ALLOWANCE FOR LOAN LOSSES - Allowance Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	$ 71,982	$ 75,312	$ 74,742
Loans charged-off	(2,830)	(1,229)	(899)
Total recoveries	473	1,463	3,667
Total provision/(credit) for loan losses	31,998	(3,564)	(2,198)
Financing Receivable, Allowance for Credit Loss, Ending Balance	81,238	71,982	75,312
Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	(20,385)
Financing Receivable, Allowance for Credit Loss, Ending Balance		(20,385)
Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	51,597
Financing Receivable, Allowance for Credit Loss, Ending Balance		51,597
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	10,048
Loans charged-off	(1,590)
Total recoveries	170
Total provision/(credit) for loan losses	922
Financing Receivable, Allowance for Credit Loss, Ending Balance	8,985	10,048
Commercial and industrial | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	(565)
Financing Receivable, Allowance for Credit Loss, Ending Balance		(565)
Commercial and Industrial Portfolio Segment, Including Tax Exempt
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	16,064	15,912	11,735
Loans charged-off		(645)	(709)
Total recoveries		891	680
Total provision/(credit) for loan losses		(94)	4,206
Financing Receivable, Allowance for Credit Loss, Ending Balance		16,064	15,912
Paycheck Protection Program
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	0
Loans charged-off	0
Total recoveries	0
Total provision/(credit) for loan losses	159
Financing Receivable, Allowance for Credit Loss, Ending Balance	159	0
Paycheck Protection Program | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	0
Financing Receivable, Allowance for Credit Loss, Ending Balance		0
Commercial tax-exempt
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	6,016
Loans charged-off	0
Total recoveries	0
Total provision/(credit) for loan losses	943
Financing Receivable, Allowance for Credit Loss, Ending Balance	2,550	6,016
Commercial tax-exempt | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	(4,409)
Financing Receivable, Allowance for Credit Loss, Ending Balance		(4,409)
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	40,765	41,934	46,820
Loans charged-off	0	0	(135)
Total recoveries	160	429	2,389
Total provision/(credit) for loan losses	24,691	(1,598)	(7,140)
Financing Receivable, Allowance for Credit Loss, Ending Balance	51,161	40,765	41,934
Commercial real estate | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	(14,455)
Financing Receivable, Allowance for Credit Loss, Ending Balance		(14,455)
Construction and land
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	5,119	6,022	4,949
Loans charged-off	0	0	0
Total recoveries	0	0	0
Total provision/(credit) for loan losses	1,080	(903)	1,073
Financing Receivable, Allowance for Credit Loss, Ending Balance	4,041	5,119	6,022
Construction and land | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	(2,158)
Financing Receivable, Allowance for Credit Loss, Ending Balance		(2,158)
Residential
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	8,857	10,026	9,773
Loans charged-off	0	0	(16)
Total recoveries	0	100	429
Total provision/(credit) for loan losses	3,322	(1,269)	(160)
Financing Receivable, Allowance for Credit Loss, Ending Balance	12,864	8,857	10,026
Residential | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	685
Financing Receivable, Allowance for Credit Loss, Ending Balance		685
Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	778	1,284	835
Loans charged-off	(1,157)	(562)	0
Total recoveries	132	10	1
Total provision/(credit) for loan losses	1,075	46	448
Financing Receivable, Allowance for Credit Loss, Ending Balance	293	778	1,284
Home equity | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	(535)
Financing Receivable, Allowance for Credit Loss, Ending Balance		(535)
Consumer and other
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	399	134	630
Loans charged-off	(83)	(22)	(39)
Total recoveries	11	33	168
Total provision/(credit) for loan losses	(194)	254	(625)
Financing Receivable, Allowance for Credit Loss, Ending Balance	1,185	399	$ 134
Consumer and other | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total Allowance for loan losses, beginning of year	$ 1,052
Financing Receivable, Allowance for Credit Loss, Ending Balance		$ 1,052