BENEFIT PLANS: (Details) $ in Millions	9 Months Ended
	Sep. 30, 2024 USD ($) plan	Sep. 30, 2023 USD ($)
Benefit plans
Number of noncontributory defined benefit pension plans | plan	2
Post retirement defined benefit plans and defined contribution plan
Defined benefit plan, net periodic benefit costs
Service cost	$ 1.9	$ 1.6
Interest cost	2.8	2.6
Expected return on plan assets	(4.7)	(4.2)
Amortization of prior service cost/(credit)	0.2	0.1
Amortization of net loss/(gain)	0.3	0.3
Net periodic benefit cost	0.5	0.4
Post-retirement health care plans
Defined benefit plan, net periodic benefit costs
Interest cost	1.5	1.6
Net periodic benefit cost	$ 1.5	$ 1.6