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Investments
 12 Months Ended
Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]  
Investments INVESTMENTS
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
)

ATC Holdings - As of December 31, 2019, Alliant Energy has a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, collectively referred to as ATC Holdings. ATC is an independent, for-profit, transmission-only company. ATC Holdco LLC holds Duke-American Transmission Company, LLC, a joint venture between Duke Energy Corporation and ATC, that owns electric transmission infrastructure in North America.

Non-utility Wind Farm in Oklahoma - The non-utility wind farm located in Oklahoma provides electricity to a third-party under a long-term PPA, and has both cash and tax equity ownership. The equity income recognized in 2018 was primarily related to the impacts of Federal Tax Reform. The liquidation method utilized to recognize Alliant Energy’s share of the wind farm’s earnings includes utilizing the federal income tax rate in effect as of the end of the measurement period. The lower
federal income tax rate effective as of January 1, 2018 resulted in an acceleration of earnings attributable to Alliant Energy’s interest in the Oklahoma wind farm. This increase in earnings is expected to reverse over time. Alliant Energy does not maintain or operate the wind farm, and provided a parent guarantee of its subsidiary’s indemnification obligations under the operating agreement and PPA. Refer to Note 17(d) for discussion of the guarantee.