LOANS - Narrative (Details) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($) modification	Dec. 31, 2018 USD ($) modification	Dec. 31, 2017 USD ($) modification
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateral value of pledged loans to Federal Reserve Bank of Boston	$ 144,200
Collateral value of pledged loans for FHLB borrowings	955,700
Loans	$ 2,641,092	$ 2,490,227
Borrower's sustained repayment performance period	6 months
Modifications that subsequently defaulted, number of contracts \| modification	0	0	0
Loan portfolio	$ 2,641,092	$ 2,490,227
Servicing loans for participants	497,200	496,500
Contractually specified servicing fees	1,300	1,300	$ 1,200
Residential real estate:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Foreclosed property	2,200	2,400
Total loans held for sale	6,500	168
Loan portfolio	1,151,857	1,144,698
Net gains on sales of loans	493	121
Residential real estate: \| Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Process of foreclosure	810	1,500
Acquired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	805,862	859,330
Acquired credit impaired	16,600
Certain loans acquired in transfer not accounted for as debt securities, note balance, net	21,800
Financing receivable not considered impaired at time of acquisition	789,200
Loan portfolio	805,862	859,330
Acquired Loans \| Residential real estate:
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	411,170	474,509
Loan portfolio	$ 411,170	$ 474,509
Product Line \| Loan concentration \| Lodging industry
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of loan portfolio	8.60%	9.00%
Loan portfolio	$ 227,000	$ 223,000

Represents the note balance amount of loans, considered impaired, at the time of acquisition.

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Represents the carrying amount of loans, not considered impaired, at the time of acquisition.

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Pledged Assets Separately Reported, Loans Pledged for Federal Reserve Bank, at Fair Value

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Represents the reasonable period of a borrower's sustained repayment performance for consideration of returning the loan to performing status.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Income from mortgage banking activities (fees and commissions) and servicing real estate mortgages, credit cards, and other financial assets held by others net of any related impairment of capitalized service costs. Also includes any premiums received in lieu of regular servicing fees on such loans only as earned over the life of the loans.

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Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

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Carrying amount as of the balance sheet date of all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings or defeasance; includes real and personal property; equity interests in corporations, partnerships, and joint ventures; and beneficial interests in trusts.

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Amount of gain (loss) on sale of financing receivable.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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The face amount of loans that will be sold to other entities.

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The fair value, as of the date of each statement of financial position presented, of loans that are restricted as collateral for the payment of the FHLB debt obligations, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

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The fair value of collateral received that was taken through foreclosure due to default by the original transferor in lieu of collection of the secured receivable.

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Aggregate amount of servicing assets that are subsequently measured at fair value and servicing assets that are subsequently measured using the amortization method.

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