Annual report pursuant to Section 13 and 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)

v3.20.4
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
 12 Months Ended
Apr. 30, 2020
USD ($)
Feb. 01, 2019
USD ($)
shares
Dec. 31, 2020
USD ($)
item
loan
shares
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Accounting Policies [Line Items]          
Number of bank branches | item     134    
Number of ATM's | item     155    
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     56,700,000    
HTM securities, Accrued interest receivable     6,800,000    
AFS securities, accrued interest receivable     11,900,000    
Cash paid in acquisition     0 12,000 $ 14,304,000
Liabilities for post retirement benefits payable to other beneficiaries     $ 13,800,000 $ 12,100,000  
Stock options granted | shares     0    
HTM securities, additional reserve for allowance for credit losses     $ 0    
Minimum          
Accounting Policies [Line Items]          
Intangible assets, amortization period (years)     4 years    
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     50 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 issued under the CARES Act     $ 1,700,000,000    
Loans, recorded investment under the CARES Act     1,200,000,000    
Loan origination fees Under the CARES Act     $ 17,600,000    
Number of loans forgiven under the CARES Act | loan     3,100    
Loans forgiven under the CARES Act     $ 429,300,000    
Paycheck Protection Program (PPP) | Minimum          
Accounting Policies [Line Items]          
Number of loans processed under the CARES Act | loan     11,000