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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Short-term investment:          
Trading securities $ 545.3   $ 545.3   $ 410.2
Weighted average interest rate (as a percent)     0.30%   0.40%
Available-for-sale 0.5   $ 0.5   $ 0.6
Weighted average interest rate (as a percent)     0.70%   0.70%
Total 545.8   $ 545.8   $ 410.8
Trading:          
Interest earned 0.2   0.7 $ 0.4  
Available-for-sale:          
Interest earned 1.3 $ 4.3 3.7 12.2  
Investment redeemed     0.1 0.1  
Maximum          
Available-for-sale:          
Interest earned $ 0.1 $ 0.1 $ 0.1 $ 0.1