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SHORT-TERM INVESTMENTS: (Details) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Investments    
Trading securities $ 127,800,000 $ 514,600,000
Weighted average interest rate (as a percent) 1.87% 1.37%
Available for sale 6,500,000 7,300,000
Weighted average interest rate (as a percent) 0.43% 0.58%
Total short-term investments 134,298,000 521,955,000
Trading securities:    
Interest earned 3,100,000 6,000,000
Unrealized gain (loss) 2,400,000 (7,600,000)
Available-for-sale:    
Interest earned 100,000 100,000
Investment redeemed 1,900,000 2,100,000
Contractual maturities of the Company's available for sale debt securities    
One year or less 400,000 500,000
Maturing after five years through ten years   600,000
Due after 10 years 6,100,000 6,200,000
Total debt securities $ 6,500,000 $ 7,300,000