EMPLOYEE RETIREMENT PLANS AND POSTRETIREMENT BENEFITS - Projected Benefit Obligation and Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in projected benefit obligation
Projected benefit obligation, beginning of year	$ 83,262	$ 33,178
Acquisition		48,350
Service cost	1,068	272	$ 841
Interest cost	1,716	1,211	802
Actuarial (gain) loss	7,591	(193)
Benefits paid	(1,199)	(1,779)
Translation adjustment	5,302	2,223
Projected benefit obligation, end of year	97,740	83,262	33,178
Changes in fair value of plan assets
Fair value of plan assets, beginning of year	14,903	13,433
Acquisitions		102
Actual return on plan assets	682	380
Contributions	1,827	644
Benefits paid	(993)	(1,176)
Actuarial gain	180	1,064
Translation adjustment	694	456
Fair value of plan assets, end of year	17,293	14,903	$ 13,433
Funded status of plan	$ (80,447)	$ (68,359)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of increase in benefit obligation of defined benefit plan from business combination.

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Amount of increase in plan assets of defined benefit plan from business combination.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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Amount of increase (decrease) in plan assets of defined benefit plan.

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Amount of foreign currency translation gain (loss) which (increases) decreases benefit obligation of defined benefit plan.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of foreign currency translation gain (loss) which increases (decreases) plan assets of defined benefit plan.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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