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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Short-term investment:          
Trading securities $ 626.4   $ 626.4   $ 410.2
Weighted average interest rate (as a percent)     0.20%   0.40%
Available-for-sale 0.4   $ 0.4   $ 0.6
Weighted average interest rate (as a percent)     0.70%   0.70%
Total 626.8   $ 626.8   $ 410.8
Trading:          
Interest earned 0.6   1.3 $ 0.1  
Available-for-sale:          
Interest earned 1.5 $ 2.8 5.2 14.9  
Investment redeemed 0.1   0.2 0.1  
Maximum          
Available-for-sale:          
Interest earned $ 0.1 $ 0.1 $ 0.1 $ 0.1