LOAN PORTFOLIO AND CREDIT QUALITY - Nonaccrual Loans by Class of Financing Receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual receivables	$ 23,851	$ 16,103
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual receivables	4,394	582
Paycheck Protection Program
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual receivables	0	0
Commercial tax-exempt
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual receivables	0	0
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual receivables	5,261	0
Construction and land
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual receivables	0	0
Residential
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual receivables	13,780	13,993
Home equity
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual receivables	415	1,525
Consumer and other
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual receivables	$ 1	$ 3