EMPLOYEE BENEFIT PLANS (Details) - Qualified Pension Plan - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
EMPLOYEE BENEFIT PLANS
Pension contributions			$ 0
Minimum
EMPLOYEE BENEFIT PLANS
Adjusted funding target percentage to be maintained			80.00%
Nonqualified Excess Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	$ 387,000	$ 334,000	$ 774,000	$ 668,000
Interest cost on projected benefit obligation	359,000	297,000	718,000	594,000
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	0	0	0	0
Amortization of actuarial loss/(gain)	265,000	118,000	530,000	236,000
Preliminary net periodic benefit cost	1,011,000	749,000	2,022,000	1,498,000
Settlement/curtailment expense	0	0	0	0
Total net periodic benefit costs	1,011,000	749,000	2,022,000	1,498,000
Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	3,441,000	3,348,000	6,882,000	6,696,000
Interest cost on projected benefit obligation	2,397,000	2,191,000	4,794,000	4,382,000
Expected return on plan assets	(4,026,000)	(3,352,000)	(8,052,000)	(6,704,000)
Amortization of prior service cost	0	0	0	0
Amortization of actuarial loss/(gain)	0	0	0	0
Preliminary net periodic benefit cost	1,812,000	2,187,000	3,624,000	4,374,000
Settlement/curtailment expense	0	0	0	0
Total net periodic benefit costs	$ 1,812,000	$ 2,187,000	$ 3,624,000	$ 4,374,000