EMPLOYEE BENEFIT PLANS - Plan summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Nonunion Defined Benefit Pension Plan
Pension and other postretirement benefit plans
Employer contributions		$ 7,710
Noncash charge to pension settlement expense		4,000
Noncash charge to pension settlement expense, tax benefit		0
Postretirement Health Benefit Plan
Pension and other postretirement benefit plans
Employer contributions	$ 615	$ 848
Unrecognized prior service credit, amortization period	9 years