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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Dec. 31, 2021
Short-term investment:      
Trading securities $ 406.1   $ 486.5
Weighted average interest rate (as a percent) 0.40%   0.30%
Available-for-sale $ 0.4   $ 0.4
Weighted average interest rate (as a percent) 0.70%   0.70%
Total $ 406.5   $ 486.9
Trading:      
Interest earned 0.8 $ 0.4  
Unrealized gain (loss) at the end of the period (0.1) 0.1  
Available-for-sale:      
Interest earned 4.6 2.4  
Investment redeemed   0.1  
Maximum      
Available-for-sale:      
Interest earned $ 0.1 $ 0.1