EMPLOYEE BENEFIT PLANS (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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
Interest cost on projected benefit obligation	2,397,000	2,191,000
Expected return on plan assets	(4,026,000)	(3,352,000)
Amortization of prior service cost	0	0
Amortization of actuarial loss/(gain)	0	0
Preliminary net periodic benefit cost	1,812,000	2,187,000
Settlement/curtailment expense	0	0
Total net periodic benefit costs	1,812,000	2,187,000
Nonqualified Excess Pension Plan | Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	387,000	334,000
Interest cost on projected benefit obligation	359,000	297,000
Expected return on plan assets	0	0
Amortization of prior service cost	0	0
Amortization of actuarial loss/(gain)	265,000	118,000
Preliminary net periodic benefit cost	1,011,000	749,000
Settlement/curtailment expense	0	0
Total net periodic benefit costs	$ 1,011,000	$ 749,000