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SHORT-TERM INVESTMENTS: (Tables)
3 Months Ended
Mar. 31, 2022
SHORT-TERM INVESTMENTS:  
Schedule of short-term investments

Short-term investments were as follows (in millions):

At March 31, 

At December 31, 

    

2022

    

2021

Trading securities

$

406.1

$

486.5

Weighted average interest rate

 

0.4

%  

 

0.3

%

Available-for-sale

$

0.4

$

0.4

Weighted average interest rate

 

0.7

%  

 

0.7

%

Total

$

406.5

$

486.9

Summary of activity investments

The following table summarizes the activity of these investments by category (in millions):

Three months ended

 

March 31, 

    

2022

    

2021

 

Trading:

Interest earned

$

0.8

$

0.4

Unrealized gain (loss) at the end of the period

$

(0.1)

$

0.1

Available-for-sale:

Interest earned

 

(*)

(*)

Investment redeemed

$

(*)

$

0.1

(*) Less than $0.1 million.