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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Dec. 31, 2019
Short-term investment:      
Trading securities $ 40.1   $ 80.0
Weighted average interest rate (as a percent) 1.90%   2.00%
Available-for-sale $ 0.6   $ 0.7
Weighted average interest rate (as a percent) 0.70%   0.70%
Total $ 40.7   $ 80.7
Trading:      
Interest earned 0.7 $ 0.1  
Unrealized gain (loss) at the end of the period   (0.1)  
Available-for-sale:      
Interest earned 7.8 3.7  
Investment redeemed 0.1    
Maximum      
Available-for-sale:      
Interest earned $ 0.1 $ 0.1