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Income Taxes
 9 Months Ended
Sep. 30, 2025
Income Taxes [Line Items]  
Income Taxes INCOME TAXES
Income Tax Rates - Overall effective income tax rates for the three and nine months ended September 30, which were computed by dividing income tax expense (benefit) by income before income taxes, were as follows. The effective income tax rates were different than the federal statutory rate primarily due to state income taxes, production tax credits, investment tax credits, amortization of excess deferred taxes and the effect of rate-making on property-related differences. Also impacting Alliant Energy’s and IPL’s effective income tax rates for the nine months ended September 30, 2024 were the pre-tax non-cash charge of $60 million for IPL’s Lansing Generating Station discussed in Note 2 and the pre-tax non-cash charge of $20 million for the AROs allocated to IPL’s steam business discussed in Note 1(d). Alliant Energy’s, IPL’s and WPL’s effective income tax rates for the three and nine months ended September 30, 2025 were also impacted by additional tax credits in 2025 from renewable generation and energy storage projects placed in service in 2024 and/or expected to be placed in service in 2025.
Alliant EnergyIPLWPL
Three MonthsNine Months Three MonthsNine Months Three MonthsNine Months
202520242025202420252024202520242025202420252024
Overall income tax rate(7%)(27%)(19%)(24%)(15%)(56%)(36%)(70%)1%3%(3%)5%

Deferred Tax Assets and Liabilities -
Carryforwards - At September 30, 2025, the carryforwards and expiration dates were estimated as follows (in millions):
Range of Expiration DatesAlliant EnergyIPLWPL
State net operating losses2025-2045$249$7$1
Federal tax credits2034-2045741464263

State Income Tax Apportionment - Deferred tax assets and liabilities are recorded for temporary differences between the tax basis of assets and liabilities and the amounts reported in the financial statements. Deferred taxes are recorded using currently enacted tax rates and estimates of state income tax apportionment. Estimates of state income tax apportionment are supported by historical data and reasonable projections. Alliant Energy currently expects an increase in total state income tax apportionment primarily due to an increase in projected electric utility revenues driven by demand for energy from commercial and industrial customers, including demand from IPL’s and WPL’s executed electric service agreements with customers who expect to build data centers in their service territories. Alliant Energy parent company’s deferred tax assets were remeasured to reflect an increase in estimated total state income tax apportionment, which resulted in a charge of $8 million recorded to income tax expense in Alliant Energy’s income statement and an increase in deferred tax liabilities on Alliant Energy’s balance sheet in the third quarter of 2025.
IPL [Member]  
Income Taxes [Line Items]  
Income Taxes INCOME TAXES
Income Tax Rates - Overall effective income tax rates for the three and nine months ended September 30, which were computed by dividing income tax expense (benefit) by income before income taxes, were as follows. The effective income tax rates were different than the federal statutory rate primarily due to state income taxes, production tax credits, investment tax credits, amortization of excess deferred taxes and the effect of rate-making on property-related differences. Also impacting Alliant Energy’s and IPL’s effective income tax rates for the nine months ended September 30, 2024 were the pre-tax non-cash charge of $60 million for IPL’s Lansing Generating Station discussed in Note 2 and the pre-tax non-cash charge of $20 million for the AROs allocated to IPL’s steam business discussed in Note 1(d). Alliant Energy’s, IPL’s and WPL’s effective income tax rates for the three and nine months ended September 30, 2025 were also impacted by additional tax credits in 2025 from renewable generation and energy storage projects placed in service in 2024 and/or expected to be placed in service in 2025.
Alliant EnergyIPLWPL
Three MonthsNine Months Three MonthsNine Months Three MonthsNine Months
202520242025202420252024202520242025202420252024
Overall income tax rate(7%)(27%)(19%)(24%)(15%)(56%)(36%)(70%)1%3%(3%)5%

Deferred Tax Assets and Liabilities -
Carryforwards - At September 30, 2025, the carryforwards and expiration dates were estimated as follows (in millions):
Range of Expiration DatesAlliant EnergyIPLWPL
State net operating losses2025-2045$249$7$1
Federal tax credits2034-2045741464263
WPL [Member]  
Income Taxes [Line Items]  
Income Taxes INCOME TAXES
Income Tax Rates - Overall effective income tax rates for the three and nine months ended September 30, which were computed by dividing income tax expense (benefit) by income before income taxes, were as follows. The effective income tax rates were different than the federal statutory rate primarily due to state income taxes, production tax credits, investment tax credits, amortization of excess deferred taxes and the effect of rate-making on property-related differences. Also impacting Alliant Energy’s and IPL’s effective income tax rates for the nine months ended September 30, 2024 were the pre-tax non-cash charge of $60 million for IPL’s Lansing Generating Station discussed in Note 2 and the pre-tax non-cash charge of $20 million for the AROs allocated to IPL’s steam business discussed in Note 1(d). Alliant Energy’s, IPL’s and WPL’s effective income tax rates for the three and nine months ended September 30, 2025 were also impacted by additional tax credits in 2025 from renewable generation and energy storage projects placed in service in 2024 and/or expected to be placed in service in 2025.
Alliant EnergyIPLWPL
Three MonthsNine Months Three MonthsNine Months Three MonthsNine Months
202520242025202420252024202520242025202420252024
Overall income tax rate(7%)(27%)(19%)(24%)(15%)(56%)(36%)(70%)1%3%(3%)5%

Deferred Tax Assets and Liabilities -
Carryforwards - At September 30, 2025, the carryforwards and expiration dates were estimated as follows (in millions):
Range of Expiration DatesAlliant EnergyIPLWPL
State net operating losses2025-2045$249$7$1
Federal tax credits2034-2045741464263