EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
EMPLOYEE BENEFIT PLANS
Defined benefit plan contributions made by employer			$ 17,400
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	$ 3,006	$ 9,342	$ 9,888
Interest cost on projected benefit obligation	2,778	2,577	8,334	7,371
Expected return on plan assets	(4,463)	(4,743)	(13,389)	(12,795)
Amortization of actuarial loss	0	0	0	0
Preliminary net periodic benefit cost	1,429	840	4,287	4,464
Settlement/curtailment expense	0	0	0	0
Total net periodic benefit costs	1,429	840	4,287	4,464
Pension Plan | Nonqualified Excess Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	285	288	855	1,062
Interest cost on projected benefit obligation	371	359	1,113	1,077
Expected return on plan assets	0	0	0	0
Amortization of actuarial loss	74	196	222	726
Preliminary net periodic benefit cost	730	843	2,190	2,865
Settlement/curtailment expense	0	986	0	986
Total net periodic benefit costs	$ 730	$ 1,829	$ 2,190	$ 3,851