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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Investments          
Trading securities $ 569,000   $ 569,000   $ 333,700
Weighted average interest rate (as a percent)     0.52%   0.78%
Available-for-sale 4,300   $ 4,300   $ 4,900
Weighted average interest rate (as a percent)     0.49%   0.44%
Total short-term investments 573,353   $ 573,353   $ 338,589
Trading:          
Interest earned 200 $ 1,100 500 $ 2,500  
Unrealized gain (loss) at the end of the period (200) 1,700 (200) 1,700  
Available-for-sale          
Investment redeemed 400 200 600 200  
Maximum          
Available-for-sale          
Interest earned $ 100 $ 100 $ 100 $ 100