SHORT-TERM INVESTMENTS: (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Short-term investment:
Trading securities	$ 410.2	$ 80.0
Weighted average interest rate (as a percent)	0.40%	2.00%
Available-for-sale	$ 0.6	$ 0.7
Weighted average interest rate (as a percent)	0.70%	0.70%
Total	$ 410.8	$ 80.7
Trading:
Interest earned	0.7	0.6
Available-for-sale:
Interest earned	19.2	21.2	$ 16.0
Investment redeemed	0.1
Contractual maturities of available-for-sale debt securities
Due after 10 years	0.6	0.7
Total debt securities	0.6	0.7
Maximum
Available-for-sale:
Interest earned	$ 0.1	$ 0.1