SHORT-TERM INVESTMENTS: (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Short-term investment:
Trading securities	$ 245.2	$ 599.1
Weighted average interest rate (as a percent)	4.70%	5.70%
Available-for-sale	$ 0.2	$ 0.2
Weighted average interest rate (as a percent)	0.90%	0.80%
Total	$ 245.3	$ 599.3
Trading:
Interest earned	25.7	21.0
Available-for-sale:
Interest earned	131.4	86.6	$ 35.0
Investment redeemed		0.1
Contractual maturities of available-for-sale debt securities
Due after 10 years	0.2	0.2
Maximum
Available-for-sale:
Interest earned	$ 0.1	$ 0.1