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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Investments      
Trading securities $ 279.6   $ 600.2
Weighted average interest rate (as a percent) 0.87%   0.71%
Available-for-sale $ 3.0   $ 3.3
Weighted average interest rate (as a percent) 0.79%   0.72%
Total short-term investments $ 282.6   $ 603.5
Trading:      
Interest earned 0.4 $ 0.3  
Unrealized gain at the end of the period 0.1 2.8  
Available-for-sale:      
Investment redeemed 0.3 0.2  
Maximum      
Available-for-sale:      
Interest earned $ 0.1 $ 0.1