RETIREMENT PLANS: (Details 7) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
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)
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain) during the period	527	(511)	(255)
Amortization of prior service cost	0	0	0
Amortization of unrecognized gain (loss)	0	(57)	(88)
Total recognized in other comprehensive income (loss)	$ 527	$ (568)	$ (343)