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SHORT-TERM INVESTMENTS: (Details) (USD $)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Investments          
Trading securities $ 373,000,000   $ 373,000,000   $ 514,600,000
Weighted average interest rate (as a percent)     1.00%   1.37%
Available for sale 6,700,000   6,700,000   7,300,000
Weighted average interest rate (as a percent)     0.51%   0.58%
Total short-term investments 379,702,000   379,702,000   521,955,000
Trading securities:          
Interest earned 900,000 1,200,000 2,500,000 2,900,000  
Unrealized gain (loss) 3,700,000 (1,100,000) 8,800,000 (1,900,000)  
Available-for-sale:          
Investment redeemed 200,000 900,000 1,400,000 1,700,000  
Maximum
          
Available-for-sale:          
Interest earned $ 100,000 $ 100,000 $ 100,000 $ 100,000