Amount of specific reserve associated with troubled debt restructuring.

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Amounts charged-off on TDRs.

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No definition available.

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Consumer Credit Exposures Period Past Due Required To Classify Loans As Special Mention Maximum.

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No definition available.

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Financing receivables commitments to lend additional funds.

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No definition available.

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Financing Receivables, Modifications, CARES Act

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Investment recorded represented by troubled debt restructuring in the current period.

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No definition available.

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- Definition

Maximum exposure to credit risk of loans or receivables.

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No definition available.

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- Definition

Maximum potential amount of future undiscounted payments that could be required under the credit derivative, before reduction for potential recoveries under recourse or collateralization provisions.

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- Definition

Amount, before allowance for credit loss, fee, and loan in process, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

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- Definition

Amount excluding accrued interest, after allowance for credit loss, of financing receivable.

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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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No definition available.

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- Definition

Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

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- Definition

The aggregate amount of the participating mortgage obligation at the end of the accounting period.

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The cash outflow to obtain servicing rights, which contractually entitle the servicer to receive fees and ancillary revenues for performing billing, collection, disbursement and recordkeeping services in connection with a mortgage portfolio. Rights may be obtained via (1) acquisition or assumption of a servicing obligation that does not relate to financial assets of the servicer or its consolidated affiliates; or (2) by originating mortgage loans and then (a) transferring the loans to a Variable Interest Entity (VIE) in a transaction that meets the necessary transfer and classification requirements, or (b) transferring the loans in a transaction that meets the requirements for sale accounting.

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The sum of the carrying amounts, as of the date of the latest financial statement presented, of loans receivable which are owned but transferred to serve as collateral for the payment of related debt obligations, primarily secured borrowings and repurchase agreements, and for which the transferees are not permitted to sell or re-pledge them to an unrelated party.

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Loans and Allowance for Loan Losses - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Changes in lines of credit,resrticted to commercial exposure
Loan syndications, amount	$ 100,000,000
Mortgage loans partially or wholly-owned by others and serviced by the Company	95,800,000	$ 13,500,000
Recorded investment	12,157,281,000	9,593,958,000
Total loans	$ 12,281,510,000	9,730,525,000	$ 8,987,046,000
Maximum number of days required for special mention	90 days
Amount of specific reserve associated with the TDR	$ 3,400,000	3,500,000
Amount of additional commitments to lend to borrowers who have been a party to a TDR	0	0
Outstanding recorded investment of loans that were new to troubled debt restructuring	800,000	3,900,000
Amounts charged-off on TDRs	0	200,000
Century Bancorp, Inc.
Changes in lines of credit,resrticted to commercial exposure
Payments to acquire mortgage servicing rights (MSR)	600,000
Payment Deferral \| CARES Act
Changes in lines of credit,resrticted to commercial exposure
Loan modifications	106,700,000	332,700,000
Commercial and industrial \| Commercial Portfolio Segment
Changes in lines of credit,resrticted to commercial exposure
Total loans	2,960,527,000	1,995,016,000	1,642,184,000
Business banking \| Commercial Portfolio Segment
Changes in lines of credit,resrticted to commercial exposure
Total loans	1,334,694,000	1,339,164,000	$ 771,498,000
FHLB advances
Changes in lines of credit,resrticted to commercial exposure
Carrying value of loans pledged to secure advances from federal banks	2,600,000,000	2,400,000,000
Funds borrowed from federal banks	14,000,000	14,600,000
Federal Reserve Bank Advances
Changes in lines of credit,resrticted to commercial exposure
Carrying value of loans pledged to secure advances from federal banks	784,000,000	884,100,000
Funds borrowed from federal banks	0	0
Nonperforming Loans
Changes in lines of credit,resrticted to commercial exposure
Total loans	9,040,062,000	7,213,518,000
Nonperforming Loans \| Commercial and industrial \| Commercial Portfolio Segment
Changes in lines of credit,resrticted to commercial exposure
Total loans	2,960,527,000	1,995,016,000
Nonperforming Loans \| Business banking \| Commercial Portfolio Segment
Changes in lines of credit,resrticted to commercial exposure
Total loans	1,334,694,000	1,339,164,000
Unrated \| Nonperforming Loans
Changes in lines of credit,resrticted to commercial exposure
Total loans	872,544,000	1,580,852,000
Unrated \| Nonperforming Loans \| Commercial and industrial \| Commercial Portfolio Segment
Changes in lines of credit,resrticted to commercial exposure
Total loans	171,537,000	655,346,000
Unrated \| Nonperforming Loans \| Business banking \| Commercial Portfolio Segment
Changes in lines of credit,resrticted to commercial exposure
Total loans	696,629,000	918,921,000
Unrated \| Nonperforming Loans \| Paycheck Protection Program \| Commercial and industrial \| Commercial Portfolio Segment
Changes in lines of credit,resrticted to commercial exposure
Total loans	112,800,000	568,800,000
Unrated \| Nonperforming Loans \| Paycheck Protection Program \| Business banking \| Commercial Portfolio Segment
Changes in lines of credit,resrticted to commercial exposure
Total loans	218,600,000	457,400,000
Unrated \| Line of Credit \| Commercial and industrial
Changes in lines of credit,resrticted to commercial exposure
Lines of credit, exposure	1,000,000
Unrated \| Line of Credit \| Business banking
Changes in lines of credit,resrticted to commercial exposure
Lines of credit, exposure	100,000
Loss
Changes in lines of credit,resrticted to commercial exposure
Recorded investment	0
Loss \| Nonperforming Loans
Changes in lines of credit,resrticted to commercial exposure
Total loans	0	0
Loss \| Nonperforming Loans \| Commercial and industrial \| Commercial Portfolio Segment
Changes in lines of credit,resrticted to commercial exposure
Total loans	0	0
Loss \| Nonperforming Loans \| Business banking \| Commercial Portfolio Segment
Changes in lines of credit,resrticted to commercial exposure
Total loans	$ 0	$ 0