LOAN PORTFOLIO AND CREDIT QUALITY - Loans by Past Due Status (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	$ 19,862	$ 25,945
Nonaccrual receivables	23,851	16,103
Current Accruing Loans	7,060,596	6,934,656
Total loans	7,104,309	6,976,704
Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	2,359	828
Nonaccrual receivables	4,394	582
Current Accruing Loans	551,590	692,624
Total loans	558,343	694,034
Paycheck Protection Program
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0
Nonaccrual receivables	0	0
Current Accruing Loans	312,356
Total loans	312,356	0
Commercial tax-exempt
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
Nonaccrual receivables	0	0
Current Accruing Loans	442,159	447,927
Total loans	442,159	447,927
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	627	1,420
Nonaccrual receivables	5,261	0
Current Accruing Loans	2,751,487	2,549,854
Total loans	2,757,375	2,551,274
Construction and land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
Nonaccrual receivables	0	0
Current Accruing Loans	159,204	225,983
Total loans	159,204	225,983
Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	15,498	20,171
Nonaccrual receivables	13,780	13,993
Current Accruing Loans	2,648,186	2,804,991
Total loans	2,677,464	2,839,155
Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	1,363	369
Nonaccrual receivables	415	1,525
Current Accruing Loans	75,586	81,763
Total loans	77,364	83,657
Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	15	3,157
Nonaccrual receivables	1	3
Current Accruing Loans	120,028	131,514
Total loans	120,044	134,674
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	14,055	22,758
30-59 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	1,837	828
30-59 Days Past Due | Paycheck Protection Program
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0
30-59 Days Past Due | Commercial tax-exempt
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
30-59 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	136	1,420
30-59 Days Past Due | Construction and land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
30-59 Days Past Due | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	10,960	19,133
30-59 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	1,107	369
30-59 Days Past Due | Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	15	1,008
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	5,807	3,187
60-89 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	522	0
60-89 Days Past Due | Paycheck Protection Program
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0
60-89 Days Past Due | Commercial tax-exempt
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
60-89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	491	0
60-89 Days Past Due | Construction and land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	0
60-89 Days Past Due | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	4,538	1,038
60-89 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	256	0
60-89 Days Past Due | Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Accruing Past Due	0	2,149
Current
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	12,345	9,814
Current | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	3,934	0
Current | Paycheck Protection Program
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0
Current | Commercial tax-exempt
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0	0
Current | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0	0
Current | Construction and land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0	0
Current | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	8,183	9,593
Current | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	228	220
Current | Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0	1
30-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	1,609	1,148
30-89 Days Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	87	241
30-89 Days Past Due | Paycheck Protection Program
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0
30-89 Days Past Due | Commercial tax-exempt
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0	0
30-89 Days Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0	0
30-89 Days Past Due | Construction and land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0	0
30-89 Days Past Due | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	1,521	759
30-89 Days Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0	148
30-89 Days Past Due | Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	1	0
90 Days or Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	9,897	5,141
90 Days or Greater Past Due | Commercial and industrial
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	373	341
90 Days or Greater Past Due | Paycheck Protection Program
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0
90 Days or Greater Past Due | Commercial tax-exempt
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0	0
90 Days or Greater Past Due | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	5,261	0
90 Days or Greater Past Due | Construction and land
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	0	0
90 Days or Greater Past Due | Residential
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	4,076	3,641
90 Days or Greater Past Due | Home equity
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	187	1,157
90 Days or Greater Past Due | Consumer and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual receivables	$ 0	$ 2