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, 2019
USD ($)
Dec. 31, 2005
item
Dec. 31, 2019
USD ($)
item
Dec. 31, 2018
USD ($)
Accounting Policies [Line Items]        
Number of bank branches | item     149  
Number of ATM's | item     170  
Number of business acquisitions | item   2    
Accruals for uncertain tax positions     $ 0 $ 0
Impairment charges for goodwill or intangible assets $ 0   0  
Goodwill     $ 935,560,000 $ 727,168,000
Commercial loans on nonaccrual status, period     90 days  
Number of days past due to charge-off real estate secured loans     180 days  
Federal Home Loan Bank requires Bank to maintain percentage of stock equal to outstanding borrowings     4.25% 4.25%
Percentage of Federal Reserve Bank of Richmond reserve     6.00% 6.00%
Future payments, contractual term     6 months  
Period of independent appraisal or valuation     12 months  
Liabilities for post retirement benefits payable to other beneficiaries     $ 12,100,000 $ 10,500,000
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%  
Tax position benefit recognition threshold     50.00%  
Maximum        
Accounting Policies [Line Items]        
Intangible assets, amortization period (years)     10 years  
Period past due to charge off consumer loans non-real estate secured     120 days  
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%