EMPLOYEE BENEFIT PLANS - Components of Net Periodic Benefit Cost (Details) - Qualified Pension Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Qualified Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	$ 13,277	$ 13,185	$ 12,011
Interest cost on projected benefit obligation	11,380	9,830	9,846
Expected return on plan assets	(18,106)	(17,058)	(13,570)
Amortization of prior service cost	0	0	0
Amortization of actuarial losses	0	0	0
Preliminary net periodic benefit cost	6,551	5,957	8,287
Settlement/curtailment expense	0	0	0
Total net periodic benefit cost	$ 6,551	5,957	8,287
Average remaining service period used	8 years 8 months 8 days
Expected amortization, next fiscal year	$ 1,200
Nonqualified Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	1,148	1,415	1,350
Interest cost on projected benefit obligation	1,572	1,436	1,480
Expected return on plan assets	0	0	0
Amortization of prior service cost	0	0	0
Amortization of actuarial losses	553	969	634
Preliminary net periodic benefit cost	3,273	3,820	3,464
Settlement/curtailment expense	0	1,526	0
Total net periodic benefit cost	$ 3,273	$ 5,346	$ 3,464
Average remaining service period used	7 years 8 months 4 days
Expected amortization, next fiscal year	$ 1,000