EMPLOYEE BENEFIT PLANS - Change in Projected Benefit Obligation, Fair Value of Plan Assets, Funded Status, and Amounts Not Yet Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Group life insurance plan
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 10,978	$ 9,634
Service cost	153	122
Interest cost	366	354
Actuarial (gain)/loss	(1,045)	1,347
Projected benefit obligation at end of year	10,012	10,978	$ 9,634
Change in plan assets:
Benefits paid	(440)	(479)
Qualified Plan | Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Accumulated benefit obligation, end of year	269,802	278,084
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	300,423	265,848
Service cost	13,185	12,011	12,791
Interest cost	9,830	9,846	9,751
Amendments	0	0
Actuarial (gain)/loss	(15,608)	26,539
Benefits paid	(19,701)	(13,821)
Projected benefit obligation at end of year	288,129	300,423	265,848
Change in plan assets:
Fair value of plan assets at beginning of year	260,926	201,843
Actual return on plan assets	(6,070)	29,404
Employer contributions	18,800	43,500
Benefits paid	(19,701)	(13,821)
Fair value of plan assets at end of year	253,955	260,926	201,843
After reflecting FASB guidance:
Funded status	(34,174)	(39,497)
Amounts recognized in the balance sheet:
Other liabilities	(34,174)	(39,497)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain)/loss	10,370	2,850
Prior service cost/(credit)	0	0
Total amounts recognized in AOCI	10,370	2,850
Nonqualified Plan | Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Accumulated benefit obligation, end of year	46,299	50,149
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	54,590	47,802
Service cost	1,415	1,350	1,413
Interest cost	1,436	1,480	1,353
Amendments	0	0
Actuarial (gain)/loss	(2,001)	7,861
Benefits paid	(8,095)	(3,903)
Projected benefit obligation at end of year	47,345	54,590	47,802
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	8,095	3,903
Benefits paid	(8,095)	(3,903)
Fair value of plan assets at end of year	0	0	$ 0
After reflecting FASB guidance:
Funded status	(47,345)	(54,590)
Amounts recognized in the balance sheet:
Other liabilities	(47,345)	(54,590)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain)/loss	9,025	13,521
Prior service cost/(credit)	0	0
Total amounts recognized in AOCI	$ 9,025	$ 13,521