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Investments (Tables)
 12 Months Ended
Dec. 31, 2019
Investment [Line Items]  
Unconsolidated Equity Investments Alliant Energy’s unconsolidated investments accounted for under the equity method of accounting are as follows (in millions):
 
Ownership Interest at
 
Carrying Value at December 31,
 
Equity (Income) / Loss
 
December 31, 2019
 
2019
 
2018
 
2019
 
2018
 
2017
ATC Holdings
16%, 20%
 

$320.1

 

$293.6

 

($45.3
)
 

($38.1
)
 

($42.4
)
Non-utility wind farm in Oklahoma
50%
 
105.3

 
105.1

 
(5.2
)
 
(15.6
)
 
(1.8
)
Other
Various
 
32.9

 
22.6

 
(2.5
)
 
(0.9
)
 
(0.6
)
 
 
 

$458.3

 

$421.3

 

($53.0
)
 

($54.6
)
 

($44.8
)