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Equity Investments (Tables)
3 Months Ended
Mar. 31, 2019
Equity Method Investments and Joint Ventures [Abstract]  
ALLETE's Investment in ATC [Table Text Block]
ALLETE’s Investment in ATC
 
Millions
 
Equity Investment Balance as of December 31, 2018

$128.1

Cash Investments
0.4

Equity in ATC Earnings
5.6

Distributed ATC Earnings
(4.4
)
Amortization of the Remeasurement of Deferred Income Taxes
0.3

Equity Investment Balance as of March 31, 2019

$130.0