.8 229.1
Electric [Member] | IPL [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 372.4 364.4 728.6 726.0
Operating income (loss) 62.4 44.5 96.2 87.9
Electric [Member] | WPL [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 308.5 311.5 629.9 618.8
Operating income (loss) 73.2 74.8 146.6 141.2
Gas [Member]        
Segment Reporting Information [Line Items]        
Operating revenues [1] 62.6 57.0 216.9 209.2
Operating income (loss) [1] 3.9 1.9 31.9 30.7
Gas [Member] | IPL [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 36.7 34.5 119.8 118.7
Operating income (loss) 2.4 0.9 16.8 16.7
Gas [Member] | WPL [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 25.9 22.5 97.1 90.5
Operating income (loss) 1.5 1.0 15.1 14.0
Other Utility [Member]        
Segment Reporting Information [Line Items]        
Operating revenues [1] 11.5 12.4 23.2 25.6
Operating income (loss) [1] 0.5 1.8 0.9 4.0
Other Utility [Member] | IPL [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 11.1 12.1 22.3 25.0
Operating income (loss) 1.5 2.6 2.9 5.4
Other Utility [Member] | WPL [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 0.4 0.3 0.9 0.6
Operating income (loss) (1.0) (0.8) (2.0) (1.4)
Utility Business [Member]        
Segment Reporting Information [Line Items]        
Operating revenues [1] 755.0 745.3 1,598.6 1,579.6
Operating income (loss) [1] 140.0 123.0 275.6 263.8
Income from continuing operations, net of tax [1]   75.1 177.2 167.2
Income (loss) from discontinued operations, net of tax [1]   0.0 0.0 0.0
Net income attributable to common shareowners [1] 87.6 75.1 177.2 167.2
Non-Regulated [Member] | Other Segments [Member]        
Segment Reporting Information [Line Items]        
Operating revenues 10.3 9.3 20.6 18.8
Operating income (loss) 9.3 5.6 16.6 10.7
Income from continuing operations, net of tax   9.3 16.1 14.8
Income (loss) from discontinued operations, net of tax   (0.5) 1.4 (1.6)
Net income attributable to common shareowners $ 6.7 $ 8.8 $ 17.5 $ 13.2
[1] Alliant Energy’s utility business segments include: a) utility electric operations, which include Alliant Energy’s entire investment in ATC; b) utility gas operations; and c) utility other, which includes steam operations and the unallocated portions of the utility business.