EMPLOYEE BENEFIT PLANS - Postretirement Benefits Other Than Pensions, Actuarial Gains and Losses, and Assumptions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net unrecognized actuarial gains and losses related to the postretirement medical benefit obligation are included in accumulated other comprehensive income
Plan amendments - prior service cost	$ 214
Amortization of prior period cost into other income (expense)	(338)	$ 288	$ (73)
Actuarial gain	(1,341)	456	106
Postretirement Medical Benefit Plans
Net unrecognized actuarial gains and losses related to the postretirement medical benefit obligation are included in accumulated other comprehensive income
Amounts included in accumulated other comprehensive income at beginning of period	375	(369)
Amortization of actuarial loss into income	(342)	367
Amortization of prior period cost into other income (expense)	(79)	(79)
Actuarial gain	(1,341)	456
Amounts included in accumulated other comprehensive income at end of period	$ (1,387)	$ 375	$ (369)
Assumptions used
Weighted-average discount rate	4.10%
Initial health care cost trend (2017)	8.00%
Ultimate health care cost trend	4.50%
Year that the rate reached the ultimate health care cost trend rate	2027
Average remaining service lives of active plan participants (years)	12 years 4 months 6 days
Impact of changes in the assumed health care cost trend rates upon the accumulated postretirement medical benefit obligation and the net periodic postretirement medical benefit expense
1% increase, Accumulated postretirement medical benefit obligation	$ 1,159
1% decrease, Accumulated postretirement medical benefit obligation	(992)
1% increase, Net periodic postretirement medical benefit expense	138
1% decrease, Net periodic postretirement medical benefit expense	$ (116)