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Investments (Tables)
 6 Months Ended
Jun. 30, 2018
Schedule of Investments [Line Items]  
Unconsolidated Equity Investments
Alliant Energy’s equity (income) loss from unconsolidated investments accounted for under the equity method of accounting for the three and six months ended June 30 was as follows (in millions):
 
Three Months
 
Six Months
 
2018
 
2017
 
2018
 
2017
ATC Holdings

($7.8
)
 

($11.1
)
 

($16.5
)
 

($22.6
)
Non-utility wind farm in Oklahoma
(2.5
)
 
—

 
(14.6
)
 
—

Other
(0.2
)
 
(0.2
)
 
(0.7
)
 
(0.2
)
 

($10.5
)
 

($11.3
)
 

($31.8
)
 

($22.8
)