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SHORT-TERM INVESTMENTS: (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Investments    
Trading securities $ 49.2 $ 600.2
Weighted average interest rate (as a percent) 2.20% 0.71%
Available-for-sale $ 2.1 $ 3.3
Weighted average interest rate (as a percent) 0.78% 0.72%
Total short-term investments $ 51.3 $ 603.5
Trading securities:    
Interest earned 1.5 1.5
Unrealized gain (loss) at the end of the period 0.4 (0.1)
Available-for-sale:    
Investment redeemed 1.2 1.6
Contractual maturities of available-for-sale debt securities    
One year or less   0.2
Due after 10 years 2.1 3.1
Total debt securities 2.1 3.3
Maximum    
Available-for-sale:    
Interest earned $ 0.1 $ 0.1