RETIREMENT PLANS: (Details 11) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,432	$ 1,882	$ 2,153
Interest cost	3,621	3,729	3,516
Recognized actuarial loss	5,305	2,839
Net periodic pension cost	2,318	4,118	4,282
Net loss (gain) during the period	5,366	(6,150)	(1,894)
Amortization of prior service cost	(1)	(1)	(1)
Total recognized in other comprehensive income (loss)	4,161	(8,086)	(3,959)
Total recognized net periodic pension cost and other comprehensive income	6,479	(3,968)	323
Postretirement Health Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	53	55	63
Interest cost	172	186	173
Net periodic pension cost	225	241	236
Net loss (gain) during the period	83	(144)	(200)
Total recognized in other comprehensive income (loss)	83	(144)	(200)
Total recognized net periodic pension cost and other comprehensive income	$ 308	$ 97	$ 36