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Investments (Tables)
 9 Months Ended
Sep. 30, 2025
Schedule of Equity Method Investments [Line Items]  
Unconsolidated Equity Investments Alliant Energy’s equity (income) loss from unconsolidated investments accounted for under the equity method of accounting for the three and nine months ended September 30 was as follows (in millions):
Three MonthsNine Months
2025202420252024
ATC Holdings($15)($13)($43)($38)
Non-utility wind farm in Oklahoma(2)(2)(6)(5)
Corporate venture investments(1)2101
Other—(1)(2)(2)
($18)($14)($41)($44)