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Income Taxes
 6 Months Ended
Jun. 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 June 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
7.5

 
6.5

 
8.3

 
7.0

 
6.2

 
6.2

Production tax credits
(8.5
)
 
(5.4
)
 
(13.2
)
 
(5.3
)
 
(4.2
)
 
(6.8
)
Effect of rate-making on property-related differences
(6.2
)
 
(6.7
)
 
(10.1
)
 
(11.5
)
 
(2.7
)
 
(2.7
)
Amortization of excess deferred taxes
(1.3
)
 
(0.4
)
 
(0.4
)
 
—

 
(2.3
)
 
(0.1
)
IPL’s tax benefit riders
(0.7
)
 
(2.1
)
 
(1.4
)
 
(4.3
)
 
—

 
—

Other items, net
(1.5
)
 
(1.2
)
 
(0.4
)
 
(2.8
)
 
(2.5
)
 
(4.1
)
Overall income tax rate
10.3
%
 
11.7
%
 
3.8
%
 
4.1
%
 
15.5
%
 
13.5
%

 
Alliant Energy
 
IPL
 
WPL
Six Months Ended June 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
7.2

 
7.1

 
8.1

 
7.7

 
6.2

 
6.2

Production tax credits
(8.9
)
 
(5.5
)
 
(13.6
)
 
(5.3
)
 
(4.6
)
 
(6.7
)
Effect of rate-making on property-related differences
(6.0
)
 
(7.2
)
 
(9.6
)
 
(12.4
)
 
(2.4
)
 
(2.5
)
Amortization of excess deferred taxes
(1.1
)
 
(0.4
)
 
(0.3
)
 
—

 
(1.8
)
 
(0.1
)
IPL’s tax benefit riders
(0.7
)
 
(2.2
)
 
(1.4
)
 
(4.5
)
 
—

 
—

Other items, net
(1.0
)
 
(1.0
)
 
(0.5
)
 
(1.5
)
 
(1.2
)
 
(1.9
)
Overall income tax rate
10.5
%
 
11.8
%
 
3.7
%
 
5.0
%
 
17.2
%
 
16.0
%


Deferred Tax Assets and Liabilities -
Carryforwards - At June 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
 

$760

 

$550

 

$122

State net operating losses
2019-2039
 
764

 
13

 
2

Federal tax credits
2022-2039
 
325

 
152

 
154
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 June 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
7.5

 
6.5

 
8.3

 
7.0

 
6.2

 
6.2

Production tax credits
(8.5
)
 
(5.4
)
 
(13.2
)
 
(5.3
)
 
(4.2
)
 
(6.8
)
Effect of rate-making on property-related differences
(6.2
)
 
(6.7
)
 
(10.1
)
 
(11.5
)
 
(2.7
)
 
(2.7
)
Amortization of excess deferred taxes
(1.3
)
 
(0.4
)
 
(0.4
)
 
—

 
(2.3
)
 
(0.1
)
IPL’s tax benefit riders
(0.7
)
 
(2.1
)
 
(1.4
)
 
(4.3
)
 
—

 
—

Other items, net
(1.5
)
 
(1.2
)
 
(0.4
)
 
(2.8
)
 
(2.5
)
 
(4.1
)
Overall income tax rate
10.3
%
 
11.7
%
 
3.8
%
 
4.1
%
 
15.5
%
 
13.5
%

 
Alliant Energy
 
IPL
 
WPL
Six Months Ended June 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
7.2

 
7.1

 
8.1

 
7.7

 
6.2

 
6.2

Production tax credits
(8.9
)
 
(5.5
)
 
(13.6
)
 
(5.3
)
 
(4.6
)
 
(6.7
)
Effect of rate-making on property-related differences
(6.0
)
 
(7.2
)
 
(9.6
)
 
(12.4
)
 
(2.4
)
 
(2.5
)
Amortization of excess deferred taxes
(1.1
)
 
(0.4
)
 
(0.3
)
 
—

 
(1.8
)
 
(0.1
)
IPL’s tax benefit riders
(0.7
)
 
(2.2
)
 
(1.4
)
 
(4.5
)
 
—

 
—

Other items, net
(1.0
)
 
(1.0
)
 
(0.5
)
 
(1.5
)
 
(1.2
)
 
(1.9
)
Overall income tax rate
10.5
%
 
11.8
%
 
3.7
%
 
5.0
%
 
17.2
%
 
16.0
%


Deferred Tax Assets and Liabilities -
Carryforwards - At June 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
 

$760

 

$550

 

$122

State net operating losses
2019-2039
 
764

 
13

 
2

Federal tax credits
2022-2039
 
325

 
152

 
154
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 June 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
7.5

 
6.5

 
8.3

 
7.0

 
6.2

 
6.2

Production tax credits
(8.5
)
 
(5.4
)
 
(13.2
)
 
(5.3
)
 
(4.2
)
 
(6.8
)
Effect of rate-making on property-related differences
(6.2
)
 
(6.7
)
 
(10.1
)
 
(11.5
)
 
(2.7
)
 
(2.7
)
Amortization of excess deferred taxes
(1.3
)
 
(0.4
)
 
(0.4
)
 
—

 
(2.3
)
 
(0.1
)
IPL’s tax benefit riders
(0.7
)
 
(2.1
)
 
(1.4
)
 
(4.3
)
 
—

 
—

Other items, net
(1.5
)
 
(1.2
)
 
(0.4
)
 
(2.8
)
 
(2.5
)
 
(4.1
)
Overall income tax rate
10.3
%
 
11.7
%
 
3.8
%
 
4.1
%
 
15.5
%
 
13.5
%

 
Alliant Energy
 
IPL
 
WPL
Six Months Ended June 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
7.2

 
7.1

 
8.1

 
7.7

 
6.2

 
6.2

Production tax credits
(8.9
)
 
(5.5
)
 
(13.6
)
 
(5.3
)
 
(4.6
)
 
(6.7
)
Effect of rate-making on property-related differences
(6.0
)
 
(7.2
)
 
(9.6
)
 
(12.4
)
 
(2.4
)
 
(2.5
)
Amortization of excess deferred taxes
(1.1
)
 
(0.4
)
 
(0.3
)
 
—

 
(1.8
)
 
(0.1
)
IPL’s tax benefit riders
(0.7
)
 
(2.2
)
 
(1.4
)
 
(4.5
)
 
—

 
—

Other items, net
(1.0
)
 
(1.0
)
 
(0.5
)
 
(1.5
)
 
(1.2
)
 
(1.9
)
Overall income tax rate
10.5
%
 
11.8
%
 
3.7
%
 
5.0
%
 
17.2
%
 
16.0
%


Deferred Tax Assets and Liabilities -
Carryforwards - At June 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
 

$760

 

$550

 

$122

State net operating losses
2019-2039
 
764

 
13

 
2

Federal tax credits
2022-2039
 
325

 
152

 
154