RETIREMENT PLANS: (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Service cost - benefits earned	$ 1,300	$ 1,218	$ 1,388
Interest cost on projected benefit obligation	3,116	3,465	3,194
Expected return on plan assets	(4,198)	(3,585)	(3,962)
Net amortization and deferral	1,968	1,558	1,448
Net periodic pension cost	2,186	2,656	2,068
Net loss (gain) during the period	3,188	6,362	(1,192)
Amortization of prior service cost	1	1	1
Amortization of unrecognized gain (loss)	(1,967)	(1,558)	(1,447)
Total recognized in other comprehensive (income) loss	(1,220)	(4,803)	2,640
Total recognized net periodic pension cost and other comprehensive income	$ 3,406	$ 7,459	$ (572)