LOANS AND ALLOWANCE FOR CREDIT LOSSES ON LOANS (Narrative) (Details)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($) loan	Jun. 30, 2021 USD ($) loan	Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($) loan	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
SBA PPP Loans, Total Originations, Number of Loans | loan			3,034	1,620
SBA PPP Loans, Total Originations			$ 244,800,000	$ 102,200,000
Loans and Leases Receivable, Related Parties, Description			At September 30, 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 September 30, 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				$ 400,812,000	$ 460,130,000
Gain on sale of mortgage loans				(11,260,000)	(10,385,000)
Loans held for sale, at fair value (book value of $10,789 and $40,499, respectively)	$ 10,826,000			10,826,000		$ 41,557,000
Allowance for Loan and Lease Losses, Period Increase (Decrease)		$ (32,300,000)		$ 5,600,000
Other industry exposures				0
Temporary Loan Modifications, COVID-Related Loan Modification, Recorded Investment	0			$ 0		26,500,000
Non-Accrual Loans	5,072,000			5,072,000		7,596,000
Loans outstanding	3,315,303,000			3,315,303,000		3,219,822,000
Interest lost on nonaccrual loans	59,000	$ 85,000		202,000	$ 251,000
Loans and Leases Receivable, Excluded Accrued Interest from Amortized Cost	8,000,000			8,000,000		10,200,000
Recorded Investment	2,783,000			2,783,000		3,066,000
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend	$ 0			$ 0		0
Loans modified | loan	0	0		0	0
FHLB advances, general debt obligations, pledged collateral	$ 1,200,000,000			$ 1,200,000,000		1,300,000,000
SBA PPP Loans, 2021 Originations, Outstanding | loan	1,230			1,230
Financing Receivable, Year One, Originated, Current Fiscal Year	$ 796,413,000			$ 796,413,000		716,230,000
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	$ 595,765,000			$ 595,765,000		527,204,000
SBA PPP Loans, 2020 Originations, Number of Contracts Outstanding | loan	120			120
CARES Act
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accrual Loans	$ 1,000,000			$ 1,000,000		457,000
CARES Act | 30 to 89 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans outstanding	310,000			310,000		1,200,000
SBA PPP Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans outstanding	81,959,000			81,959,000		135,095,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accrual Loans	2,576,000			2,576,000		3,477,000
Loans outstanding	1,222,084,000			1,222,084,000		1,054,798,000
Mortgage Loans in Process of Foreclosure, Amount	1,100,000			1,100,000		1,500,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accrual Loans	860,000			860,000		1,607,000
Loans outstanding	352,533,000			352,533,000		381,494,000
Home Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accrual Loans	1,401,000			1,401,000		1,996,000
Loans outstanding	218,797,000			218,797,000		258,573,000
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-Accrual Loans	28,000			28,000		4,000
Loans outstanding	20,253,000			20,253,000		20,392,000
Fixed Rate Residential Mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from the sale of mortgage loans	86,200,000	$ 182,700,000		389,600,000	$ 449,700,000
Gain on sale of mortgage loans	(2,200,000)	(4,300,000)		(11,300,000)	(10,400,000)
Loans held for sale, at fair value (book value of $10,789 and $40,499, respectively)	10,800,000			10,800,000		40,500,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded Investment	80,000			80,000		100,000
Financing Receivable, Year One, Originated, Current Fiscal Year	60,833,000			60,833,000		54,153,000
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	48,063,000			48,063,000		73,897,000
SBA PPP Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	78,721,000			78,721,000		135,095,000
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	3,238,000			3,238,000		0
Nonoperating Income (Expense) | Fixed Rate Residential Mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gain on sale of mortgage loans	216,000	47,000		$ 1,000,000	$ (695,000)
Unrealized gain (loss) on loans held for sale	37,000	$ 1,100,000
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%
Substandard [Member] | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0			$ 0		187,000
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	280,000			280,000		1,012,000
Substandard [Member] | SBA PPP Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0			0		0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0			0		0
Performing Financial Instruments [Member] | Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded Investment	2,600,000			2,600,000		2,800,000
Nonperforming Financial Instruments [Member] | Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Recorded Investment	$ 219,000			$ 219,000		$ 248,000