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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Dec. 31, 2022
Short-term investment:          
Trading securities $ 246.8   $ 246.8   $ 208.0
Weighted average interest rate (as a percent)     5.50%   4.50%
Available-for-sale 0.3   $ 0.3   $ 0.3
Weighted average interest rate (as a percent)     0.80%   0.80%
Total 247.1   $ 247.1   $ 208.3
Trading:          
Interest earned 3.3 $ 3.3 7.6 $ 3.9  
Available-for-sale:          
Interest earned 20.8 7.8 65.3 16.7  
Investment redeemed       0.1  
Maximum          
Available-for-sale:          
Interest earned $ 0.1 $ 0.1 $ 0.1 $ 0.1