EMPLOYEE BENEFIT PLANS (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Company contribution			$ 18,800,000.0
Qualified Pension Plan | Minimum
EMPLOYEE BENEFIT PLANS
Adjusted funding target percentage to be maintained			80.00%
Qualified Pension Plan | Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	$ 3,006,000	$ 3,348,000	$ 9,888,000	$ 10,044,000
Interest cost on projected benefit obligation	2,577,000	2,191,000	7,371,000	6,573,000
Expected return on plan assets	(4,743,000)	(3,352,000)	(12,795,000)	(10,056,000)
Amortization of prior service cost	0	0	0	0
Amortization of actuarial loss/(gain)	0	0	0	0
Preliminary net periodic benefit cost	840,000	2,187,000	4,464,000	6,561,000
Settlement/curtailment expense	0	0	0	0
Total net periodic benefit costs	840,000	2,187,000	4,464,000	6,561,000
Nonqualified Excess Pension Plan | Pension Plan
EMPLOYEE BENEFIT PLANS
Service cost — benefits earned during the period	288,000	334,000	1,062,000	1,002,000
Interest cost on projected benefit obligation	359,000	297,000	1,077,000	891,000
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	0	0	0	0
Amortization of actuarial loss/(gain)	196,000	118,000	726,000	354,000
Preliminary net periodic benefit cost	843,000	749,000	2,865,000	2,247,000
Settlement/curtailment expense	986,000	0	986,000	0
Total net periodic benefit costs	$ 1,829,000	$ 749,000	$ 3,851,000	$ 2,247,000