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SHORT-TERM INVESTMENTS: (Tables)
 3 Months Ended
Mar. 31, 2016
SHORT-TERM INVESTMENTS:  
Schedule of short-term investments

 

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

 

 

 

At March 31,

 

At December 31,

 

 

 

2016

 

2015

 

Trading securities

 

$
279.6 

 

$
600.2 

 

Weighted average interest rate

 
0.87 

%
0.71 

%

 

 

 

 

 

 

Available-for-sale

 

$
3.0 

 

$
3.3 

 

Weighted average interest rate

 
0.79 

%
0.72 

%

 

 

 

 

 

 

Total

 

$
282.6 

 

$
603.5 

 

 

 

 

 

 

 

 

 

 

Summary of activities of short-term investments

 

 

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

 

 

 

Three months ended
March 31,

 

 

 

2016

 

2015

 

Trading:

 

 

 

 

 

Interest earned

 

$
0.4 

 

$
0.3 

 

Unrealized gain at the end of the period

 

$
0.1 

 

$
2.8 

 

 

 

 

 

 

 

Available-for-sale:

 

 

 

 

 

Interest earned

 

(*

)

(*

)

Investment redeemed

 

$
0.3 

 

$
0.2 

 

 

(*) Less than $0.1 million.