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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Investments      
Trading securities $ 50.6   $ 49.2
Weighted average interest rate (as a percent) 1.70%   2.20%
Available-for-sale $ 2.1   $ 2.1
Weighted average interest rate (as a percent) 0.80%   0.78%
Total $ 52.7   $ 51.3
Trading:      
Interest earned 0.2 $ 0.4  
Unrealized (loss) gain at the end of the period (0.1) 0.1  
Available-for-sale:      
Investment redeemed   0.3  
Maximum      
Available-for-sale:      
Interest earned 0.1 $ 0.1  
Investment redeemed $ 0.1