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SHORT-TERM INVESTMENTS: (Tables)
 12 Months Ended
Dec. 31, 2015
SHORT-TERM INVESTMENTS:  
Schedule of short-term investments

 

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

 

 

 

At December 31,

 

 

 

2015

 

2014

 

Trading securities

 

$
600.2 

 

$
333.7 

 

Weighted average interest rate

 
0.71 

%
0.78 

%

 

 

 

 

 

 

Available-for-sale

 

$
3.3 

 

$
4.9 

 

Weighted average interest rate

 
0.72 

%
0.44 

%

 

 

 

 

 

 

Total

 

$
603.5 

 

$
338.6 

 

 

 

 

 

 

 

 

 

 

Summary of activities of short-term investments

 

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

 

 

 

Years ended December 31,

 

 

 

2015

 

2014

 

Trading:

 

 

 

 

 

Interest earned

 

$
1.5

 

$
4.3

 

Unrealized gain (loss) at December 31,

 

$
(0.1

)

$
2.1

 

 

 

 

 

 

 

Available-for-sale:

 

 

 

 

 

Interest earned

 

(*

)

(*

)

Investment redeemed

 

$
1.6

 

$
0.8

 

 

(*) Less than $0.1 million

Schedule of contractual maturities of the Company's available for sale debt securities

 

At December 31, 2015 and 2014, contractual maturities of the available-for-sale debt securities are as follows (in millions):

 

 

 

2015

 

2014

 

One year or less

 

$
0.2 

 

$
0.3 

 

Maturing after one year through five years

 

—

 

—

 

Maturing after five years through ten years

 

—

 
0.1 

 

Due after 10 years

 
3.1 

 
4.5 

 

 

 

 

 

 

 

Total debt securities

 

$
3.3 

 

$
4.9