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SHORT-TERM INVESTMENTS: (Tables)
 3 Months Ended
Mar. 31, 2013
SHORT-TERM INVESTMENTS:  
Schedule of short-term investments

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

 

 

 

March 31,

 

December 31,

 

 

 

2013

 

2012

 

Trading securities

 

$

198.0

 

$

127.8

 

Weighted average interest rate

 

1.43

%

1.87

%

 

 

 

 

 

 

Available for sale

 

$

6.5

 

$

6.5

 

Weighted average interest rate

 

0.44

%

0.43

%

Total

 

$

204.5

 

$

134.3

 

 
Summary of activities in short-term investments

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

 

 

 

Three months ended
March 31,

 

 

 

2013

 

2012

 

Trading:

 

 

 

 

 

Interest earned

 

$

0.6

 

$

0.8

 

Unrealized gain (loss)

 

$

1.1

 

$

5.0

 

 

 

 

 

 

 

Available for sale:

 

 

 

 

 

Interest earned

 

(*

)

(*

)

Investment redeemed

 

$

—

 

$

0.3

 

 

 

(*) Less than $0.1 million.