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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Short-term investment:          
Trading securities $ 49.9   $ 49.9   $ 49.5
Weighted average interest rate (as a percent)     2.30%   1.80%
Available-for-sale 0.8   $ 0.8   $ 1.0
Weighted average interest rate (as a percent)     0.70%   0.70%
Total 50.7   $ 50.7   $ 50.5
Trading securities:          
Interest earned 0.1 $ 0.2 0.1 $ 0.4  
Unrealized gain (loss) at the end of the period (0.1)   (0.2) (0.1)  
Available-for-sale:          
Interest earned 2.8 1.3 5.4 2.2  
Investment redeemed 0.1 1.0 0.2 1.0  
Maximum          
Trading securities:          
Unrealized gain (loss) at the end of the period   0.1      
Available-for-sale:          
Interest earned $ 0.1 $ 0.1 $ 0.1 $ 0.1