EMPLOYEE BENEFITS - Net Periodic Benefit Cost and Postretirement Benefit Cost (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Retirement plan curtailment expense	$ 163	$ 163	$ 0	$ 0
Loss recognized due to settlement		(215)	0	0
Defined Benefit Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		1,793	1,420	1,548
Interest cost		3,031	3,654	3,392
Expected return on assets		(5,676)	(6,030)	(6,483)
Net loss amortization		2,474	1,827	1,512
Prior service cost (credit) amortization		(14)	(14)	(14)
Retirement plan curtailment expense		151	0	0
Loss recognized due to settlement		0	0	0
Net periodic benefit cost		1,759	857	(45)
Defined Benefit Pension Plan Acquired
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		0	0	0
Interest cost		162	169	163
Expected return on assets		(301)	(293)	(290)
Net loss amortization		10	0	0
Prior service cost (credit) amortization		0	0	0
Retirement plan curtailment expense		0	0	0
Loss recognized due to settlement		215	0	0
Net periodic benefit cost		86	(124)	(127)
Restoration Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		429	339	293
Interest cost		568	713	597
Net loss amortization		551	559	677
Prior service cost (credit) amortization		7	6	7
Retirement plan curtailment expense		12	0	0
Loss recognized due to settlement		0	0	0
Net periodic benefit cost		$ 1,567	$ 1,617	$ 1,574