Fair Value Measurements - Summary of Carrying Values and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financial assets
Securities available for sale	$ 212,266	$ 237,030
Securities held to maturity	499,885	260,139
FHLB Boston stock	4,816	5,734
Recurring Basis \| Carrying Value
Financial assets
Cash and cash equivalents	98,789	75,785
Securities available for sale	212,266	237,030
Securities held to maturity	492,782	247,672
Loans, net	3,246,120	3,117,632
Loans held for sale	911	6,909
FHLB Boston stock	4,816	5,734
Accrued interest receivable	8,983	9,514
Mortgage servicing rights	1,281	1,219
Interest rate contracts	5,801	7,618
Loan level interest rate swaps	27,619	38,415
Risk participation agreements out to counterparties	33	51
Financial liabilities
Deposits	3,764,558	3,403,083
Borrowings	17,244	32,992
Loan level interest rate swaps	27,619	38,415
Risk participation agreements in with counterparties	327	496
Recurring Basis \| Estimated Fair Value
Financial assets
Cash and cash equivalents	98,789	75,785
Securities available for sale	212,266	237,030
Securities held to maturity	499,885	260,139
Loans, net	3,209,085	3,092,021
Loans held for sale	911	7,101
FHLB Boston stock	4,816	5,734
Accrued interest receivable	8,983	9,514
Mortgage servicing rights	1,281	1,219
Interest rate contracts	5,801	7,618
Loan level interest rate swaps	27,619	38,415
Risk participation agreements out to counterparties	33	51
Financial liabilities
Deposits	3,764,508	3,403,832
Borrowings	17,276	34,284
Loan level interest rate swaps	27,619	38,415
Risk participation agreements in with counterparties	$ 327	$ 496

Interest receivable fair value.

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Loan level interest rate swaps assets fair value disclosure.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

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Fair value portion of borrowing from Federal Home Loan Bank (FHLBank).

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Fair value as of the balance sheet date of interest rate derivative assets, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as assets.

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Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

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