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SHORT TERM INVESTMENTS: (Details) (USD $)
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Investments    
Trading securities $ 514,600,000 $ 66,900,000
Weighted average interest rate (as a percent) 1.37% 1.14%
Available for sale 7,300,000 9,300,000
Weighted average interest rate (as a percent) 0.58% 1.01%
Total short-term investments 521,955,000 76,209,000
Trading securities:    
Interest earned 6,000,000 2,600,000
Unrealized loss 7,900,000  
Available-for-sale:    
Interest earned   200,000
Investment redeemed 2,100,000 14,800,000
Contractual maturities of the Company's available for sale debt securities    
One year or less 500,000 400,000
Maturing after one year through five years   200,000
Maturing after five years through ten years 600,000  
Due after 10 years 6,200,000 8,700,000
Total debt securities 7,300,000 9,300,000
Maximum
    
Available-for-sale:    
Interest earned $ 100,000