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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Short-term investment:          
Trading securities $ 1.0   $ 1.0   $ 213.1
Weighted average interest rate (as a percent)     1.90%   2.20%
Available-for-sale 0.7   $ 0.7   $ 0.7
Weighted average interest rate (as a percent)     0.90%   0.70%
Total 1.7   $ 1.7   $ 213.8
Trading:          
Interest earned   $ 0.1 0.1 $ 0.2  
Unrealized gain (loss) at the end of the period   (0.2)   (0.2)  
Available-for-sale:          
Interest earned 5.1 4.4 13.1 9.8  
Investment redeemed   0.1   0.3  
Maximum          
Available-for-sale:          
Interest earned $ 0.1 $ 0.1 $ 0.1 $ 0.1