EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	$ 3,348	$ 2,906	$ 10,044	$ 8,717
Interest cost on projected benefit obligation	2,191	2,737	6,573	8,210
Expected return on plan assets	(3,352)	(3,605)	(10,056)	(10,816)
Amortization of prior service cost	0	0	0	0
Amortization of actuarial losses	0	0	0	0
Preliminary net periodic benefit cost	2,187	2,038	6,561	6,111
Settlement/curtailment expense	0	0	0	0
Total net periodic benefit costs	2,187	2,038	6,561	6,111
Pension contributions			$ 43,500
Defined Benefit Pension Plan | Minimum
EMPLOYEE BENEFIT PLANS
Adjusted funding target percentage to be maintained			80.00%
Excess Benefit Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	334	311	$ 1,002	1,102
Interest cost on projected benefit obligation	297	299	891	1,054
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	0	0	0	0
Amortization of actuarial losses	118	64	354	114
Preliminary net periodic benefit cost	749	674	2,247	2,270
Settlement/curtailment expense	0	635	0	2,135
Total net periodic benefit costs	$ 749	$ 1,309	$ 2,247	$ 4,405