Annual report pursuant to Section 13 and 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
 12 Months Ended
Apr. 30, 2021
USD ($)
Feb. 01, 2019
USD ($)
shares
Dec. 31, 2021
USD ($)
item
shares
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Accounting Policies [Line Items]          
Number of bank branches | item     130    
Number of ATM's | item     150    
Accruals for uncertain tax positions     $ 0 $ 0  
Impairment charges for goodwill $ 0   0    
Available for sale securities, allowance for credit losses     0    
Goodwill     935,560,000 935,560,000  
Loans held for investment, accrued interest receivable     43,300,000 56,700,000  
HTM securities, Accrued interest receivable     7,000,000.0 6,800,000  
AFS securities, accrued interest receivable     14,500,000 11,900,000  
Cash paid in acquisition     $ 0 $ 0 $ 12,000
Restricted equity securities as a percentage of outstanding borrowings, required by FHLB     3.75% 3.75%  
Restricted equity securities as a percentage of outstanding borrowings, required by FRB     6.00% 6.00%  
Debt security, duration to be placed in nonaccrual status     90 days    
ALLL, reasonable and supportable forecast period     2 years    
Commercial and consumer loans on nonaccrual status, period     90 days    
Liabilities for post retirement benefits payable to other beneficiaries     $ 14,900,000 $ 13,800,000  
Stock options granted | shares     0    
Minimum          
Accounting Policies [Line Items]          
Intangible assets, amortization period (years)     4 years    
Period past due to change TDR status     30 days    
Estimated useful life of bank premises     3 years    
Percentage of fair value of the derivative instruments for the hedge to be highly effective     80.00%    
Maximum          
Accounting Policies [Line Items]          
Intangible assets, amortization period (years)     10 years    
Estimated useful life of bank premises     40 years    
Percentage of fair value of the derivative instruments for the hedge to be highly effective     125.00%    
Access National Bank          
Accounting Policies [Line Items]          
Number of shares equivalent to each share of acquired entity | shares   0.75      
Company's common shares issued | shares   15,842,026      
Cash paid in acquisition   $ 12,000      
Paycheck Protection Program (PPP)          
Accounting Policies [Line Items]          
Loans, recorded investment under the CARES Act     $ 154,700,000 1,200,000,000  
Loan origination fees Under the CARES Act     $ 4,400,000 $ 17,600,000  
Non-real Estate Secured Loans [Member]          
Accounting Policies [Line Items]          
Consumer loans, period for past due status     120 days    
Real Estate Secured Loans [Member]          
Accounting Policies [Line Items]          
Consumer loans, period for past due status     180 days