PENSION AND OTHER BENEFIT PLANS - Funded Status and Amounts Recognized in Balance Sheets and AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2009
Amounts recognized in the consolidated statement of condition were:
AOCI at year end	$ 27,204	$ 31,301	$ 15,470
Accumulated other comprehensive income (loss), negative postretirement medical plan amendment, arising during period, net of tax				$ 3,500
Pension Benefits
Change in benefit obligation:
Benefit obligation at the beginning of year	151,361	183,270
Defined Benefit Plan, Service Cost	4,433	4,959	5,920
Defined Benefit Plan, Interest Cost	7,561	5,334	5,036
Plan amendment / acquisition	0	1,851
Participant contributions	0	0
Deferred actuarial (gain)/loss	5,530	(31,759)
Benefits paid	(10,605)	(12,294)
Benefit obligation at end of year	158,280	151,361	183,270
Change in plan assets:
Fair value of plan assets at beginning of year	244,332	290,687
Actual return of plan assets	28,746	(34,967)
Participant contributions	0	0
Employer contributions	4,989	906
Plan acquisition	0	0
Benefits paid	(10,605)	(12,294)
Fair value of plan assets at end of year	267,462	244,332	290,687
Over/(Under) funded status at year end	109,182	92,971
Amounts recognized in the consolidated statement of condition were:
Other assets	123,263	106,986
Other liabilities	(14,081)	(14,015)
Amounts recognized in the consolidated statement of condition were:
Net loss	33,977	38,894
Net prior service cost (credit)	2,292	3,112
Pre-tax AOCI	36,269	42,006
Taxes	(8,955)	(10,351)
AOCI at year end	27,314	31,655
Pension Benefits | Restoration Plan [Member]
Amounts recognized in the consolidated statement of condition were:
Initial projected benefit obligation	400
Post-retirement Benefits
Change in benefit obligation:
Benefit obligation at the beginning of year	1,656	1,529
Defined Benefit Plan, Service Cost	0	0	0
Defined Benefit Plan, Interest Cost	92	68	44
Plan amendment / acquisition	0	536
Participant contributions	0	0
Deferred actuarial (gain)/loss	165	(306)
Benefits paid	(199)	(171)
Benefit obligation at end of year	1,714	1,656	1,529
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return of plan assets	0	0
Participant contributions	0	0
Employer contributions	199	171
Plan acquisition	0	0
Benefits paid	(199)	(171)
Fair value of plan assets at end of year	0	0	$ 0
Over/(Under) funded status at year end	(1,714)	(1,656)
Amounts recognized in the consolidated statement of condition were:
Other assets	0	0
Other liabilities	(1,714)	(1,656)
Amounts recognized in the consolidated statement of condition were:
Net loss	398	255
Net prior service cost (credit)	(549)	(728)
Pre-tax AOCI	(151)	(473)
Taxes	41	119
AOCI at year end	$ (110)	(354)
Supplemental Pension Plans [Member] | Key Executives [Member]
Pension Plan [Abstract]
Defined Benefit Plan, Funding Status [Extensible List]	us-gaap:UnfundedPlanMember
Change in benefit obligation:
Benefit obligation at the beginning of year	$ 14,000
Benefit obligation at end of year	13,600	14,000
Amounts recognized in the consolidated statement of condition were:
Benefit obligation for defined benefit pension plan prior to plan revaluation	$ 144,200	$ 137,300