POSTRETIREMENT BENEFITS (obligations and funded status) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in benefit obligation
Interest cost	$ 2,727	$ 2,626	$ 2,896
Amounts recognized in balance sheets
Other non-current assets	56,743	41,259
Amounts recognized in accumulated other comprehensive loss
Unrecognized prior service credit	12,756	14,178
Unrecognized net actuarial loss	(45,319)	(57,436)
Tax effect	4,157	6,729
Amount recognized in accumulated other comprehensive loss, net of tax	(28,406)	(36,529)
Postretirement benefit plan [Member]
Change in benefit obligation
Benefit obligation, beginning of year	73,717	87,594
Interest cost	2,617	2,529
Net actuarial (gain) loss	5,012	(9,231)
Benefits paid from plan assets and company funds	(8,171)	(7,175)
Benefit obligation, end of year	73,175	73,717	87,594
Change in plan assets
Fair value of plan assets, beginning of year	114,976	127,443
Return on plan assets	21,179	(6,663)
Benefits paid	(6,237)	(5,804)
Fair value of plan assets, end of year	129,918	114,976	127,443
Funded status	56,743	41,259
Amounts recognized in balance sheets
Other non-current assets	56,743	41,259
Accrued liabilities	0	0
Other non-current liabilities	$ 0	0
Amounts recognized in accumulated other comprehensive loss
Weighted-average amortization period, prior service credit	21 years
Amortization period net actuarial loss	14 years 1 month 6 days
Pension plan [Member]
Change in benefit obligation
Benefit obligation, beginning of year	$ 3,148	3,398
Interest cost	111	97
Net actuarial (gain) loss	316	(23)
Benefits paid from plan assets and company funds	(324)	(324)
Benefit obligation, end of year	3,251	3,148	$ 3,398
Change in plan assets
Funded status	(3,251)	(3,148)
Amounts recognized in balance sheets
Other non-current assets	0	0
Accrued liabilities	324	324
Other non-current liabilities	$ 2,927	$ 2,824