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Income Taxes
 9 Months Ended
Sep. 30, 2019
Income Taxes [Line Items]  
Income Taxes INCOME TAXES
Income Tax Rates - The overall income tax rates shown in the following table were computed by dividing income taxes by income before income taxes.
 
Alliant Energy
 
IPL
 
WPL
Three Months Ended September 30
2019
 
2018
 
2019
 
2018
 
2019
 
2018
Statutory federal income tax rate
21.0
 %
 
21.0
 %
 
21.0
%
 
21.0
%
 
21.0
 %
 
21.0
 %
State income taxes, net of federal benefits
7.0

 
6.9

 
8.1

 
7.7

 
6.2

 
6.2

Production tax credits
(9.3
)
 
(5.4
)
 
(14.7
)
 
(5.5
)
 
(4.4
)
 
(6.4
)
Effect of rate-making on property-related differences
(5.6
)
 
(6.3
)
 
(9.0
)
 
(11.5
)
 
(2.8
)
 
(2.1
)
Amortization of excess deferred taxes
(1.1
)
 
(0.1
)
 
(0.3
)
 
—

 
(1.7
)
 
(0.1
)
Adjustment for prior period taxes
(1.8
)
 
(5.7
)
 
1.3

 
(10.2
)
 
0.4

 
—

IPL’s tax benefit riders
(0.8
)
 
(2.3
)
 
(1.5
)
 
(4.8
)
 
—

 
—

Federal Tax Reform adjustments
—

 
(2.5
)
 
—

 
(0.9
)
 
—

 
(6.4
)
Other items, net
(0.2
)
 
(0.3
)
 
(0.3
)
 
(0.6
)
 
(0.2
)
 
(0.4
)
Overall income tax rate
9.2
%
 
5.3
%
 
4.6
%
 
(4.8
%)
 
18.5
%
 
11.8
%

 
Alliant Energy
 
IPL
 
WPL
Nine Months Ended September 30
2019
 
2018
 
2019
 
2018
 
2019
 
2018
Statutory federal income tax rate
21.0
 %
 
21.0
 %
 
21.0
%
 
21.0
%
 
21.0
 %
 
21.0
 %
State income taxes, net of federal benefits
7.1

 
6.9

 
8.1

 
7.7

 
6.2

 
6.2

Production tax credits
(9.1
)
 
(5.4
)
 
(14.3
)
 
(5.4
)
 
(4.5
)
 
(6.6
)
Effect of rate-making on property-related differences
(5.7
)
 
(6.7
)
 
(9.3
)
 
(12.0
)
 
(2.6
)
 
(2.3
)
Amortization of excess deferred taxes
(1.1
)
 
(0.3
)
 
(0.3
)
 
—

 
(1.7
)
 
(0.1
)
Adjustment for prior period taxes
(0.7
)
 
(2.6
)
 
0.6

 
(5.4
)
 
(0.3
)
 
—

IPL’s tax benefit riders
(0.7
)
 
(2.2
)
 
(1.5
)
 
(4.6
)
 
—

 
—

Federal Tax Reform adjustments
—

 
(1.2
)
 
—

 
(0.5
)
 
—

 
(2.8
)
Other items, net
(1.0
)
 
(0.7
)
 
(0.1
)
 
(1.0
)
 
(0.4
)
 
(1.2
)
Overall income tax rate
9.8
%
 
8.8
%
 
4.2
%
 
(0.2
%)
 
17.7
%
 
14.2
%


Deferred Tax Assets and Liabilities -
Carryforwards - At September 30, 2019, carryforwards and expiration dates were estimated as follows (in millions):
 
Range of Expiration Dates
 
Alliant Energy
 
IPL
 
WPL
Federal net operating losses
2031-2037
 

$611

 

$522

 

$41

State net operating losses
2019-2039
 
677

 
12

 
2

Federal tax credits
2022-2039
 
337

 
162

 
156
IPL [Member]  
Income Taxes [Line Items]  
Income Taxes INCOME TAXES
Income Tax Rates - The overall income tax rates shown in the following table were computed by dividing income taxes by income before income taxes.
 
Alliant Energy
 
IPL
 
WPL
Three Months Ended September 30
2019
 
2018
 
2019
 
2018
 
2019
 
2018
Statutory federal income tax rate
21.0
 %
 
21.0
 %
 
21.0
%
 
21.0
%
 
21.0
 %
 
21.0
 %
State income taxes, net of federal benefits
7.0

 
6.9

 
8.1

 
7.7

 
6.2

 
6.2

Production tax credits
(9.3
)
 
(5.4
)
 
(14.7
)
 
(5.5
)
 
(4.4
)
 
(6.4
)
Effect of rate-making on property-related differences
(5.6
)
 
(6.3
)
 
(9.0
)
 
(11.5
)
 
(2.8
)
 
(2.1
)
Amortization of excess deferred taxes
(1.1
)
 
(0.1
)
 
(0.3
)
 
—

 
(1.7
)
 
(0.1
)
Adjustment for prior period taxes
(1.8
)
 
(5.7
)
 
1.3

 
(10.2
)
 
0.4

 
—

IPL’s tax benefit riders
(0.8
)
 
(2.3
)
 
(1.5
)
 
(4.8
)
 
—

 
—

Federal Tax Reform adjustments
—

 
(2.5
)
 
—

 
(0.9
)
 
—

 
(6.4
)
Other items, net
(0.2
)
 
(0.3
)
 
(0.3
)
 
(0.6
)
 
(0.2
)
 
(0.4
)
Overall income tax rate
9.2
%
 
5.3
%
 
4.6
%
 
(4.8
%)
 
18.5
%
 
11.8
%

 
Alliant Energy
 
IPL
 
WPL
Nine Months Ended September 30
2019
 
2018
 
2019
 
2018
 
2019
 
2018
Statutory federal income tax rate
21.0
 %
 
21.0
 %
 
21.0
%
 
21.0
%
 
21.0
 %
 
21.0
 %
State income taxes, net of federal benefits
7.1

 
6.9

 
8.1

 
7.7

 
6.2

 
6.2

Production tax credits
(9.1
)
 
(5.4
)
 
(14.3
)
 
(5.4
)
 
(4.5
)
 
(6.6
)
Effect of rate-making on property-related differences
(5.7
)
 
(6.7
)
 
(9.3
)
 
(12.0
)
 
(2.6
)
 
(2.3
)
Amortization of excess deferred taxes
(1.1
)
 
(0.3
)
 
(0.3
)
 
—

 
(1.7
)
 
(0.1
)
Adjustment for prior period taxes
(0.7
)
 
(2.6
)
 
0.6

 
(5.4
)
 
(0.3
)
 
—

IPL’s tax benefit riders
(0.7
)
 
(2.2
)
 
(1.5
)
 
(4.6
)
 
—

 
—

Federal Tax Reform adjustments
—

 
(1.2
)
 
—

 
(0.5
)
 
—

 
(2.8
)
Other items, net
(1.0
)
 
(0.7
)
 
(0.1
)
 
(1.0
)
 
(0.4
)
 
(1.2
)
Overall income tax rate
9.8
%
 
8.8
%
 
4.2
%
 
(0.2
%)
 
17.7
%
 
14.2
%


Deferred Tax Assets and Liabilities -
Carryforwards - At September 30, 2019, carryforwards and expiration dates were estimated as follows (in millions):
 
Range of Expiration Dates
 
Alliant Energy
 
IPL
 
WPL
Federal net operating losses
2031-2037
 

$611

 

$522

 

$41

State net operating losses
2019-2039
 
677

 
12

 
2

Federal tax credits
2022-2039
 
337

 
162

 
156
WPL [Member]  
Income Taxes [Line Items]  
Income Taxes INCOME TAXES
Income Tax Rates - The overall income tax rates shown in the following table were computed by dividing income taxes by income before income taxes.
 
Alliant Energy
 
IPL
 
WPL
Three Months Ended September 30
2019
 
2018
 
2019
 
2018
 
2019
 
2018
Statutory federal income tax rate
21.0
 %
 
21.0
 %
 
21.0
%
 
21.0
%
 
21.0
 %
 
21.0
 %
State income taxes, net of federal benefits
7.0

 
6.9

 
8.1

 
7.7

 
6.2

 
6.2

Production tax credits
(9.3
)
 
(5.4
)
 
(14.7
)
 
(5.5
)
 
(4.4
)
 
(6.4
)
Effect of rate-making on property-related differences
(5.6
)
 
(6.3
)
 
(9.0
)
 
(11.5
)
 
(2.8
)
 
(2.1
)
Amortization of excess deferred taxes
(1.1
)
 
(0.1
)
 
(0.3
)
 
—

 
(1.7
)
 
(0.1
)
Adjustment for prior period taxes
(1.8
)
 
(5.7
)
 
1.3

 
(10.2
)
 
0.4

 
—

IPL’s tax benefit riders
(0.8
)
 
(2.3
)
 
(1.5
)
 
(4.8
)
 
—

 
—

Federal Tax Reform adjustments
—

 
(2.5
)
 
—

 
(0.9
)
 
—

 
(6.4
)
Other items, net
(0.2
)
 
(0.3
)
 
(0.3
)
 
(0.6
)
 
(0.2
)
 
(0.4
)
Overall income tax rate
9.2
%
 
5.3
%
 
4.6
%
 
(4.8
%)
 
18.5
%
 
11.8
%

 
Alliant Energy
 
IPL
 
WPL
Nine Months Ended September 30
2019
 
2018
 
2019
 
2018
 
2019
 
2018
Statutory federal income tax rate
21.0
 %
 
21.0
 %
 
21.0
%
 
21.0
%
 
21.0
 %
 
21.0
 %
State income taxes, net of federal benefits
7.1

 
6.9

 
8.1

 
7.7

 
6.2

 
6.2

Production tax credits
(9.1
)
 
(5.4
)
 
(14.3
)
 
(5.4
)
 
(4.5
)
 
(6.6
)
Effect of rate-making on property-related differences
(5.7
)
 
(6.7
)
 
(9.3
)
 
(12.0
)
 
(2.6
)
 
(2.3
)
Amortization of excess deferred taxes
(1.1
)
 
(0.3
)
 
(0.3
)
 
—

 
(1.7
)
 
(0.1
)
Adjustment for prior period taxes
(0.7
)
 
(2.6
)
 
0.6

 
(5.4
)
 
(0.3
)
 
—

IPL’s tax benefit riders
(0.7
)
 
(2.2
)
 
(1.5
)
 
(4.6
)
 
—

 
—

Federal Tax Reform adjustments
—

 
(1.2
)
 
—

 
(0.5
)
 
—

 
(2.8
)
Other items, net
(1.0
)
 
(0.7
)
 
(0.1
)
 
(1.0
)
 
(0.4
)
 
(1.2
)
Overall income tax rate
9.8
%
 
8.8
%
 
4.2
%
 
(0.2
%)
 
17.7
%
 
14.2
%


Deferred Tax Assets and Liabilities -
Carryforwards - At September 30, 2019, carryforwards and expiration dates were estimated as follows (in millions):
 
Range of Expiration Dates
 
Alliant Energy
 
IPL
 
WPL
Federal net operating losses
2031-2037
 

$611

 

$522

 

$41

State net operating losses
2019-2039
 
677

 
12

 
2

Federal tax credits
2022-2039
 
337

 
162

 
156