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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2022
Sep. 30, 2021
Dec. 31, 2021
Short-term investment:          
Trading securities         $ 486.5
Weighted average interest rate (as a percent)         0.30%
Available-for-sale $ 0.3   $ 0.3   $ 0.4
Weighted average interest rate (as a percent)     0.80%   0.70%
Total 0.3   $ 0.3   $ 486.9
Trading:          
Interest earned 3.3 $ 0.6 3.9 $ 1.3  
Available-for-sale:          
Interest earned 7.8 1.5 16.7 5.2  
Investment redeemed   0.1 0.1 0.2  
Maximum          
Available-for-sale:          
Interest earned $ 0.1 $ 0.1 $ 0.1 $ 0.1