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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Investments          
Trading securities $ 98.3   $ 98.3   $ 600.2
Weighted average interest rate (as a percent)     1.34%   0.71%
Available-for-sale 2.5   $ 2.5   $ 3.3
Weighted average interest rate (as a percent)     0.80%   0.72%
Total short-term investments 100.8   $ 100.8   $ 603.5
Trading securities:          
Interest earned 0.3 $ 0.4 1.0 $ 0.9  
Unrealized gain (loss) at the end of the period 0.3   0.3    
Available-for-sale:          
Investment redeemed 0.3 0.2 0.8 0.8  
Maximum          
Trading securities:          
Unrealized gain (loss) at the end of the period   0.1   0.1  
Available-for-sale:          
Interest earned $ 0.1 $ 0.1 $ 0.1 $ 0.1