RETIREMENT PLANS: (Details 11) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,300	$ 1,218	$ 1,388
Interest cost	3,116	3,465	3,194
Recognized actuarial loss	6,845	14,233
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)
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)
Postretirement Health Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	38	34	42
Interest cost	125	146	131
Amortization of net actuarial loss (gain)	0	(16)	0
Net periodic pension cost	163	164	173
Net loss (gain) during the period	238	626	(872)
Amortization of prior service cost	0	16	0
Total recognized in other comprehensive income (loss)	238	642	(872)
Total recognized net periodic pension cost and other comprehensive income	$ 401	$ 806	$ (699)