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SHORT-TERM INVESTMENTS: (Details) (USD $)
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
Investments          
Trading securities $ 367,300,000   $ 367,300,000   $ 514,600,000
Weighted average interest rate (as a percent)     1.01%   1.37%
Available for sale 6,400,000   6,400,000   7,300,000
Weighted average interest rate (as a percent)     0.54%   0.58%
Total short-term investments 373,756,000   373,756,000   521,955,000
Trading securities:          
Interest earned 900,000 1,000,000 1,600,000 1,400,000  
Unrealized gain (loss) 100,000 (3,300,000) 5,100,000 (900,000)  
Available-for-sale:          
Investment redeemed 900,000 400,000 1,200,000 800,000  
Maximum
          
Available-for-sale:          
Interest earned $ 100,000 $ 100,000 $ 100,000 $ 100,000