LOAN PORTFOLIO AND CREDIT QUALITY - Loans by Portfolio Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 7,104,309	$ 6,976,704
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	558,343	694,034
Paycheck Protection Program
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	312,356	0
Commercial tax-exempt
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	442,159	447,927
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,757,375	2,551,274
Construction and land
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	159,204	225,983
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	2,677,464	2,839,155
Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	77,364	83,657
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 120,044	$ 134,674