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BENEFIT PLANS - Funded Status and Amounts Recognized in Balance Sheets and AOCI (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2009
Jun. 01, 2018
Change in plan assets [Roll Forward]          
Fair value of plan assets at end of year $ 1,400        
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]          
AOCI at year end 28,873 $ 34,882 $ 32,837    
Accumulated other comprehensive income (loss), negative postretirement medical plan amendment, arising during period, net of tax       $ 3,500  
Additional unamortized actuarial gain 1,100        
Director's Stock Balance Plan [Member] | Directors [Member]          
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 162,084 144,211      
Service cost 5,750 5,081 4,561    
Interest cost 5,657 6,264 5,676    
Plan amendment / acquisition 13,598 0      
Participant contributions 0 0      
Deferred actuarial (gain)/loss 13,954 16,292      
Benefits paid (10,682) (9,764)      
Benefit obligation at end of year 190,361 162,084 144,211    
Change in plan assets [Roll Forward]          
Fair value of plan assets at beginning of year 227,323 203,672      
Actual return of plan assets 33,543 25,522      
Participant contributions 0 0      
Employer contributions 7,993 7,893      
Plan acquisition 14,423 0      
Benefits paid (10,682) (9,764)      
Fair value of plan assets at end of year 272,600 227,323 203,672    
Over/(Under) funded status at year end 82,239 65,239      
Amounts recognized in the consolidated statement of condition [Abstract]          
Other assets 101,849 81,930      
Other liabilities (19,610) (16,691)      
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]          
Net loss 34,290 42,743      
Net prior service cost (credit) 4,348 4,056      
Pre-tax AOCI 38,638 46,799      
Taxes (9,488) (11,448)      
AOCI at year end 29,150 35,351      
Pension Benefits [Member] | Unfunded Plan Member          
Change in plan assets [Roll Forward]          
Fair value of plan assets at end of year 3,500        
Pension Benefits [Member] | Nonqualified Plan [Member] | Unfunded Plan Member          
Change in plan assets [Roll Forward]          
Employer contributions 3,900 7,300      
Pension Benefits [Member] | Restoration Plan [Member] | Nonqualified Plan [Member] | Unfunded Plan Member          
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]          
Initial projected benefit obligation 30 300     $ 800
Post-retirement Benefits [Member]          
Change in benefit obligation [Roll Forward]          
Benefit obligation at the beginning of year 1,677 1,657      
Service cost 0 0 0    
Interest cost 57 70 69    
Plan amendment / acquisition 0 0      
Participant contributions 0 35      
Deferred actuarial (gain)/loss 114 87      
Benefits paid (130) (172)      
Benefit obligation at end of year 1,718 1,677 1,657    
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 0 35      
Employer contributions 130 137      
Plan acquisition 0 0      
Benefits paid (130) (172)      
Fair value of plan assets at end of year 0 0 $ 0    
Over/(Under) funded status at year end (1,718) (1,677)      
Amounts recognized in the consolidated statement of condition [Abstract]          
Other assets 0 0      
Other liabilities (1,718) (1,677)      
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]          
Net loss 715 641      
Net prior service cost (credit) (1,086) (1,265)      
Pre-tax AOCI (371) (624)      
Taxes 94 155      
AOCI at year end $ (277) (469)      
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 $ 16,400        
Benefit obligation at end of year 19,600 16,400      
Amounts recognized in accumulated other comprehensive loss/(income) ("AOCI") [Abstract]          
Benefit obligation for defined benefit pension plan prior to plan revaluation $ 170,800 $ 145,400