RETIREMENT PLANS: (Details 9) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Benefit obligation at January 1	$ 93,233	$ 90,855
Service cost	1,432	1,882	$ 2,153
Interest cost	3,621	3,729	3,516
Actuarial (gain) loss	5,305	2,839
Defined Benefit Plan, Benefit Obligation, Benefits Paid	5,495	6,072
Benefit obligation at December 31	98,096	93,233	90,855
Funded status at December 31	(26,647)	(23,101)
Supplemental Employee Retirement Plans, Defined Benefit
Change in benefit obligation:
Benefit obligation at January 1	4,276	4,383
Service cost	53	55
Interest cost	172	186
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant	75	69
Actuarial (gain) loss	83	(144)
Defined Benefit Plan, Benefit Obligation, Benefits Paid	298	273
Benefit obligation at December 31	4,361	4,276	$ 4,383
Funded status at December 31	$ 4,361	$ 4,276