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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Investments    
Trading securities $ 600.2 $ 333.7
Weighted average interest rate (as a percent) 0.71% 0.78%
Available-for-sale $ 3.3 $ 4.9
Weighted average interest rate (as a percent) 0.72% 0.44%
Total short-term investments $ 603.5 $ 338.6
Trading:    
Interest earned 1.5 4.3
Unrealized gain (loss) at the end of the period (0.1) 2.1
Available-for-sale    
Investment redeemed 1.6 0.8
Contractual maturities of available-for-sale debt securities    
One year or less 0.2 0.3
Maturing after five years through ten years   0.1
Due after 10 years 3.1 4.5
Total debt securities 3.3 4.9
Maximum    
Available-for-sale    
Interest earned $ 0.1 $ 0.1