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Investment in ATC (Tables)
6 Months Ended
Jun. 30, 2016
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, 2015

$124.5

Cash Investments
1.6

Equity in ATC Earnings
8.9

Distributed ATC Earnings
(6.0
)
Equity Investment Balance as of June 30, 2016

$129.0