BENEFIT PLANS: (Details) $ in Millions	6 Months Ended
	Jun. 30, 2023 USD ($) plan	Jun. 30, 2022 USD ($)
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	$ 1.0	$ 0.9
Interest cost	1.7	1.1
Expected return on plan assets	(2.7)	(1.9)
Amortization of prior service cost / (credit)		0.1
Amortization of net loss (gain)	0.2	0.3
Net periodic benefit cost	0.2	0.5
Post-retirement Health care plans
Defined benefit plan, net periodic benefit costs
Interest cost	1.0	0.8
Amortization of net loss (gain)		0.1
Net periodic benefit cost	$ 1.0	$ 0.9