EMPLOYEE BENEFIT PLANS (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Company contribution			$ 0
Qualified Pension Plan | Minimum
EMPLOYEE BENEFIT PLANS
Adjusted funding target percentage to be maintained			80.00%
Qualified Pension Plan | 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
Nonqualified Excess Pension Plan | Qualified 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