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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Investments          
Trading securities $ 95.4   $ 95.4   $ 600.2
Weighted average interest rate (as a percent)     1.41%   0.71%
Available-for-sale 2.8   $ 2.8   $ 3.3
Weighted average interest rate (as a percent)     0.89%   0.72%
Total short-term investments 98.2   $ 98.2   $ 603.5
Trading securities:          
Interest earned 0.3 $ 0.2 0.7 $ 0.5  
Unrealized gain (loss) at the end of the period 0.4 (0.2) 0.4 (0.2)  
Available-for-sale:          
Investment redeemed 0.2 0.4 0.5 0.6  
Maximum          
Available-for-sale:          
Interest earned $ 0.1 $ 0.1 $ 0.1 $ 0.1