EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Qualified Pension Plan | Minimum
EMPLOYEE BENEFIT PLANS
Adjusted funding target percentage to be maintained			80.00%
Pension Plan | Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	$ 3,114	$ 3,441	$ 6,228	$ 6,882
Interest cost on projected benefit obligation	2,778	2,397	5,556	4,794
Expected return on plan assets	(4,463)	(4,026)	(8,926)	(8,052)
Amortization of actuarial loss	0	0	0	0
Preliminary net periodic benefit cost	1,429	1,812	2,858	3,624
Settlement/curtailment expense	0	0	0	0
Total net periodic benefit costs	1,429	1,812	2,858	3,624
Pension Plan | Nonqualified Excess Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	285	387	570	774
Interest cost on projected benefit obligation	371	359	742	718
Expected return on plan assets	0	0	0	0
Amortization of actuarial loss	74	265	148	530
Preliminary net periodic benefit cost	730	1,011	1,460	2,022
Settlement/curtailment expense	0	0	0	0
Total net periodic benefit costs	$ 730	$ 1,011	$ 1,460	$ 2,022