EMPLOYEE BENEFIT PLANS - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Postretirement Medical Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost with interest to end of year	$ 68	$ 63	$ 78
Interest cost	26	39	44
Amortization of prior service credit	74	74	74
Amortization of gain	(50)	(46)	(8)
Total net periodic benefit cost	118	130	$ 188
Pension Plan | Chemogas Defined Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost with interest to end of year	104	0
Interest cost	20	0
Expected return on plan assets	(14)	0
Amortization of prior service credit	0	0
Amortization of gain	0	0
Total net periodic benefit cost	$ 110	$ 0