BENEFIT PLANS - Funded Status and Amounts Recognized in Balance Sheets and AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2009
Change in plan assets:
Fair value of plan assets at beginning of year	$ 1,400
Fair value of plan assets at end of year		$ 1,400
Amounts recognized in the consolidated statement of condition were:
AOCI at year end	15,470	28,873	$ 34,882
Accumulated other comprehensive income (loss), negative postretirement medical plan amendment, arising during period, net of tax				$ 3,500
Additional unamortized actuarial gain	1,100
Pension Benefits [Member]
Change in benefit obligation:
Benefit obligation at the beginning of year	190,361	162,084
Service cost	5,920	5,750	5,081
Interest cost	5,036	5,657	6,264
Plan amendment / acquisition	0	13,598
Participant contributions	0	0
Deferred actuarial (gain)/loss	(4,881)	13,954
Benefits paid	(13,166)	(10,682)
Benefit obligation at end of year	183,270	190,361	162,084
Change in plan assets:
Fair value of plan assets at beginning of year	272,600	227,323
Actual return of plan assets	27,614	33,543
Participant contributions	0	0
Employer contributions	3,639	7,993
Plan acquisition	0	14,423
Benefits paid	(13,166)	(10,682)
Fair value of plan assets at end of year	290,687	272,600	227,323
Over/(Under) funded status at year end	107,417	82,239
Amounts recognized in the consolidated statement of condition were:
Other assets	127,538	101,849
Other liabilities	(20,121)	(19,610)
Amounts recognized in the consolidated statement of condition were:
Net loss	16,977	34,290
Net prior service cost (credit)	3,969	4,348
Pre-tax AOCI	20,946	38,638
Taxes	(5,236)	(9,488)
AOCI at year end	15,710	29,150
Pension Benefits [Member] | Unfunded Plan Member
Change in plan assets:
Fair value of plan assets at end of year	3,500
Pension Benefits [Member] | Nonqualified Plan [Member] | Unfunded Plan Member
Change in plan assets:
Employer contributions	2,900	3,900
Pension Benefits [Member] | Restoration Plan [Member] | Nonqualified Plan [Member] | Unfunded Plan Member
Amounts recognized in the consolidated statement of condition were:
Initial projected benefit obligation	30	30
Post-retirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at the beginning of year	1,718	1,677
Service cost	0	0	0
Interest cost	44	57	70
Plan amendment / acquisition	0	0
Participant contributions	0	0
Deferred actuarial (gain)/loss	(85)	114
Benefits paid	(148)	(130)
Benefit obligation at end of year	1,529	1,718	1,677
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	148	130
Plan acquisition	0	0
Benefits paid	(148)	(130)
Fair value of plan assets at end of year	0	0	$ 0
Over/(Under) funded status at year end	(1,529)	(1,718)
Amounts recognized in the consolidated statement of condition were:
Other assets	0	0
Other liabilities	(1,529)	(1,718)
Amounts recognized in the consolidated statement of condition were:
Net loss	585	715
Net prior service cost (credit)	(907)	(1,086)
Pre-tax AOCI	(322)	(371)
Taxes	82	94
AOCI at year end	$ (240)	(277)
Supplemental Pension Plans [Member] | Key Executives [Member]
Pension Plan [Abstract]
Defined Benefit Plan, Funding Status [Extensible List]	Unfunded Plan Member
Change in benefit obligation:
Benefit obligation at the beginning of year	$ 19,600
Benefit obligation at end of year	19,800	19,600
Amounts recognized in the consolidated statement of condition were:
Benefit obligation for defined benefit pension plan prior to plan revaluation	$ 163,100	$ 170,800