SHORT-TERM INVESTMENTS: (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term investment:
Trading securities	$ 213.1	$ 49.5
Weighted average interest rate (as a percent)	2.20%	1.80%
Available-for-sale	$ 0.7	$ 1.0
Weighted average interest rate (as a percent)	0.70%	0.70%
Total	$ 213.8	$ 50.5
Trading:
Interest earned	0.5	0.7
Unrealized gain (loss) at the end of the period	(0.2)	0.1
Available-for-sale:
Interest earned	16.0	5.5	$ 7.1
Investment redeemed	0.3	1.1
Contractual maturities of available-for-sale debt securities
Due after 10 years	0.7	1.0
Total debt securities	0.7	1.0
Maximum
Available-for-sale:
Interest earned	$ 0.1	$ 0.1