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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Investments          
Trading securities $ 878,000   $ 878,000   $ 333,700
Weighted average interest rate (as a percent)     1.04%   0.78%
Available-for-sale 4,100   $ 4,100   $ 4,900
Weighted average interest rate (as a percent)     0.53%   0.44%
Total short-term investments 882,147   $ 882,147   $ 338,589
Available-for-sale          
Investment redeemed 200 $ 300 800 $ 500  
Trading:          
Interest earned 400 1,100 900 3,600  
Unrealized gain (loss) at the end of the period   1,000   1,000  
Maximum          
Available-for-sale          
Interest earned 100 $ 100 100 $ 100  
Trading:          
Unrealized gain (loss) at the end of the period $ 100   $ 100