LOAN PORTFOLIO AND CREDIT QUALITY - Loan Participation Amounts (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Loans and Leases Receivable Disclosure [Line Items]
Loan participations serviced for others	$ 352,618	$ 230,014
Loans serviced for others	168,110	204,696
Commercial and industrial
Loans and Leases Receivable Disclosure [Line Items]
Loan participations serviced for others	110,589	14,533
Commercial tax-exempt
Loans and Leases Receivable Disclosure [Line Items]
Loan participations serviced for others	17,604	18,101
Commercial real estate
Loans and Leases Receivable Disclosure [Line Items]
Loan participations serviced for others	130,551	121,929
Construction and land
Loans and Leases Receivable Disclosure [Line Items]
Loan participations serviced for others	93,874	75,451
Residential
Loans and Leases Receivable Disclosure [Line Items]
Loans serviced for others	$ 168,110	$ 204,696