SHORT-TERM INVESTMENTS: (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Short-term investment:
Trading securities	$ 318.6		$ 318.6		$ 599.1
Weighted average interest rate (as a percent)			5.50%		5.70%
Available-for-sale	0.2		$ 0.2		$ 0.2
Weighted average interest rate (as a percent)			0.80%		0.80%
Total	318.8		$ 318.8		$ 599.3
Trading:
Interest earned	8.0	$ 3.3	18.6	$ 7.6
Available-for-sale:
Interest earned	$ 38.3	$ 20.8	92.1	65.3
Maximum
Available-for-sale:
Interest earned			$ 0.1	$ 0.1