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Income Taxes (Tables)
 9 Months Ended
Sep. 30, 2021
Income Taxes [Line Items]  
Schedule Of Effective Income Tax Rates Overall effective income tax rates, 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, amortization of excess deferred taxes and the effect of rate-making on property-related differences. The decreases in Alliant Energy’s and WPL’s overall effective income tax rates for the three and nine months ended September 30, 2021 compared to the same periods in 2020 were primarily due to increased amortization of excess deferred taxes primarily at WPL, which were used to offset increases in WPL’s 2021 increased revenue requirements.
Alliant EnergyIPLWPL
Three MonthsNine MonthsThree MonthsNine MonthsThree MonthsNine Months
202120202021202020212020202120202021202020212020
Overall income tax rate(9%)(9%)(13%)(9%)(8%)(14%)(11%)(14%)(19%)(7%)(24%)(8%)
Summary Of Tax Credit Carryforwards At September 30, 2021, carryforwards and expiration dates were estimated as follows (in millions):
Range of Expiration DatesAlliant EnergyIPLWPL
Federal net operating losses2037$170$157$1
State net operating losses2021-2041592242
Federal tax credits2022-2041547339186
IPL [Member]  
Income Taxes [Line Items]  
Schedule Of Effective Income Tax Rates Overall effective income tax rates, 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, amortization of excess deferred taxes and the effect of rate-making on property-related differences. The decreases in Alliant Energy’s and WPL’s overall effective income tax rates for the three and nine months ended September 30, 2021 compared to the same periods in 2020 were primarily due to increased amortization of excess deferred taxes primarily at WPL, which were used to offset increases in WPL’s 2021 increased revenue requirements.
Alliant EnergyIPLWPL
Three MonthsNine MonthsThree MonthsNine MonthsThree MonthsNine Months
202120202021202020212020202120202021202020212020
Overall income tax rate(9%)(9%)(13%)(9%)(8%)(14%)(11%)(14%)(19%)(7%)(24%)(8%)
Summary Of Tax Credit Carryforwards At September 30, 2021, carryforwards and expiration dates were estimated as follows (in millions):
Range of Expiration DatesAlliant EnergyIPLWPL
Federal net operating losses2037$170$157$1
State net operating losses2021-2041592242
Federal tax credits2022-2041547339186
WPL [Member]  
Income Taxes [Line Items]  
Schedule Of Effective Income Tax Rates Overall effective income tax rates, 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, amortization of excess deferred taxes and the effect of rate-making on property-related differences. The decreases in Alliant Energy’s and WPL’s overall effective income tax rates for the three and nine months ended September 30, 2021 compared to the same periods in 2020 were primarily due to increased amortization of excess deferred taxes primarily at WPL, which were used to offset increases in WPL’s 2021 increased revenue requirements.
Alliant EnergyIPLWPL
Three MonthsNine MonthsThree MonthsNine MonthsThree MonthsNine Months
202120202021202020212020202120202021202020212020
Overall income tax rate(9%)(9%)(13%)(9%)(8%)(14%)(11%)(14%)(19%)(7%)(24%)(8%)
Summary Of Tax Credit Carryforwards At September 30, 2021, carryforwards and expiration dates were estimated as follows (in millions):
Range of Expiration DatesAlliant EnergyIPLWPL
Federal net operating losses2037$170$157$1
State net operating losses2021-2041592242
Federal tax credits2022-2041547339186