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SHORT-TERM INVESTMENTS: (Details) (USD $)
 3 Months Ended 12 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2011
Investments      
Trading securities $ 220,000,000   $ 514,600,000
Weighted average interest rate (as a percent) 1.33%   1.37%
Available for sale 7,300,000   7,300,000
Weighted average interest rate (as a percent) 0.52%   0.58%
Total short-term investments 227,261,000   521,955,000
Trading securities:      
Interest earned 800,000 400,000  
Unrealized loss 2,600,000    
Available-for-sale:      
Investment redeemed 300,000 400,000  
Maximum
      
Available-for-sale:      
Interest earned $ 100,000 $ 100,000