RETIREMENT PLANS: (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Service cost - benefits earned	$ 1,432	$ 1,882	$ 2,153
Interest cost on projected benefit obligation	3,621	3,729	3,516
Expected return on plan assets	(3,940)	(3,429)	(3,452)
Net amortization and deferral	1,205	1,936	2,065
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
Amortization of unrecognized gain (loss)	(1,204)	(1,935)	(2,064)
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