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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Dec. 31, 2022
Short-term investment:      
Trading securities $ 140.3   $ 208.0
Weighted average interest rate (as a percent) 4.70%   4.50%
Available-for-sale $ 0.3   $ 0.3
Weighted average interest rate (as a percent) 0.80%   0.80%
Total $ 140.6   $ 208.3
Trading:      
Interest earned 3.7 $ 0.8  
Unrealized gain (loss) at the end of the period   (0.1)  
Available-for-sale:      
Interest earned 21.3 $ 4.6  
Maximum      
Available-for-sale:      
Interest earned $ 0.1