EMPLOYEE BENEFIT PLANS - Change in Projected Benefit Obligation, Fair Value of Plan Assets and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Group life insurance plan
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	$ 10,012	$ 10,978
Service cost	124	153
Interest cost	405	366
Actuarial (gain)/loss	2,285	(1,045)
Benefits paid	(404)	(440)
Projected benefit obligation at end of year	12,422	10,012	$ 10,978
Qualified Plan | Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Accumulated benefit obligation, end of year	320,023	269,802
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	288,129	300,423
Service cost	13,277	13,185	12,011
Interest cost	11,380	9,830	9,846
Amendments	0	0
Actuarial (gain)/loss	47,913	(15,608)
Benefits paid	(17,841)	(19,701)
Projected benefit obligation at end of year	342,858	288,129	300,423
Change in plan assets:
Fair value of plan assets at beginning of year	253,955	260,926
Actual return on plan assets	51,308	(6,070)
Employer contributions	17,400	18,800
Benefits paid	(17,841)	(19,701)
Fair value of plan assets at end of year	304,822	253,955	260,926
After reflecting FASB guidance:
Funded status	(38,036)	(34,174)
Amounts recognized in the balance sheet:
Other liabilities	(38,036)	(34,174)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain)/loss	25,082	10,370
Prior service cost/(credit)	0	0
Total amounts recognized in AOCI	25,082	10,370
Nonqualified Plan | Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Accumulated benefit obligation, end of year	49,446	46,299
Change in projected benefit obligation:
Projected benefit obligation at beginning of year	47,345	54,590
Service cost	1,148	1,415	1,350
Interest cost	1,572	1,436	1,480
Amendments	0	0
Actuarial (gain)/loss	4,569	(2,001)
Benefits paid	(3,967)	(8,095)
Projected benefit obligation at end of year	50,667	47,345	54,590
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	3,967	8,095
Benefits paid	(3,967)	(8,095)
Fair value of plan assets at end of year	0	0	$ 0
After reflecting FASB guidance:
Funded status	(50,667)	(47,345)
Amounts recognized in the balance sheet:
Other liabilities	(50,667)	(47,345)
Amounts recognized in accumulated other comprehensive income:
Net actuarial (gain)/loss	13,041	9,025
Prior service cost/(credit)	0	0
Total amounts recognized in AOCI	$ 13,041	$ 9,025