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Common Equity (Changes in Shareowners' Equity) (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance     $ 7,004  
Beginning balance $ 7,145 $ 6,791 7,004 $ 6,777
Net income attributable to Alliant Energy common shareowners 281 295 668 540
Net income     668 540
Common stock dividends (130) (123) (391) (369)
Shareowner Direct Plan issuances 6 6 18 18
Equity-based compensation plans and other 5 3 9 4
Other comprehensive income (loss), net of tax   (4) (1) (2)
Ending balance 7,307   7,307  
Ending balance $ 7,307 $ 6,968 $ 7,307 $ 6,968
Common stock dividends (in dollars per share) $ 0.5075 $ 0.48 $ 1.5225 $ 1.44
Common Stock [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance $ 3 $ 3 $ 3 $ 3
Ending balance 3 3 3 3
Additional Paid-in Capital [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance 3,075 3,042 3,060 3,030
Shareowner Direct Plan issuances 6 6 18 18
Equity-based compensation plans and other 6 4 9 4
Ending balance 3,087 3,052 3,087 3,052
Retained Earnings [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance 4,080 3,755 3,954 3,756
Net income attributable to Alliant Energy common shareowners 281 295 668 540
Common stock dividends (130) (123) (391) (369)
Ending balance 4,231 3,927 4,231 3,927
Accumulated Other Comprehensive Income (Loss) [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance 0 3 1 1
Other comprehensive income (loss), net of tax   (4) (1) (2)
Ending balance 0 (1) 0 (1)
Shares in Deferred Compensation Trust [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance (13) (12) (14) (13)
Equity-based compensation plans and other (1) (1)    
Ending balance (14) (13) (14) (13)
IPL [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance     4,461  
Beginning balance 4,635 4,080 4,461 3,974
Net income 165 190 374 272
Common stock dividends (90) (50) (270) (151)
Capital contributions from parent 140 200 285 325
Ending balance 4,850   4,850  
Ending balance 4,850 4,420 4,850 4,420
IPL [Member] | Common Stock [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance 33 33 33 33
Ending balance 33 33 33 33
IPL [Member] | Additional Paid-in Capital [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance 3,357 3,012 3,212 2,887
Capital contributions from parent 140 200 285 325
Ending balance 3,497 3,212 3,497 3,212
IPL [Member] | Retained Earnings [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance 1,245 1,035 1,216 1,054
Net income 165 190 374 272
Common stock dividends (90) (50) (270) (151)
Ending balance 1,320 1,175 1,320 1,175
WPL [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance     4,101  
Beginning balance 4,180 4,010 4,101 3,897
Net income 123 114 321 270
Common stock dividends (44) (49) (163) (147)
Capital contributions from parent 80   80 55
Ending balance 4,339   4,339  
Ending balance 4,339 4,075 4,339 4,075
WPL [Member] | Common Stock [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance 66 66 66 66
Ending balance 66 66 66 66
WPL [Member] | Additional Paid-in Capital [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance 2,533 2,533 2,533 2,478
Capital contributions from parent 80   80 55
Ending balance 2,613 2,533 2,613 2,533
WPL [Member] | Retained Earnings [Member]        
Changes in Shareowners' Equity Roll Forward [Line Items]        
Beginning balance 1,581 1,411 1,502 1,353
Net income 123 114 321 270
Common stock dividends (44) (49) (163) (147)
Ending balance $ 1,660 $ 1,476 $ 1,660 $ 1,476