LOANS AND ALLOWANCE FOR CREDIT LOSSES ON LOANS (Narrative) (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) loan	Mar. 31, 2020 USD ($) loan	Dec. 31, 2020 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
SBA PPP Loans, Total Originations, Number of Loans | loan	1,348		3,034
SBA PPP Loans, Total Originations	$ 85,100,000		$ 244,800,000
Loans and Leases Receivable, Related Parties, Description	At March 31, 2021 and December 31, 2020, outstanding loans to certain officers, directors and their associated companies was less than 5% of the Company's shareholders' equity.		At March 31, 2021 and December 31, 2020, outstanding loans to certain officers, directors and their associated companies was less than 5% of the Company's shareholders' equity.
Proceeds from the sale of mortgage loans	$ 198,799,000	$ 70,711,000
Gain on sale of mortgage loans	(6,173,000)	(1,476,000)
Loans held for sale, at fair value (book value of $22,243 and $40,499, respectively)	22,229,000		$ 41,557,000
Allowance for Loan and Lease Losses, Period Increase (Decrease)	$ 2,100,000	35,800,000
Other industry exposures	0
Interest lost on nonaccrual loans	$ 76,000	$ 79,000
Recorded Investment	3,102,000		3,066,000
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend	$ 0		0
Loans modified | loan	0	0
FHLB advances, general debt obligations, pledged collateral	$ 1,300,000,000		1,300,000,000
Temporary Loan Modifications, COVID-Related Loan Modification, Recorded Investment	2,400,000		26,500,000
Loans and Leases Receivable, Excluded Accrued Interest from Amortized Cost	9,600,000		10,200,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Mortgage Loans in Process of Foreclosure, Amount	1,100,000		1,500,000
Fixed Rate Residential Mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from the sale of mortgage loans	192,600,000	$ 69,200,000
Gain on sale of mortgage loans	(6,200,000)	(1,500,000)
Loans held for sale, at fair value (book value of $22,243 and $40,499, respectively)	22,200,000		40,500,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded Investment	95,000		100,000
Nonoperating Income (Expense) | Fixed Rate Residential Mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gain on sale of mortgage loans	(1,100,000)	(565,000)
Unrealized gain (loss) on loans held for sale	$ (14,000)	$ 1,100,000
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Months	18
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Months	24
Non-Residential Building Operators Industry Sector | Loan Concentration Risk | Total Loan Portfolio
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (percentage)	14.00%
Non-Residential Building Operators Industry Sector | Loan Concentration Risk | Commercial Real Estate(1)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk (percentage)	32.00%
Performing Financial Instruments [Member] | Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded Investment	$ 2,600,000		2,800,000
Nonperforming Financial Instruments [Member] | Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded Investment	$ 494,000		$ 248,000