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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Short-term investment:      
Trading securities $ 415.4   $ 410.2
Weighted average interest rate (as a percent) 0.30%   0.40%
Available-for-sale $ 0.5   $ 0.6
Weighted average interest rate (as a percent) 0.70%   0.70%
Total $ 415.9   $ 410.8
Trading:      
Interest earned 0.4 $ 0.7  
Unrealized gain (loss) at the end of the period 0.1    
Available-for-sale:      
Interest earned 2.4 7.8  
Investment redeemed 0.1 0.1  
Maximum      
Available-for-sale:      
Interest earned $ 0.1 $ 0.1