BENEFIT PLANS, Funded Status and Amounts Recognized in Balance Sheets and AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2009	Jun. 01, 2018
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]
AOCI at year end	$ 34,882	$ 32,837	$ 21,633
Accumulated other comprehensive income (loss), negative postretirement medical plan amendment, arising during period, net of tax				$ 3,500
Stock Balance Plan [Member] | Nonemployee Directors [Member]
Pension Plan [Abstract]
Defined Benefit Plan, Funding Status [Extensible List]	us-gaap:UnfundedPlanMember
Change in benefit obligation [Roll Forward]
Benefit obligation at the beginning of year	$ 100
Benefit obligation at end of year	100	100
Pension Benefits [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at the beginning of year	144,211	147,450
Service cost	5,081	4,561	4,181
Interest cost	6,264	5,676	5,717
Plan amendment / acquisition	0	883
Participant contributions	0	0
Deferred actuarial (gain)/loss	16,292	(4,177)
Benefits paid	(9,764)	(10,182)
Benefit obligation at end of year	162,084	144,211	147,450
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	203,672	217,107
Actual return of plan assets	25,522	(3,858)
Participant contributions	0	0
Employer contributions	7,893	605
Plan acquisition	0	0
Benefits paid	(9,764)	(10,182)
Fair value of plan assets at end of year	227,323	203,672	217,107
Over/(Under) funded status at year end	65,239	59,461
Amounts recognized in the consolidated statement of condition [Abstract]
Other assets	81,930	72,659
Other liabilities	(16,691)	(13,198)
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]
Net loss	41,924	39,410
Net prior service cost (credit)	4,875	4,939
Pre-tax AOCI	46,799	44,349
Taxes	(11,448)	(10,870)
AOCI at year end	35,351	33,479
Pension Benefits [Member] | Nonqualified Plan [Member] | us-gaap_UnfundedPlanMember
Change in plan assets [Roll Forward]
Employer contributions	7,300
Pension Benefits [Member] | Restoration Plan [Member] | Nonqualified Plan [Member] | us-gaap_UnfundedPlanMember
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]
Initial projected benefit obligation	300	100			$ 800
Post-retirement Benefits [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at the beginning of year	1,657	1,785
Service cost	0	0	0
Interest cost	70	69	76
Plan amendment / acquisition	0	0
Participant contributions	35	479
Deferred actuarial (gain)/loss	87	191
Benefits paid	(172)	(867)
Benefit obligation at end of year	1,677	1,657	1,785
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return of plan assets	0	0
Participant contributions	35	479
Employer contributions	137	388
Plan acquisition	0	0
Benefits paid	(172)	(867)
Fair value of plan assets at end of year	0	0	$ 0
Over/(Under) funded status at year end	(1,677)	(1,657)
Amounts recognized in the consolidated statement of condition [Abstract]
Other assets	0	0
Other liabilities	(1,677)	(1,657)
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]
Net loss	641	592
Net prior service cost (credit)	(1,265)	(1,444)
Pre-tax AOCI	(624)	(852)
Taxes	155	210
AOCI at year end	$ (469)	(642)
Supplemental Pension Plans [Member] | Key Executives [Member]
Pension Plan [Abstract]
Defined Benefit Plan, Funding Status [Extensible List]	us-gaap:UnfundedPlanMember
Change in benefit obligation [Roll Forward]
Benefit obligation at the beginning of year	$ 13,200
Benefit obligation at end of year	16,400	13,200
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]
Benefit obligation for defined benefit pension plan prior to plan revaluation	$ 145,400	$ 131,000