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SHORT-TERM INVESTMENTS: (Details) (USD $)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Dec. 31, 2013
Investments          
Trading securities $ 284,400,000   $ 284,400,000   $ 202,600,000
Weighted average interest rate (as a percent)     1.80%   3.78%
Available for sale 5,500,000   5,500,000   5,700,000
Weighted average interest rate (as a percent)     0.41%   0.42%
Total short-term investments 289,883,000   289,883,000   208,268,000
Trading securities:          
Interest earned 1,100,000 1,400,000 2,500,000 2,000,000  
Unrealized gain (loss) at the end of the period 1,700,000 3,400,000 1,700,000 3,400,000  
Available-for-sale          
Investment redeemed 200,000 400,000 200,000 400,000  
Maximum
          
Available-for-sale          
Interest earned $ 100,000 $ 100,000 $ 100,000 $ 100,000