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SHORT-TERM INVESTMENTS: (Details) (USD $)
3 Months Ended 12 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
Investments      
Trading securities $ 198,000,000   $ 127,800,000
Weighted average interest rate (as a percent) 1.43%   1.87%
Available for sale 6,500,000   6,500,000
Weighted average interest rate (as a percent) 0.44%   0.43%
Total short-term investments 204,488,000   134,298,000
Trading securities:      
Interest earned 600,000 800,000  
Unrealized gain (loss) 1,100,000 5,000,000  
Available-for-sale:      
Investment redeemed   300,000  
Maximum
     
Available-for-sale:      
Interest earned $ 100,000 $ 100,000