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%