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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Dec. 31, 2023
Short-term investment:      
Trading securities $ 329.1   $ 599.1
Weighted average interest rate (as a percent) 5.50%   5.70%
Available-for-sale $ 0.2   $ 0.2
Weighted average interest rate (as a percent) 0.80%   0.80%
Total $ 329.3   $ 599.3
Trading:      
Interest earned 7.7 $ 3.7  
Available-for-sale:      
Interest earned 27.3 21.3  
Maximum      
Available-for-sale:      
Interest earned $ 0.1 $ 0.1