EMPLOYEE BENEFIT PLANS (Details) - Pension Plan - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Minimum
EMPLOYEE BENEFIT PLANS
Adjusted funding target percentage to be maintained	80.00%
Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	$ 3,114	$ 3,441
Interest cost on projected benefit obligation	2,778	2,397
Expected return on plan assets	(4,463)	(4,026)
Amortization of prior service cost	0	0
Amortization of actuarial loss	0	0
Total net periodic benefit costs	1,429	1,812
Nonqualified Excess Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	285	387
Interest cost on projected benefit obligation	371	359
Expected return on plan assets	0	0
Amortization of prior service cost		0
Amortization of actuarial loss	74	265
Total net periodic benefit costs	$ 730	$ 1,011