Century Bancorp (CNBKA) 10-K 10 Mar 21 2020 FY Annual report Financial data

Cover Page

Cover Page - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 28, 2021	Jun. 30, 2020
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Registrant Name	CENTURY BANCORP INC
Entity Central Index Key	0000812348
Entity Filer Category	Accelerated Filer
Entity Tax Identification Number	04-2498617
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Trading Symbol	CNBKA
Entity Address, State or Province	MA
Document Annual Report	true
Document Transition Report	false
Entity Well-known Seasoned Issuer	No
Entity File Number	0-15752
Entity Incorporation, State or Country Code	MA
Entity Address, City or Town	MEDFORD
Entity Address, Address Line One	400 MYSTIC AVENUE
Entity Address, Postal Zip Code	02155
City Area Code	781
Local Phone Number	391-4000
Title of 12(b) Security	Class A Common Stock
Security Exchange Name	NASDAQ
Entity Voluntary Filers	No
Entity Public Float			$ 282,567,303
ICFR Auditor Attestation Flag	true
Class A Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		3,656,469
Class B Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		1,911,440

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
ASSETS
Cash and due from banks (Note 2)	$ 136,735	$ 44,420
Federal funds sold and interest-bearing deposits in other banks	237,265	214,273
Total cash and cash equivalents	374,000	258,693
Securities available-for-sale, amortized cost $282,273 in 2020 and $260,924 in 2019 (Notes 3, 9 and 11)	282,448	260,502
Securities held-to-maturity, fair value $2,579,103 in 2020 and $2,361,304 in 2019 (Notes 4 and 11)	2,509,088	2,351,120
Federal Home Loan Bank of Boston, stock at cost	13,361	19,471
Equity securities, cost $1,635 in 2020 and $1,635 in 2019, respectively	1,668	1,688
Loans, net:
Loans, net (Note 5)	2,995,829	2,426,119
Less: allowance for loan losses (Note 6)	35,486	29,585
Net loans	2,960,343	2,396,534
Bank premises and equipment (Note 7)	39,062	33,952
Accrued interest receivable	13,283	13,110
Other assets (Notes 5, 6, 8, 16, 17, 23)	165,581	157,354
Total assets	6,358,834	5,492,424
Deposits:
Demand deposits	1,103,878	712,842
Savings and NOW deposits	1,728,092	1,678,250
Money market accounts	2,074,108	1,453,572
Time deposits (Note 10)	546,143	555,447
Total deposits	5,452,221	4,400,111
Securities sold under agreements to repurchase (Note 11)	232,090	266,045
Other borrowed funds (Note 12)	177,009	370,955
Subordinated debentures (Note 12)	36,083	36,083
Other liabilities (Note 17)	91,022	86,649
Total liabilities	5,988,425	5,159,843
Commitments and contingencies (Notes 18 and 19)
Stockholders' equity (Note 15):
Preferred Stock – $1.00 par value; 100,000 shares authorized; no shares issued and outstanding
Additional paid-in capital	12,292	12,292
Retained earnings	378,699	338,980
Stockholders' equity before adjustment of accumulated other comprehensive income (loss)	396,559	356,840
Unrealized gains(losses) on securities available-for-sale, net of taxes	130	(308)
Unrealized losses on securities transferred to held-to-maturity, net of taxes	(1,221)	(1,812)
Pension liability, net of taxes	(25,059)	(22,139)
Total accumulated other comprehensive loss, net of taxes (Notes 3, 13 and 15)	(26,150)	(24,259)
Total stockholders' equity	370,409	332,581
Total liabilities and stockholders' equity	6,358,834	5,492,424
Class A Common Stock [Member]
Stockholders' equity (Note 15):
Common stock value	3,656	3,651
Total stockholders' equity	3,656	3,651
Class B Common Stock [Member]
Stockholders' equity (Note 15):
Common stock value	1,912	1,917
Total stockholders' equity	$ 1,912	$ 1,917

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized cost	$ 282,273	$ 260,924
Held-to-maturity securities, fair value	2,579,103	2,361,304
Equity securities, amortized cost	$ 1,635	$ 1,635
Preferred stock, par value	$ 1	$ 1
Preferred stock, shares authorized	100,000	100,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Class A Common Stock [Member]
Common stock, par value	$ 1	$ 1
Common stock, shares authorized	10,000,000	10,000,000
Common stock, shares issued	3,655,469	3,650,949
Class B Common Stock [Member]
Common stock, par value	$ 1	$ 1
Common stock, shares authorized	5,000,000	5,000,000
Common stock, shares issued	1,912,440	1,916,960

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
INTEREST INCOME
Loans, taxable	$ 57,554	$ 54,720	$ 46,615
Loans, non-taxable	28,329	33,167	31,936
Securities available-for-sale, taxable	3,264	7,125	6,748
Securities available-for-sale, non-taxable	348	1,087	1,587
Federal Home Loan Bank of Boston dividends	626	953	1,116
Securities held-to-maturity	58,072	58,036	45,556
Federal funds sold, interest-bearing deposits in other banks and short-term investments	843	4,051	3,498
Total interest income	149,036	159,139	137,056
INTEREST EXPENSE
Savings and NOW deposits	8,880	21,183	11,757
Money market accounts	15,012	21,170	13,922
Time deposits	11,573	11,804	10,208
Securities sold under agreements to repurchase	1,376	2,347	976
Other borrowed funds and subordinated debentures	5,366	6,846	7,617
Total interest expense	42,207	63,350	44,480
Net interest income	106,829	95,789	92,576
Provision for loan losses (Note 6)	5,825	1,250	1,350
Net interest income after provision for loan losses	101,004	94,539	91,226
OTHER OPERATING INCOME
Service charges on deposit accounts	8,818	9,220	8,560
Lockbox fees	3,745	3,973	3,274
Brokerage commissions	271	277	348
Net gains on sales of securities	0	61	302
Gains on sales of mortgage loans		412
Other income	6,266	4,456	3,764
Total other operating income	19,100	18,399	16,248
OPERATING EXPENSES
Salaries and employee benefits (Note 17)	45,303	44,014	42,710
Occupancy	6,040	6,246	6,092
Equipment	3,495	3,238	3,132
FDIC assessments	1,174	729	1,471
Other (Note 20)	17,476	17,902	16,288
Total operating expenses	73,488	72,129	69,693
Income before income taxes	46,616	40,809	37,781
Provision for income taxes (Note 16)	4,407	1,110	1,568
Net income	$ 42,209	$ 39,699	$ 36,213
Class A Common Stock [Member]
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	3,653,939	3,633,044	3,608,179
Weighted average number of shares outstanding, diluted	5,567,909	5,567,909	5,567,909
Basic earnings per share	$ 9.15	$ 8.63	$ 7.89
Diluted earnings per share	$ 7.58	$ 7.13	$ 6.50
Class B Common Stock [Member]
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	1,913,970	1,934,865	1,959,730
Weighted average number of shares outstanding, diluted	1,913,970	1,934,865	1,959,730
Basic earnings per share	$ 4.58	$ 4.31	$ 3.95
Diluted earnings per share	$ 4.58	$ 4.31	$ 3.95

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
NET INCOME	$ 42,209	$ 39,699	$ 36,213
Unrealized gains (losses) on securities:
Unrealized holding gains (losses) arising during period	438	(270)	326
Less: reclassification adjustment for gains included in net income		(44)	(217)
Total unrealized gains (losses) on securities	438	(314)	109
Accretion of net unrealized losses transferred during period	591	753	1,086
Defined benefit pension plans:
Amortization of prior service cost and loss included in net periodic benefit cost	1,441	1,053	1,167
Pension liability adjustment, net of tax:
Net (loss) gain	(4,361)	(6,842)	3,770
Total pension liability adjustment	(2,920)	(5,789)	4,937
Other comprehensive (loss) income	(1,891)	(5,350)	6,132
Comprehensive income	$ 40,318	$ 34,349	$ 42,345

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Class A Common Stock [Member]	Class B Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Retained Earnings [Member]Class A Common Stock [Member]	Retained Earnings [Member]Class B Common Stock [Member]	Accumulated Other Comprehensive Loss [Member]
Beginning balance at Dec. 31, 2017	$ 260,297	$ 3,606	$ 1,962	$ 12,292	$ 263,666			$ (21,229)
Net income	36,213				36,213
Other comprehensive income, net of tax:
Unrealized holding gains arising during period, net of taxes	109							109
Accretion of net unrealized losses transferred during the period, net of taxes	1,086							1,086
Pension liability adjustment, net of taxes	4,937							4,937
Adoption of ASU 2018-2, Income Statement-Reporting Comprehensive Income (Topic 220)-Reclassification of Certain Tax Effects from AOCI					3,783			(3,783)
Adoption of ASU 2016-1, Financial Instruments-Overall (Subtopic 825-10) Recognition and Measurement of Financial Assets and Financial Liabilities					29			(29)
Conversion of Class B Common Stock to Class A Common Stock		2	(2)
Cash dividends		(1,732)	(471)			$ (1,732)	$ (471)
Ending balance at Dec. 31, 2018	300,439	3,608	1,960	12,292	301,488			(18,909)
Net income	39,699				39,699
Other comprehensive income, net of tax:
Unrealized holding gains arising during period, net of taxes	(314)							(314)
Accretion of net unrealized losses transferred during the period, net of taxes	753							753
Pension liability adjustment, net of taxes	(5,789)							(5,789)
Conversion of Class B Common Stock to Class A Common Stock		43	(43)
Cash dividends		(1,742)	(465)			(1,742)	(465)
Ending balance at Dec. 31, 2019	332,581	3,651	1,917	12,292	338,980			(24,259)
Net income	42,209				42,209
Other comprehensive income, net of tax:
Unrealized holding gains arising during period, net of taxes	438							438
Accretion of net unrealized losses transferred during the period, net of taxes	591							591
Pension liability adjustment, net of taxes	(2,920)							(2,920)
Conversion of Class B Common Stock to Class A Common Stock		5	(5)
Cash dividends		(1,973)	(517)			$ (1,973)	$ (517)
Ending balance at Dec. 31, 2020	$ 370,409	$ 3,656	$ 1,912	$ 12,292	$ 378,699			$ (26,150)

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Unrealized holding gains arising during period, taxes	$ 159	$ 116	$ 16
Realized net gains		61	302
Accretion of net unrealized losses transferred during the period, taxes	207	269	391
Pension liability adjustment, taxes	$ 1,142	$ 2,263	$ 1,930
Class A Common Stock [Member]
Conversion of Class B Common Stock to Class A Common Stock, shares	4,520	42,620	2,500
Cash dividends, per share	$ 0.54	$ 0.48	$ 0.48
Class B Common Stock [Member]
Conversion of Class B Common Stock to Class A Common Stock, shares	4,520	42,620	2,500
Cash dividends, per share	$ 0.27	$ 0.24	$ 0.24

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 42,209	$ 39,699	$ 36,213
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Gain on sales of mortgage loans		(412)
Loss on sales of other real estate owned		79
Net gains on sales of securities	0	(61)	(302)
Net (gain) loss on equity securities	20	(92)	67
Provision for loan losses	5,825	1,250	1,350
Deferred tax (expense) benefit	(1,089)	(2,135)	(1,766)
Net depreciation and amortization	(1,070)	(2,382)	885
(Increase) decrease in accrued interest receivable	(173)	1,296	(3,227)
Decrease in other assets	(413)	8,532	2,326
(Decrease) increase in other liabilities	(934)	2,075	5,242
Net cash provided by operating activities	44,375	47,849	40,788
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from redemptions of Federal Home Loan Bank of Boston stock	10,836	14,380	18,388
Purchase of Federal Home Loan Bank of Boston stock	(4,726)	(15,877)	(14,583)
Proceeds from calls/maturities of securities available-for-sale	86,691	144,739	215,406
Proceeds from sales of securities available-for-sale		16,285	27,517
Purchase of securities available-for-sale	(107,744)	(85,123)	(183,588)
Proceeds from calls/maturities of securities held-to-maturity	902,981	458,915	234,741
Purchase of securities held-to-maturity	(1,056,023)	(757,997)	(576,140)
Proceeds from sales of securities held-to-maturity	3,900	1,193
Proceeds from life insurance policies		5,461	375
Proceeds from sales of portfolio loans		22,120
Net increase in loans	(569,595)	(162,415)	(110,874)
Bank owned life insurance purchases	(8,731)	(33,664)
Proceeds from sales of other real estate owned		2,146
Capital expenditures	(8,376)	(13,144)	(3,601)
Net cash used in investing activities	(750,787)	(402,981)	(392,359)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net decrease in time deposit accounts	(9,304)	(5,132)	(64,782)
Net increase (decrease) in demand, savings, money market and NOW deposits	1,061,414	(1,721)	554,779
Cash dividends	(2,490)	(2,207)	(2,203)
Net (decrease) increase in securities sold under agreements to repurchase	(33,955)	111,805	(4,750)
Net (decrease) increase in other borrowed funds	(193,946)	168,577	(145,400)
Net cash provided by financing activities	821,719	271,322	337,644
Net increase (decrease) in cash and cash equivalents	115,307	(83,810)	(13,927)
Cash and cash equivalents at beginning of period	258,693	342,503	356,430
Cash and cash equivalents at end of period	374,000	258,693	342,503
Cash paid (received) during the period for:
Interest	42,655	63,345	44,289
Income tax refunds		(6,504)
Income taxes	4,300		590
Change in unrealized losses on securities available-for-sale, net of taxes	438	(314)	109
Change in unrealized losses on securities transferred to held-to-maturity, net of taxes	591	753	1,086
Pension liability adjustment, net of taxes	$ (2,920)	$ (5,789)	4,937
Transfer of loans to other real estate owned			$ 2,225

Summary of Significant Accounti

Summary of Significant AccountingPolicies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	1. Summary of Significant Accounting Policies BASIS OF FINANCIAL STATEMENT PRESENTATION The consolidated financial statements include the accounts of Century Bancorp, Inc. (the “Company”) and its wholly owned subsidiary, Century Bank and Trust Company (the “Bank”). The consolidated financial statements also include the accounts of the Bank’s wholly owned subsidiaries, Century Subsidiary Investments, Inc. (“CSII”), Century Subsidiary Investments, Inc. II (“CSII II”), Century Subsidiary Investments, Inc. III (“CSII III”) and Century Financial Services Inc. (“CFSI”). CSII, CSII II, and CSII III are engaged in buying, selling, and holding investment securities. CFSI has the power to engage in financial agency, securities brokerage, and investment and financial advisory services and related securities credit. The Company also owns 100% of Century Bancorp Capital Trust II (“CBCT II”). The entity is an unconsolidated subsidiary of the Company. All significant intercompany accounts and transactions have been eliminated in consolidation. The Company provides a full range of banking services to individual, business, and municipal customers in Massachusetts, New Hampshire, Rhode Island, Connecticut, and New York. As a bank holding company, the Company is subject to the regulation and supervision of the Federal Reserve Board. The Bank, a state chartered financial institution, is subject to supervision and regulation by applicable state and federal banking agencies, including the Federal Reserve Board, the Federal Deposit Insurance Corporation (the “FDIC”) and the Commonwealth of Massachusetts Commissioner of Banks. The Bank is also subject to various requirements and restrictions under federal and state law, including requirements to maintain reserves against deposits, restrictions on the types and amounts of loans that may be granted and the interest that may be charged thereon, and limitations on the types of investments that may be made and the types of services that may be offered. Various consumer laws and regulations also affect the operations of the Bank. In addition to the impact of regulation, commercial banks are affected significantly by the actions of the Federal Reserve Board as it attempts to control the money supply and credit availability in order to influence the economy. All aspects of the Company’s business are highly competitive. The Company faces aggressive competition from other lending institutions and from numerous other providers of financial services. The Company has one reportable operating segment. The financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America and general practices within the banking industry. In preparing the financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the balance sheet and revenues and expenses for the period. Actual results could differ from those estimates. Material estimates that are susceptible to change in the near term relate to the allowance for loan losses. Management believes that the allowance for loan losses is adequate based on a review of factors, including historical charge-off FAIR VALUE MEASUREMENTS The Company follows FASB ASC 820-10, Fair Value Measurements and Disclosures, 820-10 value. The three broad levels of the hierarchy are as follows: Level I—Quoted prices are available in active markets for identical assets or liabilities as of the reported date. The type of financial instruments included in Level I are highly liquid cash instruments with quoted prices, such as listed equities and money market securities, as well as listed derivative instruments. Level II—Pricing inputs are other than quoted prices in active markets, which are either directly or indirectly observable as of the reported date. The nature of these financial instruments includes cash instruments for which quoted prices are available but traded less frequently, derivative instruments whose fair value has been derived using a model where inputs to the model are directly observable in the market or can be derived principally from or corroborated by observable market data, and instruments that are fair valued using other financial instruments, the parameters of which can be directly observed. Instruments that are generally included in this category are G-7 Level III—These instruments have little to no pricing observability as of the reported date. These financial instruments do not have two-way CASH AND CASH EQUIVALENTS For purposes of reporting cash flows, cash equivalents include highly liquid assets with an original maturity of three months or less. Highly liquid assets include cash and due from banks, federal funds sold and certificates of deposit. INVESTMENT SECURITIES Debt securities that the Company has the positive intent and ability to hold to maturity are classified as held-to-maturity held-to-maturity available-for-sale Premiums and discounts on investment securities are amortized or accreted into income by use of the level-yield method. Gains and losses on the sale of investment securities are recognized on the trade date on a specific identification basis. Management also considers the Company’s capital adequacy, interest-rate risk, liquidity, and business plans in assessing whether it is more likely than not that the Company will sell or be required to sell the investment securities before recovery. Other-than-temporary-impairment (OTTI) arises when a security’s fair value is less than its amortized cost and, based on specific factors, the loss is considered OTTI. If the Company determines that a decline in fair value is OTTI and that it is more likely than not that the Company will not sell or be required to sell the investment security before recovery of its amortized cost, the credit portion of the impairment loss is recognized in the Company’s consolidated statement of income and the noncredit portion is recognized in accumulated other comprehensive income. The credit portion of the OTTI impairment represents the difference between the amortized cost and the present value of the expected future cash flows of the investment security. If the Company determines that a decline in fair value is OTTI and it is more likely than not that it will sell or be required to sell the investment security before recovery of its amortized cost, the entire difference between the amortized cost and the fair value of the security will be recognized in the Company’s consolidated statement of income. The transfer of a security between categories of investments shall be accounted for at fair value. For a debt security transferred into the held-to-maturity available-for-sale held-to-maturity The sale of a security held-to-maturity FEDERAL HOME LOAN BANK STOCK The Bank, as a member of the Federal Home Loan Bank of Boston (“FHLBB”), is required to maintain an investment in capital stock of the FHLBB. Based on redemption provisions, the stock has no quoted market value and is carried at cost. At its discretion, the FHLBB may declare dividends on the stock. The Company reviews for impairment based on the ultimate recoverability of the cost basis of the stock. As of December 31, 2020, no impairment has been recognized. LOANS HELD FOR SALE Loans originated and intended for sale in the secondary market are carried at the lower of cost or estimated fair value in the aggregate. Net unrealized losses, if any, are recognized through a valuation allowance by charges to income. LOANS Loans are stated at the principal amount outstanding, net of amounts charged off, unamortized premiums or discounts, and deferred loan fees or costs. Interest on loans is recognized based on the daily principal amount outstanding. Accrual of interest is discontinued when loans become ninety days delinquent unless the collateral is sufficient to cover both principal and interest and the loan is in the process of collection. Past-due Loan origination fees and related direct loan origination costs are offset, and the resulting net amount is deferred and amortized over the life of the related loans using the level-yield method. Prepayments are not initially considered when amortizing premiums and discounts. The Bank measures impairment for impaired loans at either the fair value of the loan, the present value of the expected future cash flows discounted at the loan’s effective interest rate or the fair value of the collateral if the loan is collateral dependent. This method applies to all loans, uncollateralized as well as collateralized, except large groups of smaller-balance homogeneous loans such as residential real estate and consumer loans that are collectively evaluated for impairment. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off uncollectible. Management considers the payment status, net worth and earnings potential of the borrower, and the value and cash flow of the collateral as factors to determine if a loan will be paid in accordance with its contractual terms. Management does not set any minimum delay of payments as a factor in reviewing for impaired classification. Loans are charged-off in-substance pre-modification TRANSFERS OF FINANCIAL ASSETS Transfers of financial assets, typically residential mortgages, and loan participations for the Company, are accounted for as sales when control over the assets has been surrendered. Control over transferred assets is deemed to be surrendered when (1) the assets have been isolated from the Company, (2) the transferee obtains the right (free of conditions that constrain it from taking advantage of that right) to pledge or exchange the transferred assets, and (3) the Company does not maintain effective control over the transferred assets. ACQUIRED LOANS In accordance with FASB ASC 310-30, Loans and Debt Securities Acquired with Deteriorated Credit Quality No. 03-3, loan’s contractually required payments received in excess of the amount of its cash flows expected at acquisition, or nonaccretable discount, is not accreted into income. FASB ASC 310-30 Loans which, at acquisition, do not have evidence of deterioration of credit quality since origination are outside the scope of FASB ASC 310-30. When a loan is paid off, the excess of any cash received over the net investment is recorded as interest income. In addition to the amount of purchase discount that is recognized at that time, income may include interest owed by the borrower prior to the Company’s acquisition of the loan, interest collected if on nonperforming status, prepayment fees and other loan fees. NONPERFORMING ASSETS In addition to nonperforming loans, nonperforming assets include other real estate owned. Other real estate owned is comprised of properties acquired through foreclosure or acceptance of a deed in lieu of foreclosure. Other real estate owned is recorded initially at the lower of cost or the estimated fair value less costs to sell. When such assets are acquired, the excess of the loan balance over the estimated fair value of the asset is charged to the allowance for loan losses. An allowance for losses on other real estate owned is established by a charge to earnings when, upon periodic evaluation by management, further declines in the estimated fair value of properties have occurred. Such evaluations are based on an analysis of individual properties as well as a general assessment of current real estate market conditions. Holding costs and rental income on properties are included in current operations, while certain costs to improve such properties are capitalized. Gains and losses from the sale of other real estate owned are reflected in earnings when realized.​​​​​​​ ALLOWANCE FOR LOAN LOSSES The allowance for loan losses is based on management’s evaluation of the quality of the loan portfolio and is used to provide for losses resulting from loans that ultimately prove uncollectible. The components of the allowance for loan losses represent estimates based upon Accounting Standards Codification (“ASC”) Topic 450, contingencies, and ASC Topic 310 Receivables. ASC Topic 450 applies to homogenous loan pools such as consumer installment, residential mortgages, consumer lines of credit and commercial loans that are not individually evaluated for impairment under ASC Topic 310. In determining the level of the allowance, periodic evaluations are made of the loan portfolio, which takes into account factors such as the characteristics of the loans, loan status, financial strength of the borrowers, value of collateral securing the loans and other relevant information sufficient to reach an informed judgment. The allowance is increased by provisions charged to income and reduced by loan charge-offs, net of recoveries. Management maintains an allowance for loan losses to absorb losses inherent in the loan portfolio. The allowance is based on assessments of the probable estimated losses inherent in the loan portfolio. Management’s methodology for assessing the appropriateness of the allowance consists of several key elements, which include the specific allowances, if appropriate, for identified problem loans, formula allowance, and possibly an unallocated allowance. Arriving at an appropriate level of allowance for loan losses necessarily involves a high degree of judgment. While management uses available information in establishing the allowance for loan losses, future adjustments to the allowance may be necessary if economic conditions differ substantially from the assumptions used in making the evaluations. Loans are charged-off Under ASC Topic 310, a loan is impaired, based upon current information and in management’s opinion, when it is probable that the loan will not be repaid according to its original contractual terms, including both principal and interest, or if a loan is designated as a Troubled Debt Restructuring (“TDR”). Specific allowances for loan losses entail the assignment of allowance amounts to individual loans on the basis of loan impairment. Under this method, loans are selected for evaluation based upon a change in internal risk rating, occurrence of delinquency, loan classification or nonaccrual status. A specific allowance amount is allocated to an individual loan when such loan has been deemed impaired and when the amount of a probable loss is able to be estimated on the basis of: (a) present value of anticipated future cash flows, (b) the loan’s observable fair market price or (c) fair value of collateral if the loan is collateral dependent. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off In estimating probable loan loss under ASC Topic 450, management considers numerous factors, including historical charge-offs and subsequent recoveries. The formula allowances are based on evaluations of homogenous loans to determine the allocation appropriate within each portfolio segment. Formula allowances are based on internal risk ratings or credit ratings from external sources. Individual loans within the commercial and industrial, commercial real estate and real estate construction loan portfolio segments are assigned internal risk ratings to group them with other loans possessing similar risk characteristics. Changes in risk grades affect the amount of the formula allowance. Risk grades are determined by reviewing current collateral value, financial information, cash flow, payment history and other relevant facts surrounding the particular credit. On these loans, the formula allowances are based on the risk ratings, the historical loss experience, and the loss emergence period. Historical loss data and loss emergence periods are developed based on the Company’s historical experience. For larger loans with available external credit ratings, these ratings are utilized rather than the Company’s risk ratings. The historical loss factor and loss emergence periods for these loans are based on data published by the rating agencies for similar credits as the Company has limited internal historical data. For the residential real estate and consumer loan portfolios, the formula allowances are calculated by applying historical loss experience and the loss emergence period to the outstanding balance in each loan category. Loss factors and loss emergence periods are based on the Company’s historical net loss experience. Additional allowances are added to portfolio segments based on qualitative factors. Management considers potential factors identified in regulatory guidance. Management has identified certain qualitative factors, which could impact the degree of loss sustained within the portfolio. These include market risk factors and unique portfolio risk factors that are inherent characteristics of the Company’s loan portfolio. Market risk factors may consist of changes to general economic and business conditions, such as unemployment and GDP that may impact the Company’s loan portfolio customer base in terms of ability to repay and that may result in changes in value of underlying collateral. Unique portfolio risk factors may include the outlooks for business segments in which the Company’s borrowers operate and loan size. The potential ranges for qualitative factors are based on historical volatility in losses. The actual amount utilized is based on management’s assessment of current conditions. After considering the above components, an unallocated component may be generated to cover uncertainties that could affect management’s estimate of probable losses. These uncertainties include the effects of loans in new geographical areas and new industries. The unallocated component of the allowance reflects the margin of imprecision inherent in the underlying assumptions used in the methodologies for estimating allocated and general reserves in the portfolio.​​​​​​​ BANK PREMISES AND EQUIPMENT Bank premises and equipment are stated at cost less accumulated depreciation and amortization. Land is stated at cost. Depreciation is computed using the straight-line method over the estimated useful lives of the assets or the terms of leases, if shorter. It is general practice to charge the cost of maintenance and repairs to operations when incurred; major expenditures for improvements are capitalized and depreciated. GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS Goodwill represents the excess of the cost of an acquisition over the fair value of the net assets acquired. Goodwill is not subject to amortization. Identifiable intangible assets consist of core deposit intangibles and are assets resulting from acquisitions that are being amortized over their estimated useful lives. Goodwill and identifiable intangible assets are included in other assets on the consolidated balance sheets. The Company tests goodwill for impairment on an annual basis, or more often if events or circumstances indicate there may be impairment. Goodwill impairment testing is performed at the segment (or “reporting unit”) level. Currently, the Company’s goodwill is evaluated at the entity level as there is only one reporting unit. Goodwill is assigned to reporting units at the date the goodwill is initially recorded. Once goodwill has been assigned to reporting units, it no longer retains its association with a particular acquisition, and all of the activities within a reporting unit, whether acquired or organically grown, are available to support the value of the goodwill. Goodwill impairment is evaluated by first assessing qualitative factors (events and circumstances) to determine whether it is more likely than not (meaning a likelihood of more than 50 percent) that the fair value of a reporting unit is less than its carrying amount. If, after considering all relevant events and circumstances, an entity determines it is not more​​​​​​​ likely than not that the fair value of a reporting unit is less than its carrying amount, then performing the quantitative impairment test will be unnecessary.​​​​​​​​​​​​​​ The quantitative impairment test, used to identify potential impairment, involves comparing each reporting unit’s fair value to its carrying value including goodwill. If the fair value of a reporting unit exceeds its carrying value, applicable goodwill is considered not to be impaired. If the carrying value exceeds fair value, the difference is recorded as impairment. SERVICING The Company services mortgage loans for others. Mortgage servicing assets are recognized as separate assets when rights are acquired through purchase or through sale of financial assets. Fair value is determined using prices for similar assets with similar characteristics, when available, or based upon discounted cash flows using market-based assumptions. The valuation model incorporates assumptions that market participants would use in estimating future net servicing income, such as the cost to service, the discount rate, an inflation rate, ancillary income, prepayment speeds and default rates and losses. Capitalized servicing rights are reported in other assets and are amortized into loan servicing fee income in proportion to, and over the period of, the estimated future net servicing income of the underlying financial assets. Servicing assets are evaluated for impairment based upon the fair value of the rights as compared to amortized cost. Impairment is determined by stratifying rights by predominant risk characteristics, such as interest rates and terms. Impairment is recognized through a valuation allowance for an individual stratum, to the extent that fair value is less than the capitalized amount for the stratum. Changes in the valuation allowance are reported in loan servicing fee income. INCOME TAXES The Company uses the asset and liability method in accounting for income taxes. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which temporary differences are expected to be recovered or settled. Under this method, the effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. The Company accounts for uncertain tax positions in accordance with FASB ASC 740. The Company classifies interest resulting from underpayment of income taxes as income tax expense in the first period the interest would begin accruing according to the provisions of the relevant tax law. The Company classifies penalties resulting from underpayment of income taxes as income tax expense in the period for which the Company claims or expects to claim an uncertain tax position or in the period in which the Company’s judgment changes regarding an uncertain tax position. Also, for tax years beginning after December 31, 2017, the corporate Alternative Minimum Tax (“AMT”) has been repealed. For 2018 through 2021, the AMT credit carryforward can offset regular tax liability and is refundable in an amount equal to 50% (100% for 2021) of the excess of the minimum tax credit for the tax year over the amount of the credit allowable for the year against regular tax liability. On March 27, 2020, the Coronavirus Aid, Relief and Economic Security (CARES) Act was signed into law. As a result of the CARES Act, the full balance of the AMT credit was refunded during 2020. The Tax Act also contains other provisions that may affect the Company currently or in future years. Among these are changes to the deductibility of meals and entertainment, the deductibility of executive compensation, the dividend received deduction and net operating loss carryforwards. Tax Act changes for individuals include lower tax rates, mortgage interest and state and local tax limitations as well as an increase in the standard deduction, among others. EARNINGS PER SHARE (“EPS”) Class A and Class B shares participate equally in undistributed earnings. Under the Company’s Articles of Organization, the holders of Class A Common Stock are entitled to receive dividends per share equal to at least 200% of dividends paid, if any, from time to time, on each share of Class B Common Stock. Diluted EPS includes the dilutive effect of common stock equivalents; basic EPS excludes all common stock equivalents. The Company utilizes the two class method for reporting EPS. The two-class PENSION The Company provides pension benefits to certain of its employees under a noncontributory, defined benefit plan, which is funded on a current basis in compliance with the requirements of the Employee Retirement Income Security Act of 1974 (“ERISA”) and recognizes costs over the estimated employee service period. The Company also has a Supplemental Executive Insurance/Retirement Plan (“the Supplemental Plan”), which is limited to certain officers and employees of the Company. The Supplemental Plan is accrued on a current basis and recognizes costs over the estimated employee service period. Executive officers of the Company or its subsidiaries who have at least one year of service may participate in the Supplemental Plan. The Supplemental Plan is voluntary. Individual life insurance policies, which are owned by the Company, are purchased covering the life of each participant. Prior to December 31, 2019, the Company utilized a full yield curve approach in the estimation of the service and interest components by applying the specific spot rates along the yield curve used in the determination of the benefit obligation to the underlying projected cash flows. Effective December 31, 2019, the discount rate is determined by preparing an analysis of the respective plan’s expected future cash flows and high-quality fixed-income investments currently available and expected to be available during the period to maturity of the benefits. LEASING A right-of-use For real estate leases, lease components and non-lease non-interest RECENT ACCOUNTING DEVELOPMENTS Recently Adopted Accounting Standards Updates In August 2018, Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2018-15, Internal Use Software (Subtopic 350-40): internal-use In August 2018, FASB issued ASU 2018-13, In January 2017, the FASB issued ASU 2017-04, Securities and Exchange Commission (SEC) Ruling: In August 2018, the SEC issued a final rule that amends certain of the Commission’s disclosure requirements “that have become redundant, duplicative, overlapping, outdated, or superseded, in light of other Commission disclosure requirements, U.S. GAAP, or changes in the information environment.” The financial reporting implications of year-to-date year-to-date

Cash and Due from Banks

Cash and Due from Banks	12 Months Ended
	Dec. 31, 2020
Cash and Cash Equivalents [Abstract]
Cash and Due from Banks	2. Cash and Due from Banks The Company is required to maintain a portion of its cash and due from banks as a reserve balance under the Federal Reserve Act. Such reserve is calculated based upon deposit levels and amounted to $0 at December 31, 2020, and $0 at December 31, 2019.

Securities Available-for-Sale

Securities Available-for-Sale	12 Months Ended
	Dec. 31, 2020
Text Block [Abstract]
Securities Available-for-Sale	3. Securities Available-for-Sale December 31, 2020 December 31, 2019 Amortized Gross Gross Estimated Amortized Gross Gross Estimated (dollars in thousands) SBA Backed Securities $ 44,328 $ — $ 289 $ 44,039 $ 54,331 $ 23 $ 143 $ 54,211 U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 177,239 819 317 177,741 184,580 139 532 184,187 Privately Issued Residential Mortgage-Backed Securities 330 2 4 328 397 1 2 396 Obligations Issued by States and Political Subdivisions 52,276 — — 52,276 18,016 60 — 18,076 Other Debt Securities 8,100 24 60 8,064 3,600 51 19 3,632 Total $ 282,273 $ 845 $ 670 $ 282,448 $ 260,924 $ 274 $ 696 $ 260,502 Included in SBA Backed Securities and U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities are securities at fair value pledged to secure public deposits and repurchase agreements amounting to $183,269,000 and $186,245,000 at December 31, 2020 and 2019, respectively. Also included in securities available-for-sale available-for-sale Debt securities of U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities primarily refer to debt securities of Fannie Mae and Freddie Mac. The following table shows the estimated maturity distribution of the Company’s securities available-for-sale Amortized Fair Value (dollars in thousands) Within one year $ 52,501 $ 52,497 After one but within five years 114,877 115,236 After five but within ten years 97,911 97,880 More than ten years 16,984 16,835 Total $ 282,273 $ 282,448 The weighted average remaining life of investment securities available-for-sale As of December 31, 2020 and December 31, 2019, management concluded that the unrealized losses of its investment securities are temporary in nature since they are not related to the underlying credit quality of the issuers, and the Company does not intend to sell these debt securities and it is not more likely than not that it will be required to sell these debt securities before the anticipated recovery of its remaining amortized cost. In making its other-than-temporary impairment evaluation, the Company considered the fact that the principal and interest on these securities are from issuers that are investment grade. The change in the unrealized losses on the Obligations Issued by States and Political Subdivisions, Privately Issued Residential Mortgage-Backed Securities and Other Debt Securities was primarily caused by changes in credit spreads and liquidity issues in the marketplace. The unrealized loss on SBA Backed Securities and U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities related primarily to interest rates and not credit quality. The Company has the ability and intent to hold these investments until recovery of fair value, which may be maturity. The Company does not consider these investments to be other-than-temporarily impaired at December 31, 2020 and December 31, 2019. In evaluating the underlying credit quality of a security, management considers several factors such as the credit rating of the obligor and the issuer, if applicable. Internal reviews of issuer financial statements are performed as deemed necessary. In the case of privately issued mortgage-backed securities, the performance of the underlying loans is analyzed as deemed necessary to determine the estimated future cash flows of the securities. Factors considered include the level of subordination, current and estimated future default rates, current and estimated prepayment rates, estimated loss severity rates, geographic concentrations, and origination dates of underlying loans. The following table shows the temporarily impaired securities of the Company’s available-for-sale Temporarily Impaired Investments December 31, 2020 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) SBA Backed Securities $ 13,839 $ 42 $ 30,200 $ 247 $ 44,039 $ 289 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 18,188 50 33,617 267 51,805 317 Privately Issued Residential Mortgage-Backed Securities — — 210 4 210 4 Obligations Issued by States and Political Subdivisions — — — — — — Other Debt Securities 3,942 58 1,298 2 5,240 60 Total temporarily impaired securities $ 35,969 $ 150 $ 65,325 $ 520 $ 101,294 $ 670 The following table shows the temporarily impaired securities of the Company’s available-for-sale There are and securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of holdings at December 31, 2019. Temporarily Impaired Investments December 31, 2019 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) SBA Backed Securities $ 14,560 $ 30 $ 22,092 $ 113 $ 36,652 $ 143 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 108,806 379 29,178 153 137,984 532 Privately Issued Residential Mortgage-Backed Securities 252 2 — — 252 2 Obligations Issued by States and Political Subdivisions — — — — — — Other Debt Securities 800 1 481 18 1,281 19 Total temporarily impaired securities $ 124,418 $ 412 $ 51,751 $ 284 $ 176,169 $ 696

Investment Securities Held-to-M

Investment Securities Held-to-Maturity	12 Months Ended
	Dec. 31, 2020
Text Block [Abstract]
Investment Securities Held-to-Maturity	4. Investment Securities Held-to-Maturity December 31, 2020 December 31, 2019 Amortized Gross Gross Estimated Amortized Gross Gross Estimated (dollars in thousands) U.S. Government Sponsored Enterprises $ 244,220 $ 389 $ 866 $ 243,743 $ 98,867 $ 527 $ 96 $ 99,298 SBA Backed Securities 37,783 2,002 — 39,785 44,379 182 303 44,258 U.S. Government Sponsored Enterprises Mortgage-Backed Securities 2,227,085 69,522 1,032 2,295,575 2,207,874 20,720 10,846 2,217,748 Total $ 2,509,088 $ 71,913 $ 1,898 $ 2,579,103 $ 2,351,120 $ 21,429 $ 11,245 $ 2,361,304 Included in U.S. Government Sponsored Enterprises and U.S. Government Sponsored Enterprise Mortgage-Backed Securities are securities pledged to secure public deposits and repurchase agreements at fair value amounting to $1,866,989,000 and $1,776,399,000 at December 31, 2020, and 2019, respectively. Also included are securities pledged for borrowing at the Federal Home Loan Bank at fair value amounting to $537,367,000 and $399,646,000 at December 31, 2020, and 2019, respectively. The Company did not realize any gains of sales of securities for the year ending December 31, 2020. The Company realized gains on sales of securities of $48,000 from the proceeds of sales of held-to-maturity held-to-maturity

Loans

Loans	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Loans	5. Loans The majority of the Bank’s lending activities are conducted in Massachusetts with other lending activity principally in New Hampshire, Rhode Island, Connecticut, New York, Virginia, Washington DC, and Pennsylvania. The Bank originates construction, commercial and residential real estate loans, commercial and industrial loans, municipal loans, consumer, home equity and other loans for its portfolio. The following summary shows the composition of the loan portfolio at the dates indicated. December 31, 2020 2019 (dollars in thousands) Construction and land development $ 10,909 $ 8,992 Commercial and industrial 1,314,245 812,417 Municipal 137,607 120,455 Commercial real estate 789,836 786,102 Residential real estate 448,436 371,897 Consumer 20,007 21,071 Home equity 274,357 304,363 Overdrafts 432 822 Total $ 2,995,829 $ 2,426,119 At December 31, 2020, and December 31, 2019, loans were carried net of (premiums) discounts of $(74,000) and $(292,000), respectively. Net deferred fees included in loans at December 31, 2020, and December 31, 2019, were $4,444,000 and $220,000, respectively. The Company was servicing mortgage loans sold to others without recourse of approximately $125,998,000 and $204,690,000 at December 31, 2020, and December 31, 2019, respectively. The Company had no residential real estate loans held for sale at December 31, 2020 and December 31, 2019. The Company’s mortgage servicing rights totaled $773,000 and $1,202,000 at December 31, 2020 and December 31, 2019, respectively and are classified in other assets on the consolidated balance sheets. As of December 31, 2020, and 2019, the Company’s recorded investment in impaired loans was $5,379,000 and $3,252,000, respectively. If an impaired loan is placed on nonaccrual, the loan may be returned to an accrual status when principal and interest payments are not delinquent, and the risk characteristics have improved to the extent that there no longer exists a concern as to the collectibility of principal and interest. At December 31, 2020, there were $5,103,000 of impaired loans with specific reserves of $589,000. At December 31, 2019, there were $2,322,000 of impaired loans with specific reserves of $102,000. Loans are designated as troubled debt restructures when a concession is made on a credit as a result of financial difficulties of the borrower. Typically, such concessions consist of a reduction in interest rate to a below-market rate, taking into account the credit quality of the note, or a deferment of payments, principal or interest, which materially alters the Bank’s position or significantly extends the note’s maturity date, such that the present value of cash flows to be received is materially less than those contractually established at the loan’s origination. With the exception of COVID-19 The composition of nonaccrual loans and impaired loans is as follows: December 31, 2020 2019 2018 (dollars in thousands) Loans on nonaccrual $ 3,996 $ 2,014 $ 1,313 Loans 90 days past due and still accruing 90 — — Impaired loans on nonaccrual included above 3,178 891 296 Total recorded investment in impaired loans 5,379 3,252 3,051 Average recorded investment of impaired loans 3,641 3,161 5,491 Accruing troubled debt restructures 2,202 2,361 2,559 Interest income not recorded on nonaccrual loans according to their original terms 95 67 64 Interest income on nonaccrual loans actually recorded — — — Interest income recognized on impaired loans 94 103 196 Directors and officers of the Company and their associates are customers of, and have other transactions with, the Company in the normal course of business. All loans and commitments included in such transactions were made on substantially the same terms, including interest rates and collateral, as those prevailing at the time for comparable transactions with other persons and do not involve more than normal risk of collection or present other unfavorable features. The following table shows the aggregate amount of loans to directors and officers of the Company and their associates during 2020. Balance at Additions Repayments Balance at (dollars in thousands) $12,031 $6,340 $2,389 $15,982

Allowance for Loan Losses

Allowance for Loan Losses	12 Months Ended
	Dec. 31, 2020
Receivables [Abstract]
Allowance for Loan Losses	6. Allowance for Loan Losses The Company maintains an allowance for loan losses in an amount determined by management on the basis of the character of the loans, loan performance, financial condition of borrowers, the value of collateral securing loans and other relevant factors. The following table summarizes the changes in the Company’s allowance for loan losses for the years indicated. An analysis of the allowance for loan losses for each of the three years ending December 31, 2020, 2019 and 2018 is as follows: 2020 2019 2018 (dollars in thousands) Allowance for loan losses, beginning of year $ 29,585 $ 28,543 $ 26,255 Loans charged-off (238 ) (454 ) (833 ) Recoveries on loans previously charged-off 314 246 1,771 Net recoveries (charge-offs ) 76 (208 ) 938 Provision charged to expense 5,825 1,250 1,350 Allowance for loan losses, end of year $ 35,486 $ 29,585 $ 28,543 Further information pertaining to the allowance for loan losses at December 31, 2020 follows: Construction Commercial Municipal Commercial Residential Consumer Home Unallocated Total (dollars in thousands) Allowance for Loan Losses: Ending balance at December 31, 2019 $ 331 $ 11,596 $ 2,566 $ 11,464 $ 2,194 $ 312 $ 1,065 $ 57 $ 29,585 Charge-offs — (29 ) — — — (209 ) — — (238 ) Recoveries — 197 — — — 112 5 — 314 Provision 98 4,949 238 287 (83 ) 26 138 172 5,825 Ending balance at December 31, 2020 $ 429 $ 16,713 $ 2,804 $ 11,751 $ 2,111 $ 241 $ 1,208 $ 229 $ 35,486 Amount of allowance for loan losses for loans deemed to be impaired $ — $ 140 $ — $ 449 $ — $ — $ — $ — $ 589 Amount of allowance for loan losses for loans not deemed to be impaired $ 429 $ 16,573 $ 2,804 $ 11,302 $ 2,111 $ 241 $ 1,208 $ 229 $ 34,897 Loans: Ending balance $ 10,909 $ 1,314,245 $ 137,607 $ 789,836 $ 448,436 $ 20,439 $ 274,357 $ — $ 2,995,829 Loans deemed to be impaired $ — $ 439 $ — $ 4,940 $ — $ — $ — $ — $ 5,379 Loans not deemed to be impaired $ 10,909 $ 1,313,806 $ 137,607 $ 784,896 $ 448,436 $ 20,439 $ 274,357 $ — $ 2,990,450 Further information pertaining to the allowance for loan losses at December 31, 2019 follows: Construction Commercial Municipal Commercial Residential Consumer Home Unallocated Total (dollars in thousands) Allowance for Loan Losses: Ending balance at December 31, 2018 $ 1,092 $ 10,998 $ 1,838 $ 10,663 $ 2,190 $ 365 $ 1,111 $ 286 $ 28,543 Charge-offs — (137 ) — — — (295 ) (22 ) — (454 ) Recoveries — 60 — — — 186 — — 246 Provision (761 ) 675 728 801 4 56 (24 ) (229 ) 1,250 Ending balance at December 31, 2019 $ 331 $ 11,596 $ 2,566 $ 11,464 $ 2,194 $ 312 $ 1,065 $ 57 $ 29,585 Amount of allowance for loan losses for loans deemed to be impaired $ — $ 15 $ — $ 87 $ — $ — $ — $ — $ 102 Amount of allowance for loan losses for loans not deemed to be impaired $ 331 $ 11,581 $ 2,566 $ 11,377 $ 2,194 $ 312 $ 1,065 $ 57 $ 29,483 Loans: Ending balance $ 8,992 $ 812,417 $ 120,455 $ 786,102 $ 371,897 $ 21,893 $ 304,363 $ — $ 2,426,119 Loans deemed to be impaired $ — $ 906 $ — $ 2,346 $ — $ — $ — $ — $ 3,252 Loans not deemed to be impaired $ 8,992 $ 811,511 $ 120,455 $ 783,756 $ 371,897 $ 21,893 $ 304,363 $ — $ 2,422,867 CREDIT QUALITY INFORMATION The Company utilizes a six-grade Loans rated 1-3 Loans rated 4 (Monitor)—These loans represent classified loans that management is closely monitoring for credit quality. These loans have had or may have minor credit quality deterioration as of December 31, 2020. Loans rated 5 (Substandard)—Substandard loans represent classified loans that management is closely monitoring for credit quality. These loans have had more significant credit quality deterioration as of December 31, 2020. Loans rated 6 (Doubtful)—Doubtful loans represent classified loans that management is closely monitoring for credit quality. These loans had more significant credit quality deterioration as of December 31, 2020 and are doubtful for full collection. Impaired—Impaired loans represent classified loans that management is closely monitoring for credit quality. A loan is classified as impaired when it is probable that the Company will be unable to collect all amounts due. The following table presents the Company’s loans by risk rating at December 31, 2020. Construction Commercial Municipal Commercial (dollars in thousands) Grade: 1-3 $10,909 $ 1,309,861 $ 137,607 $761,101 4 (Monitor) — 3,945 — 23,795 5 (Substandard) — — — — 6 (Doubtful) — — — — Impaired — 439 — 4,940 Total $10,909 $ 1,314,245 $ 137,607 $789,836 The Company has increased its exposure to larger loans to large institutions with publicly available credit ratings. These ratings are tracked as a credit quality indicator for these loans. The following table presents the Company’s loans by credit rating at December 31, 2020. Commercial Municipal Commercial Total (dollars in thousands) Credit Rating: Aaa-Aa3 $ 710,955 $ 74,291 $ 38,035 $ 823,281 A1-A3 183,123 7,103 145,583 335,809 Baa1-Baa3 50,000 51,133 140,905 242,038 Ba2 — 5,080 — 5,080 Total $ 944,078 $ 137,607 $ 324,523 $ 1,406,208 The following table presents the Company’s loans by risk rating at December 31, 2019. Construction Commercial Municipal Commercial (dollars in thousands) Grade: 1-3 $ 8,992 $ 807,486 $ 120,455 $ 759,402 4 (Monitor) — 4,025 — 24,354 5 (Substandard) — — — — 6 (Doubtful) — — — — Impaired — 906 — 2,346 Total $ 8,992 $ 812,417 $ 120,455 $ 786,102 The following table presents the Company’s loans by credit rating at December 31, 2019. Commercial Municipal Commercial Total (dollars in thousands) Credit Rating: Aaa-Aa3 $ 523,644 $ 53,273 $ 40,437 $ 617,354 A1-A3 186,044 7,354 148,346 341,744 Baa1-Baa3 — 51,133 144,711 195,844 Ba2 — 5,895 — 5,895 Total $ 709,688 $ 117,655 $ 333,494 $ 1,160,837 The Company utilized payment performance as credit quality indicators for residential real estate, consumer and overdrafts, and the home equity portfolio. The indicators are depicted in the table “aging of past-due AGING OF PAST-DUE At December 31, 2020, the aging of past due loans are as follows: Accruing 30-89 Days Non Accruing Total Current Total (dollars in thousands) Construction and land development $ — $ — $ — $ — $ 10,909 $ 10,909 Commercial and industrial 56 297 90 443 1,313,802 1,314,245 Municipal — — — — 137,607 137,607 Commercial real estate — 2,881 — 2,881 786,955 789,836 Residential real estate 390 527 — 917 447,519 448,436 Consumer and overdrafts 21 1 — 22 20,417 20,439 Home equity 1,001 290 — 1,291 273,066 274,357 Total $ 1,468 $ 3,996 $ 90 $ 5,554 $ 2,990,275 $ 2,995,829 At December 31, 2019 the aging of past due loans are as follows: Accruing 30-89 Days Non Accruing Total Current Total (dollars in thousands) Construction and land development $ — $ — $ — $ — $ 8,992 $ 8,992 Commercial and industrial 227 400 — 627 811,790 812,417 Municipal — — — — 120,455 120,455 Commercial real estate 840 492 — 1,332 784,770 786,102 Residential real estate 1,563 683 — 2,246 369,651 371,897 Consumer and overdrafts 18 4 — 22 21,871 21,893 Home equity 603 435 — 1,038 303,325 304,363 Total $ 3,251 $ 2,014 $ — $ 5,265 $ 2,420,854 $ 2,426,119 IMPAIRED LOANS A loan is impaired when, based on current information and events, it is probable that a creditor will be unable to collect all amounts due according to the contractual terms of the loan agreement. When a loan is impaired, the Company measures impairment based on the present value of expected future cash flows discounted at the loan’s effective interest rate, except that as a practical expedient, the Company measures impairment based on a loan’s observable market price or the fair value of the collateral if the loan is collateral dependent. Loans are charged-off charged-off The following is information pertaining to impaired loans at December 31, 2020: Carrying Unpaid Required Average Interest (dollars in thousands) With no required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 4 5 — 633 — Municipal — — — — — Commercial real estate 272 306 — 437 — Residential real estate — — — 127 — Consumer — — — — — Home equity — — — — — Total $ 276 $ 311 $ — $ 1,197 $ — With required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 435 454 140 121 8 Municipal — — — — — Commercial real estate 4,668 4,797 449 2,323 86 Residential real estate — — — — — Consumer — — — — — Home equity — — — — — Total $ 5,103 $ 5,251 $ 589 $ 2,444 $ 94 Total Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 439 459 140 754 8 Municipal — — — — — Commercial real estate 4,940 5,103 449 2,760 86 Residential real estate — — — 127 — Consumer — — — — — Home equity — — — — — Total $ 5,379 $ 5,562 $ 589 $ 3,641 $ 94 The following is information pertaining to impaired loans at December 31, 2019: Carrying Unpaid Required Average Interest (dollars in thousands) With no required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 770 976 — 138 6 Municipal — — — — — Commercial real estate 160 189 — 445 — Residential real estate — — — — — Consumer — — — — — Home equity — — — — — Total $ 930 $ 1,165 $ — $ 583 $ 6 With required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 136 137 15 264 7 Municipal — — — — — Commercial real estate 2,186 2,306 87 2,314 90 Residential real estate — — — — — Consumer — — — — — Home equity — — — — — Total $ 2,322 $ 2,443 $ 102 $ 2,578 $ 97 Total Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 906 1,113 15 402 13 Municipal — — — — — Commercial real estate 2,346 2,495 87 2,759 90 Residential real estate — — — — — Consumer — — — — — Home equity — — — — — Total $ 3,252 $ 3,608 $ 102 $ 3,161 $ 103 Troubled Debt Restructurings (TDRs) are identified as a modification in which a concession was granted to a customer who was having financial difficulties. This concession may be below market rate, longer amortization/term, or a lower payment amount. The present value calculation of the modification did not result in an increase in the allowance for these loans beyond any previously established allocations. There was one commercial and industrial loan that was modified during the first quarter of 2019. The loan was modified by reducing the interest rates as well as extending the term on the loan. The pre-modification There were no TDRs that occurred during the year 2020. Also, there were no commitments to lend additional funds to troubled debt restructuring borrowers. There were no troubled debt restructurings that subsequently defaulted during 2020. Under Section 4013 of the CARES Act, loans less than 30 days past due as of December 31, 2019 will be considered current for COVID-19 COVID-19 COVID-19 As of December 31, 2020, and as a result of COVID-19

Bank Premises and Equipment

Bank Premises and Equipment	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Bank Premises and Equipment	7. Bank Premises and Equipment December 31, 2020 2019 Estimated Useful Life (dollars in thousands) Land $ 7,905 $ 7,246 — Bank premises 34,556 28,175 30-39 Furniture and equipment 34,381 33,259 3-10 Leasehold improvements 12,830 12,674 30-39 89,672 81,354 Accumulated depreciation and amortization (50,610 ) (47,402 ) Total $ 39,062 $ 33,952 Depreciation and amortization amounted to $3,266,000, $3,235,000, and $3,206,000 at December 31, 2020, 2019 and 2018, respectively.

Goodwill and Identifiable Intan

Goodwill and Identifiable Intangible Assets	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Identifiable Intangible Assets	8. Goodwill and Identifiable Intangible Assets At December 31, 2020 and 2019, the Company concluded that it is not more likely than not that fair value of the reporting unit is less than its carrying value, and goodwill is not considered to be impaired. The changes in goodwill and identifiable intangible assets for the years ended December 31, 2020 and 2019 are shown in the table below. Carrying Amount of Goodwill and Intangibles Goodwill Mortgage Total (dollars in thousands) Balance at December 31, 2018 $ 2,714 $ 1,226 $ 3,940 Additions — 237 237 Amortization Expense — (261 ) (261 ) Balance at December 31, 2019 $ 2,714 $ 1,202 $ 3,916 Additions — — — Amortization Expense — (429 ) (429 ) Balance at December 31, 2020 $ 2,714 $ 773 $ 3,487

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	9. Fair Value Measurements The Company follows FASB ASC 820-10, Fair Value Measurements and Disclosures, 820-10 Level I—Quoted prices are available in active markets for identical assets or liabilities as of the reported date. The type of financial instruments included in Level I are highly liquid cash instruments with quoted prices such as listed equities and money market securities, as well as listed derivative instruments. Level II—Pricing inputs are other than quoted prices in active markets, which are either directly or indirectly observable as of the reported date. The nature of these financial instruments include cash instruments for which quoted prices are available but traded less frequently, derivative instruments whose fair value have been derived using a model where inputs to the model are directly observable in the market or can be derived principally from or corroborated by observable market data, and instruments that are fair valued using other financial instruments, the parameters of which can be directly observed. Instruments which are generally included in this category are G-7 Level III—These instruments have little to no pricing observability as of the reported date. These financial instruments do not have two-way The results of the fair value hierarchy as of December 31, 2020, are as follows: Fair Value Measurements Using Carrying Quoted Prices Significant Significant Other (dollars in thousands) Financial Instruments Measured at Fair Value on a Recurring Basis Securities AFS SBA Backed Securities $ 44,039 $ — $ 44,039 $ — U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 177,741 — 177,741 — Privately Issued Residential Mortgage-Backed Securities 328 — 328 — Obligations Issued by States and Political Subdivisions 52,276 — — 52,276 Other Debt Securities 8,064 — 8,064 — Total $ 282,448 $ — $ 230,172 $ 52,276 Equity Securities $ 1,668 $ 303 $ 1,365 $ — Financial Instruments Measured at Fair Value on a Non-recurring Impaired Loans $ 3,178 $ — $ — $ 3,178 The changes in Level 3 securities for the year ended December 31, 2019 are as shown in the table below: Obligations (dollars in thousands) Balance at December 31, 2018 $ 88,728 Purchases 21,408 Maturities/redemptions (96,812 ) Transfer to Level 2 — Amortization (23 ) Change in fair value — Balance at December 31, 2019 $ 13,301 The amortized cost of Level 3 securities was $13,301,000 with an unrealized loss of $0 at December 31, 2019. The securities in this category are generally municipal securities with no readily determinable fair value or failed auction rate securities. Management evaluated the fair value of these securities based on an evaluation of the underlying issuer, prevailing rates and market liquidity.

Deposits

Deposits	12 Months Ended
	Dec. 31, 2020
Deposits [Abstract]
Deposits	10. Deposits The following is a summary of remaining maturities or re-pricing 2020 Percent 2019 Percent (dollars in thousands) Within one year $ 465,080 85 % $ 383,497 69 % Over one year to two years 50,456 9 % 123,016 22 % Over two years to three years 19,104 4 % 27,223 5 % Over three years to five years 11,503 2 % 21,711 4 % Total $ 546,143 100 % $ 555,447 100 % Time deposits of more than $250,000 totaled $350,290,000 and $342,809,000 in 2020 and 2019, respectively. Deposits totaling $34,622,000 and $34,964,000 were attributable to related parties at December 31, 2020 and December 31, 2019, respectively.

Securities Sold Under Agreement

Securities Sold Under Agreements to Repurchase	12 Months Ended
	Dec. 31, 2020
Brokers and Dealers [Abstract]
Securities Sold Under Agreements to Repurchase	11. Securities Sold Under Agreements to Repurchase The following is a summary of securities sold under agreements to repurchase as of December 31, 2020 2019 2018 (dollars in thousands) Amount outstanding at December 31 $ 232,090 $ 266,045 $ 154,240 Weighted average rate at December 31 0.34 % 0.96 % 0.82 % Maximum amount outstanding at any month end $ 253,980 $ 307,235 $ 174,150 Daily average balance outstanding during the year $ 221,609 $ 224,361 $ 147,944 Weighted average rate during the year 0.62 % 1.05 % 0.66 %

Other Borrowed Funds and Subord

Other Borrowed Funds and Subordinated Debentures	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Other Borrowed Funds and Subordinated Debentures	12. Other Borrowed Funds and Subordinated Debentures The following is a summary of other borrowed funds and subordinated debentures as of December 31, 2020 2019 2018 (dollars in thousands) Amount outstanding at December 31 $ 213,092 $ 407,038 $ 238,461 Weighted average rate at December 31 2.38 % 2.37 % 2.76 % Maximum amount outstanding at any month end $ 348,203 $ 487,502 $ 542,913 Daily average balance outstanding during the year $ 201,656 $ 231,926 $ 291,674 Weighted average rate during the year 2.66 % 2.95 % 2.61 % FEDERAL HOME LOAN BANK BORROWINGS Federal Home Loan Bank of Boston (“FHLBB”) borrowings are collateralized by a blanket pledge agreement on the Bank’s FHLBB stock, certain qualified investment securities, deposits at the FHLBB and residential mortgages held in the Bank’s portfolios. The Bank’s remaining term borrowing capacity at the FHLBB at December 31, 2020, was approximately $591,974,000. In addition, the Bank has a $14,500,000 line of credit with the FHLBB. A schedule of the maturity distribution of FHLBB advances with the weighted average interest rates is as follows:​​​​​​​ December 31, 2020 2019 2018 Amount Weighted Amount Weighted Amount Weighted (dollars in thousands) Within one year $ 67,500 1.78 % $ 218,000 1.86 % $ 63,000 2.17 % Over one year to two years $ 3,500 2.15 % $ 42,500 2.58 % $ 28,000 2.29 % Over two years to three years $ 30,000 3.38 % $ 3,500 2.15 % $ 25,000 3.34 % Over three years to five years $ 67,000 2.64 % $ 70,000 2.85 % $ 33,500 2.23 % Over five years $ 9,009 2.83 % $ 36,955 2.88 % $ 52,878 2.47 % Total $ 177,009 2.44 % $ 370,955 2.23 % $ 202,378 2.42 % Included in the table above are $40,000,000, $40,000,000, and $40,000,000, respectively, of FHLBB advances at December 31, 2020, 2019 and 2018, that are puttable at the discretion of FHLBB. These put dates were not utilized in the table above. During 2019, the Company restructured $15,000,000 of FHLBB advances. Prior to the restructure, the weighted average rate on these advances was 3.33% and the weighted average maturity was 14 months. Subsequent to the restructure, the weighted average rate was 2.37% and the weighted average maturity was 60 months. SUBORDINATED DEBENTURES Subordinated debentures totaled $36,083,000 at December 31, 2020, 2019, and 2018. In December 2004, the Company consummated the sale of a trust preferred securities offering, in which it issued $36,083,000 of subordinated debt securities due 2034 to its newly formed unconsolidated subsidiary Century Bancorp Capital Trust II. Century Bancorp Capital Trust II then issued 35,000 shares of Cumulative Trust Preferred Securities with a liquidation value of $1,000 per share. These securities paid dividends at an annualized rate of 6.65% for the first ten years and then converted to the three-month LIBOR rate plus 1.87% for the remaining 20 years. The coupon rate on these securities was 2.09% at December 31, 2020 and 3.76% at December 31, 2019. OTHER BORROWED FUNDS There were no overnight federal funds purchased at December 31, 2020, 2019, and 2018.

Reclassifications Out of Accumu

Reclassifications Out of Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2020
Text Block [Abstract]
Reclassifications Out of Accumulated Other Comprehensive Income	13. Reclassifications Out of Accumulated Other Comprehensive Income (a) Amount Reclassified from Accumulated Details About Accumulated Other Year Ended (a) Year Ended (a) Affected Line Item in the Statement Unrealized gains and losses on available-for-sale $ — $ 61 Net gains on sales of investments — (17 ) Provision for income taxes $ — $ 44 Net income Accretion of unrealized losses transferred $ (798 ) $ (1,022 ) Interest income securities held-to-maturity 207 269 Provision for income taxes $ (591 ) $ (753 ) Net income Amortization of defined benefit pension items Prior-service costs $ (114 ) $ (114 ) Salaries and employee benefits (b) Actuarial gains (losses) (1,889 ) (1,351 ) Salaries and employee benefits (b) Total before tax (2,003 ) (1,465 ) Income before taxes Tax (expense) or benefit 562 412 Provision for income taxes Net of tax $ (1,441 ) $ (1,053 ) Net income (a) Amounts in parentheses indicate decreases to profit/loss. (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see employee benefits footnote (Note 17) for additional details).

Earnings Per Share ("EPS")

Earnings Per Share ("EPS")	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Earnings Per Share ("EPS")	14. Earnings Per Share (“EPS”) Class A and Class B shares participate equally in undistributed earnings. Under the Company’s Articles of Organization, the holders of Class A Common Stock are entitled to receive dividends per share equal to at least 200% of dividends paid, if any, from time to time, on each share of Class B Common Stock. Diluted EPS includes the dilutive effect of common stock equivalents; basic EPS excludes all common stock equivalents. There were no common stock equivalents for 2020, 2019 and 2018, respectively. The following table is a reconciliation of basic EPS and diluted EPS: Year Ended December 31, 2020 2019 2018 (in thousands except share and per share data) BASIC EPS COMPUTATION Numerator: Net income, Class A $ 33,449 $ 31,351 $ 28,479 Net income, Class B 8,760 8,348 7,734 Denominator: Weighted average shares outstanding, Class A 3,653,939 3,633,044 3,608,179 Weighted average shares outstanding, Class B 1,913,970 1,934,865 1,959,730 Basic EPS, Class A $ 9.15 $ 8.63 $ 7.89 Basic EPS, Class B $ 4.58 $ 4.31 $ 3.95 DILUTED EPS COMPUTATION Numerator: Net income, Class A $ 33,449 $ 31,351 $ 28,479 Net income, Class B 8,760 8,348 7,734 Total net income, for diluted EPS, Class A computation 42,209 39,699 36,213 Denominator: Weighted average shares outstanding, basic, Class A 3,653,939 3,633,044 3,608,179 Weighted average shares outstanding, Class B 1,913,970 1,934,865 1,959,730 Weighted average shares outstanding diluted, Class A 5,567,909 5,567,909 5,567,909 Weighted average shares outstanding, Class B 1,913,970 1,934,865 1,959,730 Diluted EPS, Class A $ 7.58 $ 7.13 $ 6.50 Diluted EPS, Class B $ 4.58 $ 4.31 $ 3.95

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Stockholders' Equity	15. Stockholders’ Equity DIVIDENDS Holders of the Class A common stock may not vote in the election of directors but may vote as a class to approve certain extraordinary corporate transactions. Holders of Class B common stock may vote in the election of directors. Class A common stockholders are entitled to receive dividends per share equal to at least 200% per share of that paid, if any, on each share of Class B common stock. Class A common stock is publicly traded. Class B common stock is not publicly traded; however, it can be converted on a per share basis to Class A common stock at any time at the option of the holder. Dividend payments by the Company are dependent in part on the dividends it receives from the Bank, which are subject to certain regulatory restrictions. CAPITAL RATIOS The Bank and the Company are subject to various regulatory requirements administered by federal banking agencies. Failure to meet minimum capital requirements can initiate certain mandatory and possibly additional discretionary actions by regulators that, if undertaken, could have a direct material effect on the Bank and Company’s financial statements. Under capital adequacy guidelines and regulatory framework for prompt corrective action, the Bank and Company must meet specific capital guidelines that involve quantitative measures of the Bank and Company’s assets and liabilities, and certain off-balance-sheet Quantitative measures established by regulation to ensure capital adequacy require the Bank and the Company to maintain minimum amounts and ratios (set forth in the table below) of total and Tier 1 capital (as defined in the regulation) to risk-weighted assets (as defined) and Tier 1 capital (as defined) to average assets (as defined). Management believes, as of December 31, 2020, that the Bank and the Company meet all capital adequacy requirements to which they are subject. The Basel Committee has issued capital standards entitled “Basel III: A global framework for more resilient banks and banking systems” (Basel III). The Federal Reserve has finalized its rule implementing the Basel III regulatory capital framework. The rule was effective in January 2015 and sets the Basel III minimum Regulatory capital requirements. To be categorized as well capitalized, the Bank must maintain minimum total risk-based, Common Equity tier 1, tier 1 risk-based, and Tier 1 leverage ratios as set forth in the table. There are no conditions or events since that notification that management believes would cause a change in the Bank’s categorization. The Bank’s actual capital amounts and ratios are presented in the following table: Actual For Capital To Be Well Capitalized Amount Ratio Amount Ratio Amount Ratio As of December 31, 2020 (Basel III) Total Capital (to Risk-Weighted Assets) $ 448,695 13.03 % $ 275,403 8.00 % $ 344,254 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 413,209 12.00 % 206,553 6.00 % 275,403 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 413,209 12.00 % 154,914 4.50 % 223,765 6.50 % Tier 1 Capital (to 4th Quarter Average Assets) 413,209 6.41 % 257,763 4.00 % 322,204 5.00 % As of December 31, 2019 (Basel III) Total Capital (to Risk-Weighted Assets) $ 401,850 13.57 % $ 236,830 8.00 % $ 296,037 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 372,265 12.57 % 177,622 6.00 % 236,830 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 372,265 12.57 % 133,217 4.50 % 192,424 6.50 % Tier 1 Capital (to 4th Quarter Average Assets) 372,265 7.01 % 212,549 4.00 % 265,686 5.00 % The Company’s actual capital amounts and ratios are presented in the following table: Actual For Capital To Be Well Capitalized Amount Ratio Amount Ratio Amount Ratio As of December 31, 2020 (Basel III) Total Capital (to Risk-Weighted Assets) $ 464,331 13.43 % $ 276,673 8.00 % $ 345,842 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 428,845 12.40 % 207,505 6.00 % 276,673 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 393,845 11.39 % 155,629 4.50 % 224,797 6.50 % Tier 1 Capital (to 4th Quarter Average Assets) 428,845 6.64 % 258,414 4.00 % 323,017 5.00 % As of December 31, 2019 (Basel III) Total Capital (to Risk-Weighted Assets) $ 415,863 13.97 % $ 238,132 8.00 % $ 297,665 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 386,308 12.98 % 178,599 6.00 % 238,132 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 351,308 11.80 % 133,949 4.50 % 193,482 6.50 % Tier 1 Capital (to 4th Quarter Average Assets) 386,308 7.25 % 213,222 4.00 % 266,528 5.00 %

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	16. Income Taxes The current and deferred components of income tax expense (benefit) for the years ended December 31, are as follows: 2020 2019 2018 (dollars in thousands) Current expense: Federal $ 3,651 $ 2,548 $ 2,637 State 1,845 697 697 Total current expense 5,496 3,245 3,334 Deferred (benefit) expense: Federal (491 ) (1,660 ) (1,238 ) State (598 ) (367 ) (528 ) Valuation Allowance — (108 ) — Total deferred (benefit) expense (1,089 ) (2,135 ) (1,766 ) Provision for income taxes $ 4,407 $ 1,110 $ 1,568 Income tax accounts included in other assets at December 31, are as follows: 2020 2019 (dollars in thousands) Income tax receivable $ 2,250 $ 3,446 Deferred income tax asset, net 26,431 24,566 Total $ 28,681 $ 28,012 Differences between income tax expense (benefit) at the statutory federal income tax rate and total income tax expense are summarized as follows: 2020 2019 2018 (dollars in thousands) Federal income tax expense at statutory rate $ 9,789 $ 8,570 $ 7,934 State income tax, net of federal income tax benefit 930 261 134 Life insurance income (834 ) (265 ) (176 ) Tax-exempt (5,490 ) (6,737 ) (6,510 ) Tax credits, net (231 ) (292 ) (349 ) Valuation allowance reversal — (108 ) — Sequestration (reversal) accrual — (438 ) 438 Other, net 243 119 97 Total $ 4,407 $ 1,110 $ 1,568 Effective tax rate 9.45 % 2.72 % 4.15 % The following table sets forth the Company’s gross deferred income tax assets and gross deferred income tax liabilities at December 31: 2020 2019 (dollars in thousands) Deferred income tax assets: Allowance for loan losses $ 10,025 $ 8,354 Deferred compensation 8,833 8,910 Pension and SERP liability 9,911 8,770 Operating lease liabilities 3,917 3,567 Unrealized losses on securities transferred to held-to-maturity 436 643 Depreciation 1,214 1,060 QZAB credit — 812 PPP loan origination fees 919 — Accrued bonus — 708 Charitable contributions carryforward — 276 Nonaccrual interest 75 115 Other 305 206 Gross deferred income tax asset 35,635 33,421 Deferred income tax liabilities: Pension liability (4,142 ) (4,258 ) Operating lease right-of-use (3,855 ) (3,520 ) Deferred origination costs (584 ) (516 ) Prepaid expenses (361 ) (337 ) Mortgage servicing rights (217 ) (338 ) Unrealized (gains) losses on securities available-for-sale (45 ) 114 Gross deferred income tax liability (9,204 ) (8,855 ) Deferred income tax asset net $ 26,431 $ 24,566 Based on the Company’s historical and current pre-tax On December 22, 2017, the Tax Cuts and Jobs Act (the “Tax Act”) was enacted, which represents the most comprehensive reform to the U.S. tax code in over thirty years. The majority of the provisions of the Tax Act took effect on January 1, 2018. The Tax Act lowered the Company’s federal tax rate from 34% to 21%. Also, for tax years beginning after December 31, 2017, the corporate Alternative Minimum Tax (“AMT”) has been repealed. For 2018 through 2021, the AMT credit carryforward can offset regular tax liability and is refundable in an amount equal to 50% (100% for 2021) of the excess of the minimum tax credit for the tax year over the amount of the credit allowable for the year against regular tax liability. During 2019, a reduction in tax accruals related to sequestration of the refundable portion of our AMT credit carryforward amounted to $438,000. On March 27, 2020, the Coronavirus Aid, Relief and Economic Security (CARES) Act was signed into law. As a result of the CARES Act, the full balance of the AMT credit was refunded during 2020. The other provisions under the CARES Act did not have a material impact on the Company’s income taxes for 2020. The Tax Act also contains other provisions that may affect the Company currently or in future years. Among these are changes to the deductibility of meals and entertainment, the deductibility of executive compensation, the dividend received deduction and net operating loss carryforwards. Tax Act changes for individuals include lower tax rates, mortgage interest and state and local tax limitations as well as an increase in the standard deduction, among others. The Company did not have any unrecognized tax benefits nor a balance of accrued interest and penalties related to uncertain tax positions. The Company and its subsidiaries file a consolidated federal tax return. The Company is subject to federal and Massachusetts examinations for tax years after December 31, 2016.

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Employee Benefits	17. Employee Benefits The Company has a Qualified Defined Benefit Pension Plan (the “Plan”), which had been offered to all employees reaching minimum age and service requirements. In 2006, the Bank became a member of the Savings Bank Employees Retirement Association (“SBERA”) within which it then began maintaining the Qualified Defined Benefit Pension Plan. SBERA offers a common and collective trust as the underlying investment structure for its retirement plans. The target allocation mix for the common and collective trust portfolio calls for an equity-based investment deployment range of 47% to 61% of total portfolio assets. The remainder of the portfolio is allocated to fixed income securities with target range of 24% to 38% and other investments including global asset allocation and hedge funds from 9% to 15%. The Trustees of SBERA, through its Investment Committee, select investment managers for the common and collective trust portfolio. A professional investment advisory firm is retained by the Investment Committee to provide allocation analysis, performance measurement and to assist with manager searches. The overall investment objective is to diversify investments across a spectrum of investment types to limit risks from large market swings. The Company closed the plan to employees hired after March 31, 2006. The measurement date for the Plan is December 31 for each year. The benefits expected to be paid in each year from 2021 to 2025 are $2,008,000, $2,188,000, $2,322,000, $2,521,000, and $2,707,000, respectively. The aggregate benefits expected to be paid in the five years from 2026 to 2030 are $16,087,000. The framework for measuring fair value provides a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (level 1) and the lowest priority to unobservable inputs (level 3). Valuation techniques maximize the use of relevant observable inputs and minimize the use of unobservable inputs. The three levels of the fair value hierarchy under Topic 820 are described as follows: LEVEL 1 Inputs to the valuation methodology are quoted market prices (unadjusted) for identical assets or liabilities in active markets that the Company has the ability to access at the measurement date. LEVEL 2 Inputs other than quoted prices included within level 1 that are observable for the asset or liability, either directly or indirectly, such as: quoted prices for similar assets or liabilities in active markets; quoted prices for identical or similar assets or liabilities in inactive markets; inputs other that quoted prices that are observable for the asset or liability; or inputs that are derived principally from or corroborated by observable market data by correlation or other means. If the asset or liability has specified (contractual) term, the level 2 input must be observable for substantially the full term of the asset or liability. LEVEL 3 Inputs that are unobservable inputs for the asset or liability. Below is a description of the valuation methodologies used for assets measured at fair value. Collective Funds Valued at either the closing price reported on the active market on which the individual securities are traded or valued at the net asset value (NAV) of units of a collective trust. The NAV, as provided by the trustee, is used as a practical expedient to estimate fair value. The NAV is based on the fair value of the underlying investments held by the fund less its liabilities. This practical expedient is not used when it is determined to be probable that the fund will sell the investment for an amount different than the reported NAV. Participant transactions (purchases and sales) may occur daily. Were SBERA to initiate a full redemption of the collective trust, the investment advisor reserves the right to temporarily delay withdrawal from the trust in order to ensure that securities liquidations will be carried out in an orderly business manner. Equity Securities Valued at the closing price reported on the active market on which the individual securities are traded. Mutual Funds Valued at the daily closing price as reported by the fund. Mutual funds held open-end The mutual funds held are deemed to be actively traded. Limited Partnerships and Hedge Funds The funds are valued at NAV, without further adjustment, as calculated by the fund’s manager based upon the terms and conditions of the organization documents of the underlying investments, with further consideration to portfolio risks. The following table sets forth by level, within the fair value hierarchy, the plan’s assets at fair value. Classification within the fair value hierarchy table is based upon the lowest level of any input that is significant to the fair value measurement: The fair value of plan assets and major categories as of December 31, 2020, is as follows: Description Percent NAV Level 1 Level 2 Level 3 Total (dollars in thousands) Collective Funds 10.6 % $ — $ 5,999 $ — $ — $ 5,999 Equity Securities 26.6 % — 14,952 — — 14,952 Diversified Mutual Funds 12.3 % — 6,949 — — 6,949 Short Term Investments 0.2 % — 121 — — 121 Total investments measured in the fair value hierarchy 49.7 % — 28,021 — — 28,021 Investments measured at net asset value (1) 50.3 % 28,412 — — — 28,412 100.0 % $28,412 $28,021 $ $ $56,433 (1) In accordance with Subtopic 820-10, The fair value of plan assets and major categories as of December 31, 2019, is as follows: Description Percent NAV Level 1 Level 2 Level 3 Total (dollars in thousands) Collective Funds 8.3 % $ — $ 4,289 $ — $ — $ 4,289 Equity Securities 9.7 % — 5,016 — — 5,016 Diversified Mutual Funds 31.1 % — 16,081 — — 16,081 Total investments measured in the fair value hierarchy 49.1 % — 25,386 — — 25,386 Investments measured at net asset value (1) 50.9 % 26,274 — — — 26,274 100.0 % $ 26,274 $ 25,386 $ — $ — $ 51,660 (1) In accordance with Subtopic 820-10, INVESTMENTS MEASURED USING THE NET ASSET VALUE PER SHARE PRACTICAL EXPEDIENT The following table summarizes investments for which fair value is measured using the net asset value per share practical expedient. There are no participant redemption restrictions for these investments. The investments measured using the net asset value per share practical expedient as of December 31, 2020, is as follows: Percent Fair Value (dollars in thousands) Collective Funds by Category: Equity 12.6 % $ 7,113 US debt securities 18.0 % 10,131 International equities 14.1 % 7,964 Limited Partnerships by Category: Emerging markets 4.1 % 2,308 Hedge Funds by Category: Global opportunities (1) 0.3 % 197 Private investment entities and/or separately managed accounts (2) 1.2 % 699 50.3 % $ 28,412 The investments measured using the net asset value per share practical expedient as of December 31, 2019, is as follows: Percent Fair Value (dollars in thousands) Collective Funds by Category: Equity 19.2 % $ 9,932 US debt securities 15.2 % 7,874 International equities 10.1 % 5,208 Limited Partnerships by Category: Emerging markets 3.2 % 1,635 Multi-strategy 1.2 % 644 Hedge Funds by Category: Global opportunities (1) 0.5 % 259 Private investment entities and/or separately managed accounts (2) 1.4 % 722 50.9 % $ 26,274 (1) This category has an investment strategy to pursue a hybrid absolute return via portfolio managers, secondaries, and co-investments (2) The Fund’s investment objective is to invest in highly attractive, select investment opportunities by maintaining investments through private investment entities and/or separately managed accounts (each, an Investment or a Portfolio and collectively, the Investments or the Portfolios) with investment management professionals (each a Manager and collectively, the Managers) specializing in various alternative investment strategies. The Managers have broad investment experience and the ability to leverage their existing relationships with corporate management teams, investment banks and other institutions to gain access to certain investment opportunities. As such, the Manager is presented with “best idea” investment opportunities, typically in asset classes where market dislocations or other events have created attractive investment opportunities. The Managers are not restricted in the investment strategies that they may employ across different asset classes and regions. The Manager anticipates that any number of strategies will be eligible for consideration for investment by the Fund and the Fund reserves the right to invest in any particular strategy or asset class it deems appropriate. The Company has a Supplemental Executive Insurance/Retirement Plan (the Supplemental Plan), which is limited to certain officers and employees of the Company. The Supplemental Plan is voluntary. Under the Supplemental Plan, each participant will receive a retirement benefit based on compensation and length of service. Life insurance policies, which are owned by the Company, are purchased covering the lives of each participant. The benefits expected to be paid in each year from 2021 to 2025 are $2,570,000, $2,507,000, $2,439,000, $2,595,000, and $3,102,000, respectively. The aggregate benefits expected to be paid in the five years from 2026 to 2030 are $17,913,000. Defined Benefit Pension Plan Supplemental Insurance/ Retirement Plan 2020 2019 2020 2019 (dollars in thousands) Change projected in benefit obligation Benefit obligation at beginning of year $ 49,434 $ 40,509 $ 49,976 $ 40,405 Service cost 1,377 1,103 1,412 1,024 Interest cost 1,801 1,892 1,862 1,926 Prior service cost — — 501 — Actuarial (gain)/loss 3,245 7,099 4,703 7,537 Benefits paid (1,418 ) (1,169 ) (4,511 ) (916 ) Projected benefit obligation at end of year $ 54,439 $ 49,434 $ 53,943 $ 49,976 Change in plan assets Fair value of plan assets at beginning of year $ 51,660 $ 44,437 Actual return on plan assets 6,191 8,392 Employer contributions — — Benefits paid (1,418 ) (1,169 ) Fair value of plan assets at end of year $ 56,433 $ 51,660 (Unfunded) Funded status $ 1,994 $ 2,226 $ (53,943 ) $ (49,976 ) Accumulated benefit obligation $ 54,439 $ 49,434 $ 48,767 $ 45,238 Weighted-average assumptions as of December 31 Discount rate—Liability 3.25 % 3.71 % 3.21 % 3.71 % Discount rate—Expense 3.71 % 4.76 % 3.71 % 4.79 % Expected return on plan assets 7.50 % 7.50 % NA NA Rate of compensation increase 4.00 % 4.00 % 4.00 % 4.00 % Components of net periodic benefit cost Service cost $ 1,377 $ 1,103 $ 1,412 $ 1,024 Interest cost 1,801 1,892 1,862 1,926 Expected return on plan assets (3,808 ) (3,275 ) — — Recognized prior service cost — — 114 114 Recognized net losses 1,041 916 848 435 Net periodic cost (benefit) $ 411 $ 636 $ 4,236 $ 3,499 Other changes in plan assets and benefit obligations recognized in other comprehensive income Amortization of prior service cost $ — $ — $ (114) $ (114 ) Net (gain) loss (179 ) 1,066 3,809 7,101 Total recognized in other comprehensive income (179 ) 1,066 3,695 6,987 Total recognized in net periodic benefit cost and other comprehensive income $ 232 $ 1,702 $ 7,931 $ 10,486 The following table represents the unrecognized pension liability at the time period indicated: Plan December 31, 2020 Plan Total Plan December 31, 2019 Total (dollars in thousands) Prior service cost $ — $ (739 ) $ (739 ) $ — $ (307 ) $ (307 ) Net actuarial loss (12,741 ) (21,780 ) (34,521 ) (12,920 ) (17,971 ) (30,891 ) Total $ (12,741 ) $ (22,519 ) $ (35,260 ) $ (12,920 ) $ (18,278 ) $ (31,198 ) The following table summarizes the amounts included in Accumulated Other Comprehensive Loss at December 31, 2020, expected to be recognized as components of net periodic benefit cost in the next year: Plan Supplemental Amortization of prior service cost to be recognized in 2021 $ — $ 214 Amortization of loss to be recognized in 2021 965 1,079 Assumptions for the expected return on plan assets and discount rates in the Company’s Plan and Supplemental Plan are periodically reviewed. As part of the review, management in consultation with independent consulting actuaries performs an analysis of expected returns based on the plan’s asset allocation. This forecast reflects the Company’s and actuarial firm’s expected return on plan assets for each significant asset class or economic indicator. The range of returns developed relies on forecasts and on broad market historical benchmarks for expected return, correlation and volatility for each asset class. Also, as a part of the review, the Company’s management in consultation with independent consulting actuaries performs an analysis of discount rates based on expected returns of high-grade fixed income debt securities. Prior to December 31, 2019, the Company utilized a full yield curve approach in the estimation of the service and interest components of the net periodic pension cost by applying the specific spot rates along the yield curve used in the determination of the benefit obligation to their underlying projected cash flows. Beginning December 31, 2019, the discount rate was determined by preparing an analysis of the respective plan’s expected future cash flows and high-quality fixed-income investments currently available and expected to be available during the period to maturity of the benefits. Mortality assumptions are based on the “RP 2015 Mortality Table projected with Scale MP 2016” in 2019 and the “Amount-weighted Pri-2012 White Collar Mortality Table, adjusted for mortality improvements with the Scale MP—2020 mortality improvement scale on a generational basis in 2020.” This methodology more accurately matches yields to the expected benefit payments than the previous method. The discount rate used is an estimate of the rate at which the plans could settle their obligations. Rather than using a rate and curve developed using a bond portfolio, this method selects individual bonds to match to the expected cash flows of the Plans. This provides a more accurate depiction of the true cost to the plans to settle the obligations as the Plans could theoretically go into the marketplace and purchase the specific bonds used in the analysis in order to settle the obligations of the Plans. The financial impact of the enhanced estimate to the discount rate amounted to approximately $6,800,000 decrease in the projected benefit obligations for the combined plans at December 31, 2019. The Company offers a 401(k) defined contribution plan for all employees reaching minimum age and service requirements. The plan is voluntary and employee contributions are matched by the Company at a rate of 33.3% for the first 6% of compensation contributed by each employee. The Company’s match totaled $533,000 for 2020, $458,000 for 2019 and $454,000 for 2018. Administrative costs associated with the plan are absorbed by the Company. During 2020, the Company entered into split dollar life insurance agreements with two of its key executives. These agreements require that insurance policies be maintained that would provide an aggregate of $15,000,000 of insurance to the executives. This benefit vests over ten years. The expense related to these agreements totaled $155,000 for 2020. The Company has a cash incentive plan that is designed to reward our executives and officers for the achievement of annual financial performance goals of the Company as well as business line, department and individual performance. The plan supports the philosophy that management be measured for their performance as a team in the attainment of these goals. Discretionary bonus expense amounted to $2,529,000, $2,364,000, and $2,355,000 in 2020, 2019, and 2018, respectively. The Company does not offer any postretirement programs other than pensions.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	18. Commitments and Contingencies A number of legal claims against the Company arising in the normal course of business were outstanding at December 31, 2020. Management, after reviewing these claims with legal counsel, is of the opinion that their resolution will not have a material adverse effect on the Company’s consolidated financial position or results of operations.

Financial Instruments with Off-

Financial Instruments with Off-Balance-Sheet Risk	12 Months Ended
	Dec. 31, 2020
Risks and Uncertainties [Abstract]
Financial Instruments with Off-Balance-Sheet Risk	19. Financial Instruments with Off-Balance-Sheet The Company is party to financial instruments with off-balance reflect the extent of involvement the Company has in these particular classes of financial instruments. The Company’s exposure to credit loss in the event of non-performance on-balance off-balance Contract or Notional Amount 2020 2019 (dollars in thousands) Financial instruments whose contract amount represents credit risk Commitments to originate 1–4 family mortgages $ 28,163 $ 13,806 Standby and commercial letters of credit 5,344 5,779 Unused lines of credit 787,206 625,524 Unadvanced portions of construction loans 54,553 11,062 Unadvanced portions of other loans 13,767 15,801 Commitments to originate loans, unadvanced portions of construction loans and unused letters of credit are generally agreements to lend to a customer provided there is no violation of any condition established in the contract. Commitments generally have fixed expiration dates or other termination clauses and may require payment of a fee. Since many of the commitments are expected to expire without being drawn upon, the total commitment amounts do not necessarily represent future cash requirements. The Company evaluates each customer’s credit worthiness on a case-by-case

Other Operating Expenses

Other Operating Expenses	12 Months Ended
	Dec. 31, 2020
Other Income and Expenses [Abstract]
Other Operating Expenses	20. Other Operating Expenses Year ended December 31, 2020 2019 2018 (dollars in thousands) Marketing $ 1,295 $ 2,132 $ 2,346 Software maintenance/amortization 2,580 2,409 2,002 Legal and audit 1,668 1,514 1,444 Contributions 870 813 1,077 Processing services 1,712 1,875 1,740 Consulting 1,151 1,552 1,464 Postage and delivery 896 1,002 1,021 Supplies 1,096 985 987 Telephone 1,012 956 946 Directors’ fees 500 456 478 Insurance 464 456 420 Pension 1,859 2,008 678 Other 2,373 1,744 1,685 Total $ 17,476 $ 17,902 $ 16,288

Fair Values of Financial Instru

Fair Values of Financial Instruments	12 Months Ended
	Dec. 31, 2020
Investments, All Other Investments [Abstract]
Fair Values of Financial Instruments	21. Fair Values of Financial Instruments The following methods and assumptions were used by the Company in estimating fair values of its financial instruments. Excluded from this disclosure are all non-financial The assumptions used below are expected to approximate those that market participants would use in valuing these financial instruments. Fair value estimates are made at a specific point in time, based on available market information and judgments about the financial instrument, including estimates of timing, amount of expected future cash flows and the credit standing of the issuer. Such estimates do not consider the tax impact of the realization of unrealized gains or losses. In some cases, the fair value estimates cannot be substantiated by comparison to independent markets. In addition, the disclosed fair value may not be realized in the immediate settlement of the financial instrument. Care should be exercised in deriving conclusions about our business, its value or financial position based on the fair value information of financial instruments presented below. SECURITIES HELD-TO-MATURITY The fair values of these securities were based on quoted market prices, where available, as provided by third-party investment portfolio pricing vendors. If quoted market prices were not available, fair values provided by the vendors were based on quoted market prices of comparable instruments in active markets and/or based on a matrix pricing methodology which employs The Bond Market Association’s standard calculations for cash flow and price/yield analysis, live benchmark bond pricing and terms/condition data available from major pricing sources. Management regards the inputs and methods used by third party pricing vendors to be “Level 2 inputs and methods” as defined in the “fair value hierarchy” provided by FASB. LOANS The fair value of loans is estimated using the exit price notion consistent with Topic 820, Fair Value Measurement. Fair value is determined based on a discounted cash flow analysis. The discounted cash flow analysis was based on the contractual maturity of the loan and market indications of rates, prepayment speeds, defaults and credit risk. For certain non-performing TIME DEPOSITS The fair value of time deposits was estimated using a discounted cash flow approach that applies prevailing market interest rates for similar maturity instruments. The fair values of the Company’s time deposit liabilities do not take into consideration the value of the Company’s long-term relationships with depositors, which may have significant value. OTHER BORROWED FUNDS The fair value of other borrowed funds is based on the discounted value of contractual cash flows. The discount rate used is estimated based on the rates currently offered for other borrowed funds of similar remaining maturities. SUBORDINATED DEBENTURES The fair value of subordinated debentures is based on the discounted value of contractual cash flows. The discount rate used is estimated based on the rates currently offered for other subordinated debentures of similar remaining maturities. Fair Value Measurements Carrying Estimated Level 1 Level 2 Level 3 (dollars in thousands) December 31, 2020 Financial assets: Securities held-to-maturity $ 2,509,088 $ 2,579,103 $ — $ 2,579,103 $ — Loans (1) 2,960,343 2,902,390 — — 2,902,390 Financial liabilities: Time deposits 546,143 556,470 — 556,470 — Other borrowed funds 177,009 183,000 — 183,000 — Subordinated debentures 36,083 36,083 — 36,083 — December 31, 2019 Financial assets: Securities held-to-maturity $ 2,351,120 $ 2,361,304 $ — $ 2,361,304 $ — Loans (1) 2,396,534 2,424,770 — — 2,424,770 Financial liabilities: Time deposits 555,447 560,746 — 560,746 — Other borrowed funds 370,955 374,531 — 374,531 — Subordinated debentures 36,083 36,083 — 36,083 — (1) Comprised of loans (including collateral dependent impaired loans), net of deferred loan costs and the allowance for loan losses. LIMITATIONS Fair value estimates are made at a specific point in time, based on relevant market information and information about the type of financial instrument. These estimates do not reflect any premium or discount that could result from offering for sale at one time the Bank’s entire holdings of a particular financial instrument. Because no active market exists for some of the Bank’s financial instruments, fair value estimates are based on judgments regarding

Revenue from Contracts with Cus

Revenue from Contracts with Customers	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Revenue from Contracts with Customers	22. Revenue from Contracts with Customers Revenue from contracts with customers in the scope of ASC Topic 606 is measured based on the consideration specified in the contract with a customer, and excludes amounts collected on behalf of third parties. The Company recognizes revenue from contracts with customers when it satisfies its performance obligations. The Company’s performance obligations are typically satisfied as services are rendered, and our contracts do not include multiple performance obligations. Payment is generally collected at the time services are rendered, or monthly. Unsatisfied performance obligations at the report date are not material to our consolidated financial statements. The Company pays sales commissions to its employees in accordance with certain incentive plans. The Company expenses sales commissions when incurred if we do not expect to recover these costs from the terms of the contract with the customer. Sales commissions are included in compensation expense. In certain cases, other parties are involved with providing products and services to our customers. If the Company is a principal in the transaction (providing goods or services itself), revenues are reported based on the gross consideration received from the customer and any related expenses are reported gross in noninterest expense. If the Company is an agent in the transaction (arranging for another party to provide goods or services), the Company reports its net fee or commission retained as revenue. Waivers and reversals are recorded as a reduction of revenue either when the revenue is recognized by the Company or at the time the waiver or reversal is earned by the customer. A. Change in Accounting Policy The Company adopted Topic 606 Revenue from Contracts with Customers with a date of initial application of January 1, 2018 and has applied the guidance to all contracts within the scope of Topic 606 as of that date. As a result, the Company has changed its accounting policy for revenue recognition as detailed in this footnote. The Company applied Topic 606 using the cumulative effect method. Therefore, the comparative information has not been adjusted and continues to be reported under Topic 605. There was no cumulative effect adjustment as of January 1, 2018, and there were no material changes to the financial statements at or for the year ended December 31, 2018 as a result of adopting Topic 606. B. Practical Expedients The Company applies the practical expedient in paragraph 606-10-50-14 The Company applies the practical expedient in paragraph 606-10-32-18 C. Nature of Goods and Services The vast majority of the Company’s revenue is specifically out-of-scope in-scope, 1. Revenue earned at a point in time—Examples of revenue earned at a point in time are ATM transaction fees, wire transfer fees, NSF fees, credit and debit card interchange fees and foreign exchange transaction fees. Revenue is generally derived from transactional information accumulated by our systems and is recognized as revenue immediately as the transactions occur or upon providing the service to complete the customer’s transaction. The Company is the principal in each of these contracts, with the exception of credit and debit card interchange fees, in which case we are acting as the agent and record revenue net of expenses paid to the principal. 2. Revenue earned over time—The Company earns revenue from contracts with customers in a variety of ways in which the revenue is earned over a period of time—generally monthly or quarterly. Examples of this type of revenue are deposit account service fees, lockbox fees, investment management fees, merchant referral services, and safe deposit box fees. Account service charges, management fees and referral fees are recognized on a monthly basis while any transaction based income is recorded as the activity occurs. Revenue is primarily based on the number and type of transactions or assets managed and is generally derived from transactional information accumulated by our systems. Revenue is recorded in the same period as the related transactions occur or services are rendered to the customer. D. Disaggregation of Revenue The following table presents total revenues as presented in the Consolidated Statements of Income and the related amounts which are from contracts with customers within the scope of Topic 606. As illustrated here, the vast majority of our revenues are specifically excluded from the scope of Topic 606.​​​​​​​ Year ended Revenue from Year ended Revenue from Year ended Revenue from (dollars in thousands) Total net interest income $ 106,829 $ — $ 95,789 $ — $ 92,576 $ — Noninterest income: Service charges on deposit accounts 8,818 8,818 9,220 9,220 8,560 8,560 Lockbox fees 3,745 3,745 3,973 3,973 3,274 3,274 Brokerage commissions 271 — 277 — 348 — Net gains on sales of securities — — 61 — 302 — Gains on sales of mortgage loans — — 412 — — — Other income 6,266 2,132 4,456 2,799 3,764 2,536 Total noninterest income 19,100 14,695 18,399 15,992 16,248 14,370 Total revenues $ 125,929 $ 14,695 $ 114,188 $ 15,992 $ 108,824 $ 14,370 The following table provides information about receivables with customers. December 31, 2020 2019 (dollars in thousands) Receivables, which are included in “Other assets” $ 1,397 $ 1,200

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	23. Leases The Company has operating leases primarily for branch locations as well as data processing centers. The Company’s operating leases have remaining lease terms of 1 year to 31 years, some of which include options to extend the leases for up to 10 years, and some of which include options to terminate the leases within 1 year. The Company also has one sublease for part of a data processing center that the Company currently leases from a lessor. The sublease expires in 2022 with an option to terminate and no option to extend. Lease income, for the sublease, totaled approximately $40,000 for the year ended December 31, 2020. Variable lease costs​​​​​​​​​​​​​​​​​​​​​ include costs that are not included in the lease liability. The components of lease expense were as follows: Year Ended December 31, 2020 2019 (in thousands) Operating lease cost $ 2,179 $ 2,216 Variable lease cost 579 528 Total lease cost $ 2,758 $ 2,744 Supplemental cash flow information related to leases was as follows: Year Ended December 31, 2020 2019 (in thousands) Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 2,085 $ 2,130 Right-of-use Operating leases $ 3,044 $ 138 Supplemental balance sheet information related to leases was as follows: Year Ended December 31, 2020 2019 (in thousands, except lease term Operating Leases: Operating lease right-of-use $ 13,713 $ 12,521 Operating lease $ 13,935 $ 12,690 Weighted Average Remaining Lease Term: Operating Leases 10 Years 11 Years Weighted Average Discount Rate: Operating Leases 3.1 % 3.5 % The Company is obligated under a number of non-cancelable operating leases for premises and equipment expiring in various years through 2030. Total lease expense approximated $2,758,000, $2,744,000 and $2,601,000 for the years ended December 31, 2020, 2019 and 2018, respectively. Included in lease expense are amounts paid to a company affiliated with Barry R. Sloane, Chairman, President and CEO, and Linda Sloane Kay, Vice Chair, amounting to $439,000, $458,000 and $444,000, respectively. Rental income approximated $658,000, $419,000 and $373,000 in 2020, 2019 and 2018, respectively. A summary of future payments of lease liabilities were as follows: Minimum Rental Year Ending December 31, 2020 2019 (in thousands) 2020 $ — $ 2,030 2021 2,156 1,754 2022 1,995 1,603 2023 1,962 1,545 2024 1,692 1,277 2025 1,471 — Thereafter 7,394 7,312 Total lease payments $ 16,670 $ 15,521 Less imputed interest (2,735 ) (2,831 ) Present value of lease liability $ 13,935 $ 12,690

Quarterly Results of Operations

Quarterly Results of Operations (Unaudited)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results of Operations (Unaudited)	24. Quarterly Results of Operations (unaudited) 2020 Quarters Fourth Third Second First (in thousands, except share data) Interest income $ 36,617 $ 36,504 $ 36,120 $ 39,795 Interest expense 8,138 9,173 10,302 14,594 Net interest income 28,479 27,331 25,818 25,201 Provision for loan losses 2,150 900 1,700 1,075 Net interest income after provision for loan losses 26,329 26,431 24,118 24,126 Other operating income 6,580 4,169 4,041 4,310 Operating expenses 20,106 18,167 17,042 18,173 Income before income taxes 12,803 12,433 11,117 10,263 Provision for income taxes 1,203 1,546 1,061 597 Net income $ 11,600 $ 10,887 $ 10,056 $ 9,666 Share data: Average shares outstanding, basic Class A 3,655,469 3,655,469 3,652,469 3,652,349 Class B 1,912,440 1,912,440 1,915,440 1,915,560 Average shares outstanding, diluted Class A 5,567,909 5,567,909 5,567,909 5,567,909 Class B 1,912,440 1,912,440 1,915,440 1,915,560 Earnings per share, basic Class A $ 2.52 $ 2.36 $ 2.18 $ 2.10 Class B $ 1.26 $ 1.18 $ 1.09 $ 1.05 Earnings per share, diluted Class A $ 2.08 $ 1.96 $ 1.81 $ 1.74 Class B $ 1.26 $ 1.18 $ 1.09 $ 1.05 2019 Quarters Fourth Third Second First (in thousands, except share data) Interest income $ 40,518 $ 39,852 $ 39,692 $ 39,077 Interest expense 15,187 16,082 16,442 15,639 Net interest income 25,331 23,770 23,250 23,438 Provision for loan losses 550 75 250 375 Net interest income after provision for loan losses 24,781 23,695 23,000 23,063 Other operating income 4,689 4,286 4,997 4,427 Operating expenses 18,212 17,462 18,264 18,190 Income before income taxes 11,258 10,519 9,733 9,300 Provision for income taxes 526 435 267 (118 ) Net income $ 10,732 $ 10,084 $ 9,466 $ 9,418 Share data: Average shares outstanding, basic Class A 3,650,949 3,650,449 3,620,449 3,610,329 Class B 1,916,960 1,917,460 1,947,460 1,957,580 Average shares outstanding, diluted Class A 5,567,909 5,567,909 5,567,909 5,567,909 Class B 1,916,960 1,917,460 1,947,460 1,957,580 Earnings per share, basic Class A $ 2.33 $ 2.19 $ 2.06 $ 2.05 Class B $ 1.16 $ 1.09 $ 1.03 $ 1.03 Earnings per share, diluted Class A $ 1.93 $ 1.81 $ 1.70 $ 1.69 Class B $ 1.16 $ 1.09 $ 1.03 $ 1.03

Parent Company Financial Statem

Parent Company Financial Statements	12 Months Ended
	Dec. 31, 2020
Condensed Financial Information Disclosure [Abstract]
Parent Company Financial Statements	25. Parent Company Financial Statements The balance sheets of Century Bancorp, Inc. (“Parent Company”) as of December 31, 2020 and 2019 and the statements of income and cash flows for each of the years in the three-year period ended December 31, 2020, are presented below. The statements of changes in stockholders’ equity are identical to the consolidated statements of changes in stockholders’ equity and are therefore not presented here. BALANCE SHEETS December 31, 2020 2019 (dollars in thousands) ASSETS: Cash $ 5,141 $ 3,177 Investment in subsidiary, at equity 389,724 353,489 Other assets 15,920 16,325 Total assets $ 410,785 $ 372,991 LIABILITIES AND STOCKHOLDERS’ EQUITY: Liabilities $ 4,293 $ 4,327 Subordinated debentures 36,083 36,083 Stockholders’ equity 370,409 332,581 Total liabilities and stockholders’ equity $ 410,785 $ 372,991 STATEMENTS OF INCOME Year Ended December 31, 2020 2019 2018 (dollars in thousands) Income: Dividends from subsidiary $ 5,000 $ 5,000 $ 4,750 Other income 25 65 53 Total income 5,025 5,065 4,803 Interest expense 951 1,577 1,474 Operating expenses 235 215 225 Income before income taxes and equity in undistributed income of subsidiary 3,839 3,273 3,104 Benefit from income taxes (244 ) (363 ) (347 ) Income before equity in undistributed income of subsidiary 4,083 3,636 3,451 Equity in undistributed income of subsidiary 38,126 36,063 32,762 Net income $ 42,209 $ 39,699 $ 36,213 STATEMENTS OF CASH FLOWS December 31, 2020 2019 2018 (dollars in thousands) CASH FLOWS FROM OPERATING ACTIVITIES: Net income $ 42,209 $ 39,699 $ 36,213 Adjustments to reconcile net income to net cash provided by operating activities Undistributed income of subsidiary (38,126 ) (36,063 ) (32,762 ) Decrease (increase) in other assets 405 665 (158 ) (Decrease) increase in liabilities (34 ) (180 ) (1,808 ) Net cash provided by (used in) operating activities 4,454 4,121 1,485 CASH FLOWS FROM FINANCING ACTIVITIES: Cash dividends paid (2,490 ) (2,207 ) (2,203 ) Net cash used in financing activities (2,490 ) (2,207 ) (2,203 ) Net (decrease) in cash 1,964 1,914 (718 ) Cash at beginning of year 3,177 1,263 1,981 Cash at end of year $ 5,141 $ 3,177 $ 1,263

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
BASIS OF FINANCIAL STATEMENT PRESENTATION	BASIS OF FINANCIAL STATEMENT PRESENTATION The consolidated financial statements include the accounts of Century Bancorp, Inc. (the “Company”) and its wholly owned subsidiary, Century Bank and Trust Company (the “Bank”). The consolidated financial statements also include the accounts of the Bank’s wholly owned subsidiaries, Century Subsidiary Investments, Inc. (“CSII”), Century Subsidiary Investments, Inc. II (“CSII II”), Century Subsidiary Investments, Inc. III (“CSII III”) and Century Financial Services Inc. (“CFSI”). CSII, CSII II, and CSII III are engaged in buying, selling, and holding investment securities. CFSI has the power to engage in financial agency, securities brokerage, and investment and financial advisory services and related securities credit. The Company also owns 100% of Century Bancorp Capital Trust II (“CBCT II”). The entity is an unconsolidated subsidiary of the Company. All significant intercompany accounts and transactions have been eliminated in consolidation. The Company provides a full range of banking services to individual, business, and municipal customers in Massachusetts, New Hampshire, Rhode Island, Connecticut, and New York. As a bank holding company, the Company is subject to the regulation and supervision of the Federal Reserve Board. The Bank, a state chartered financial institution, is subject to supervision and regulation by applicable state and federal banking agencies, including the Federal Reserve Board, the Federal Deposit Insurance Corporation (the “FDIC”) and the Commonwealth of Massachusetts Commissioner of Banks. The Bank is also subject to various requirements and restrictions under federal and state law, including requirements to maintain reserves against deposits, restrictions on the types and amounts of loans that may be granted and the interest that may be charged thereon, and limitations on the types of investments that may be made and the types of services that may be offered. Various consumer laws and regulations also affect the operations of the Bank. In addition to the impact of regulation, commercial banks are affected significantly by the actions of the Federal Reserve Board as it attempts to control the money supply and credit availability in order to influence the economy. All aspects of the Company’s business are highly competitive. The Company faces aggressive competition from other lending institutions and from numerous other providers of financial services. The Company has one reportable operating segment. The financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America and general practices within the banking industry. In preparing the financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the balance sheet and revenues and expenses for the period. Actual results could differ from those estimates. Material estimates that are susceptible to change in the near term relate to the allowance for loan losses. Management believes that the allowance for loan losses is adequate based on a review of factors, including historical charge-off
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS The Company follows FASB ASC 820-10, Fair Value Measurements and Disclosures, 820-10 value. The three broad levels of the hierarchy are as follows: Level I—Quoted prices are available in active markets for identical assets or liabilities as of the reported date. The type of financial instruments included in Level I are highly liquid cash instruments with quoted prices, such as listed equities and money market securities, as well as listed derivative instruments. Level II—Pricing inputs are other than quoted prices in active markets, which are either directly or indirectly observable as of the reported date. The nature of these financial instruments includes cash instruments for which quoted prices are available but traded less frequently, derivative instruments whose fair value has been derived using a model where inputs to the model are directly observable in the market or can be derived principally from or corroborated by observable market data, and instruments that are fair valued using other financial instruments, the parameters of which can be directly observed. Instruments that are generally included in this category are G-7 Level III—These instruments have little to no pricing observability as of the reported date. These financial instruments do not have two-way
CASH AND CASH EQUIVALENTS	CASH AND CASH EQUIVALENTS For purposes of reporting cash flows, cash equivalents include highly liquid assets with an original maturity of three months or less. Highly liquid assets include cash and due from banks, federal funds sold and certificates of deposit.
INVESTMENT SECURITIES	INVESTMENT SECURITIES Debt securities that the Company has the positive intent and ability to hold to maturity are classified as held-to-maturity held-to-maturity available-for-sale Premiums and discounts on investment securities are amortized or accreted into income by use of the level-yield method. Gains and losses on the sale of investment securities are recognized on the trade date on a specific identification basis. Management also considers the Company’s capital adequacy, interest-rate risk, liquidity, and business plans in assessing whether it is more likely than not that the Company will sell or be required to sell the investment securities before recovery. Other-than-temporary-impairment (OTTI) arises when a security’s fair value is less than its amortized cost and, based on specific factors, the loss is considered OTTI. If the Company determines that a decline in fair value is OTTI and that it is more likely than not that the Company will not sell or be required to sell the investment security before recovery of its amortized cost, the credit portion of the impairment loss is recognized in the Company’s consolidated statement of income and the noncredit portion is recognized in accumulated other comprehensive income. The credit portion of the OTTI impairment represents the difference between the amortized cost and the present value of the expected future cash flows of the investment security. If the Company determines that a decline in fair value is OTTI and it is more likely than not that it will sell or be required to sell the investment security before recovery of its amortized cost, the entire difference between the amortized cost and the fair value of the security will be recognized in the Company’s consolidated statement of income. The transfer of a security between categories of investments shall be accounted for at fair value. For a debt security transferred into the held-to-maturity available-for-sale held-to-maturity The sale of a security held-to-maturity
FEDERAL HOME LOAN BANK STOCK	FEDERAL HOME LOAN BANK STOCK The Bank, as a member of the Federal Home Loan Bank of Boston (“FHLBB”), is required to maintain an investment in capital stock of the FHLBB. Based on redemption provisions, the stock has no quoted market value and is carried at cost. At its discretion, the FHLBB may declare dividends on the stock. The Company reviews for impairment based on the ultimate recoverability of the cost basis of the stock. As of December 31, 2020, no impairment has been recognized.
LOANS HELD FOR SALE	LOANS HELD FOR SALE Loans originated and intended for sale in the secondary market are carried at the lower of cost or estimated fair value in the aggregate. Net unrealized losses, if any, are recognized through a valuation allowance by charges to income.
LOANS	LOANS Loans are stated at the principal amount outstanding, net of amounts charged off, unamortized premiums or discounts, and deferred loan fees or costs. Interest on loans is recognized based on the daily principal amount outstanding. Accrual of interest is discontinued when loans become ninety days delinquent unless the collateral is sufficient to cover both principal and interest and the loan is in the process of collection. Past-due Loan origination fees and related direct loan origination costs are offset, and the resulting net amount is deferred and amortized over the life of the related loans using the level-yield method. Prepayments are not initially considered when amortizing premiums and discounts. The Bank measures impairment for impaired loans at either the fair value of the loan, the present value of the expected future cash flows discounted at the loan’s effective interest rate or the fair value of the collateral if the loan is collateral dependent. This method applies to all loans, uncollateralized as well as collateralized, except large groups of smaller-balance homogeneous loans such as residential real estate and consumer loans that are collectively evaluated for impairment. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off charged-off in-substance pre-modification
TRANSFERS OF FINANCIAL ASSETS	TRANSFERS OF FINANCIAL ASSETS Transfers of financial assets, typically residential mortgages, and loan participations for the Company, are accounted for as sales when control over the assets has been surrendered. Control over transferred assets is deemed to be surrendered when (1) the assets have been isolated from the Company, (2) the transferee obtains the right (free of conditions that constrain it from taking advantage of that right) to pledge or exchange the transferred assets, and (3) the Company does not maintain effective control over the transferred assets.
ACQUIRED LOANS	ACQUIRED LOANS In accordance with FASB ASC 310-30, Loans and Debt Securities Acquired with Deteriorated Credit Quality No. 03-3, loan’s contractually required payments received in excess of the amount of its cash flows expected at acquisition, or nonaccretable discount, is not accreted into income. FASB ASC 310-30 Loans which, at acquisition, do not have evidence of deterioration of credit quality since origination are outside the scope of FASB ASC 310-30. When a loan is paid off, the excess of any cash received over the net investment is recorded as interest income. In addition to the amount of purchase discount that is recognized at that time, income may include interest owed by the borrower prior to the Company’s acquisition of the loan, interest collected if on nonperforming status, prepayment fees and other loan fees.
NONPERFORMING ASSETS	NONPERFORMING ASSETS In addition to nonperforming loans, nonperforming assets include other real estate owned. Other real estate owned is comprised of properties acquired through foreclosure or acceptance of a deed in lieu of foreclosure. Other real estate owned is recorded initially at the lower of cost or the estimated fair value less costs to sell. When such assets are acquired, the excess of the loan balance over the estimated fair value of the asset is charged to the allowance for loan losses. An allowance for losses on other real estate owned is established by a charge to earnings when, upon periodic evaluation by management, further declines in the estimated fair value of properties have occurred.
ALLOWANCE FOR LOAN LOSSES	ALLOWANCE FOR LOAN LOSSES The allowance for loan losses is based on management’s evaluation of the quality of the loan portfolio and is used to provide for losses resulting from loans that ultimately prove uncollectible. The components of the allowance for loan losses represent estimates based upon Accounting Standards Codification (“ASC”) Topic 450, contingencies, and ASC Topic 310 Receivables. ASC Topic 450 applies to homogenous loan pools such as consumer installment, residential mortgages, consumer lines of credit and commercial loans that are not individually evaluated for impairment under ASC Topic 310. In determining the level of the allowance, periodic evaluations are made of the loan portfolio, which takes into account factors such as the characteristics of the loans, loan status, financial strength of the borrowers, value of collateral securing the loans and other relevant information sufficient to reach an informed judgment. The allowance is increased by provisions charged to income and reduced by loan charge-offs, net of recoveries. Management maintains an allowance for loan losses to absorb losses inherent in the loan portfolio. The allowance is based on assessments of the probable estimated losses inherent in the loan portfolio. Management’s methodology for assessing the appropriateness of the allowance consists of several key elements, which include the specific allowances, if appropriate, for identified problem loans, formula allowance, and possibly an unallocated allowance. Arriving at an appropriate level of allowance for loan losses necessarily involves a high degree of judgment. While management uses available information in establishing the allowance for loan losses, future adjustments to the allowance may be necessary if economic conditions differ substantially from the assumptions used in making the evaluations. Loans are charged-off Under ASC Topic 310, a loan is impaired, based upon current information and in management’s opinion, when it is probable that the loan will not be repaid according to its original contractual terms, including both principal and interest, or if a loan is designated as a Troubled Debt Restructuring (“TDR”). Specific allowances for loan losses entail the assignment of allowance amounts to individual loans on the basis of loan impairment. Under this method, loans are selected for evaluation based upon a change in internal risk rating, occurrence of delinquency, loan classification or nonaccrual status. A specific allowance amount is allocated to an individual loan when such loan has been deemed impaired and when the amount of a probable loss is able to be estimated on the basis of: (a) present value of anticipated future cash flows, (b) the loan’s observable fair market price or (c) fair value of collateral if the loan is collateral dependent. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off In estimating probable loan loss under ASC Topic 450, management considers numerous factors, including historical charge-offs and subsequent recoveries. The formula allowances are based on evaluations of homogenous loans to determine the allocation appropriate within each portfolio segment. Formula allowances are based on internal risk ratings or credit ratings from external sources. Individual loans within the commercial and industrial, commercial real estate and real estate construction loan portfolio segments are assigned internal risk ratings to group them with other loans possessing similar risk characteristics. Changes in risk grades affect the amount of the formula allowance. Risk grades are determined by reviewing current collateral value, financial information, cash flow, payment history and other relevant facts surrounding the particular credit. On these loans, the formula allowances are based on the risk ratings, the historical loss experience, and the loss emergence period. Historical loss data and loss emergence periods are developed based on the Company’s historical experience. For larger loans with available external credit ratings, these ratings are utilized rather than the Company’s risk ratings. The historical loss factor and loss emergence periods for these loans are based on data published by the rating agencies for similar credits as the Company has limited internal historical data. For the residential real estate and consumer loan portfolios, the formula allowances are calculated by applying historical loss experience and the loss emergence period to the outstanding balance in each loan category. Loss factors and loss emergence periods are based on the Company’s historical net loss experience. Additional allowances are added to portfolio segments based on qualitative factors. Management considers potential factors identified in regulatory guidance. Management has identified certain qualitative factors, which could impact the degree of loss sustained within the portfolio. These include market risk factors and unique portfolio risk factors that are inherent characteristics of the Company’s loan portfolio. Market risk factors may consist of changes to general economic and business conditions, such as unemployment and GDP that may impact the Company’s loan portfolio customer base in terms of ability to repay and that may result in changes in value of underlying collateral. Unique portfolio risk factors may include the outlooks for business segments in which the Company’s borrowers operate and loan size. The potential ranges for qualitative factors are based on historical volatility in losses. The actual amount utilized is based on management’s assessment of current conditions. After considering the above components, an unallocated component may be generated to cover uncertainties that could affect management’s estimate of probable losses. These uncertainties include the effects of loans in new geographical areas and new industries. The unallocated component of the
BANK PREMISES AND EQUIPMENT	BANK PREMISES AND EQUIPMENT Bank premises and equipment are stated at cost less accumulated depreciation and amortization. Land is stated at cost. Depreciation is computed using the straight-line method over the estimated useful lives of the assets or the terms of leases, if shorter. It is general practice to charge the cost of maintenance and repairs to operations when incurred; major expenditures for improvements are capitalized and depreciated.
GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS	GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS Goodwill represents the excess of the cost of an acquisition over the fair value of the net assets acquired. Goodwill is not subject to amortization. Identifiable intangible assets consist of core deposit intangibles and are assets resulting from acquisitions that are being amortized over their estimated useful lives. Goodwill and identifiable intangible assets are included in other assets on the consolidated balance sheets. The Company tests goodwill for impairment on an annual basis, or more often if events or circumstances indicate there may be impairment. Goodwill impairment testing is performed at the segment (or “reporting unit”) level. Currently, the Company’s goodwill is evaluated at the entity level as there is only one reporting unit. Goodwill is assigned to reporting units at the date the goodwill is initially recorded. Once goodwill has been assigned to reporting units, it no longer retains its association with a particular acquisition, and all of the activities within a reporting unit, whether acquired or organically grown, are available to support the value of the goodwill. Goodwill impairment is evaluated by first assessing qualitative factors (events and circumstances) to determine whether it is more likely than not (meaning a likelihood of more than 50 percent) that the fair value of a reporting unit is less than its carrying amount. If, after considering all relevant events and circumstances, an entity determines it is not more​​​​​​​ likely than not that the fair value of a reporting unit is less than its carrying amount, then performing the quantitative impairment test will be unnecessary.​​​​​​​​​​​​​​ The quantitative impairment test, used to identify potential impairment, involves comparing each reporting unit’s fair value to its carrying value including goodwill. If the fair value of a reporting unit exceeds its carrying value, applicable goodwill is considered not to be impaired. If the carrying value exceeds fair value, the difference is recorded as impairment.
SERVICING	SERVICING The Company services mortgage loans for others. Mortgage servicing assets are recognized as separate assets when rights are acquired through purchase or through sale of financial assets. Fair value is determined using prices for similar assets with similar characteristics, when available, or based upon discounted cash flows using market-based assumptions. The valuation model incorporates assumptions that market participants would use in estimating future net servicing income, such as the cost to service, the discount rate, an inflation rate, ancillary income, prepayment speeds and default rates and losses. Capitalized servicing rights are reported in other assets and are amortized into loan servicing fee income in proportion to, and over the period of, the estimated future net servicing income of the underlying financial assets. Servicing assets are evaluated for impairment based upon the fair value of the rights as compared to amortized cost. Impairment is determined by stratifying rights by predominant risk characteristics, such as interest rates and terms. Impairment is recognized through a valuation allowance for an individual stratum, to the extent that fair value is less than the capitalized amount for the stratum. Changes in the valuation allowance are reported in loan servicing fee income.
INCOME TAXES	INCOME TAXES The Company uses the asset and liability method in accounting for income taxes. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which temporary differences are expected to be recovered or settled. Under this method, the effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. The Company accounts for uncertain tax positions in accordance with FASB ASC 740. The Company classifies interest resulting from underpayment of income taxes as income tax expense in the first period the interest would begin accruing according to the provisions of the relevant tax law. The Company classifies penalties resulting from underpayment of income taxes as income tax expense in the period for which the Company claims or expects to claim an uncertain tax position or in the period in which the Company’s judgment changes regarding an uncertain tax position. Also, for tax years beginning after December 31, 2017, the corporate Alternative Minimum Tax (“AMT”) has been repealed. For 2018 through 2021, the AMT credit carryforward can offset regular tax liability and is refundable in an amount equal to 50% (100% for 2021) of the excess of the minimum tax credit for the tax year over the amount of the credit allowable for the year against regular tax liability. On March 27, 2020, the Coronavirus Aid, Relief and Economic Security (CARES) Act was signed into law. As a result of the CARES Act, the full balance of the AMT credit was refunded during 2020. The Tax Act also contains other provisions that may affect the Company currently or in future years. Among these are changes to the deductibility of meals and entertainment, the deductibility of executive compensation, the dividend received deduction and net operating loss carryforwards. Tax Act changes for individuals include lower tax rates, mortgage interest and state and local tax limitations as well as an increase in the standard deduction, among others.
EARNINGS PER SHARE ("EPS")	EARNINGS PER SHARE (“EPS”) Class A and Class B shares participate equally in undistributed earnings. Under the Company’s Articles of Organization, the holders of Class A Common Stock are entitled to receive dividends per share equal to at least 200% of dividends paid, if any, from time to time, on each share of Class B Common Stock. Diluted EPS includes the dilutive effect of common stock equivalents; basic EPS excludes all common stock equivalents. The Company utilizes the two class method for reporting EPS. The two-class
PENSION	PENSION The Company provides pension benefits to certain of its employees under a noncontributory, defined benefit plan, which is funded on a current basis in compliance with the requirements of the Employee Retirement Income Security Act of 1974 (“ERISA”) and recognizes costs over the estimated employee service period. The Company also has a Supplemental Executive Insurance/Retirement Plan (“the Supplemental Plan”), which is limited to certain officers and employees of the Company. The Supplemental Plan is accrued on a current basis and recognizes costs over the estimated employee service period. Executive officers of the Company or its subsidiaries who have at least one year of service may participate in the Supplemental Plan. The Supplemental Plan is voluntary. Individual life insurance policies, which are owned by the Company, are purchased covering the life of each participant. Prior to December 31, 2019, the Company utilized a full yield curve approach in the estimation of the service and interest components by applying the specific spot rates along the yield curve used in the determination of the benefit obligation to the underlying projected cash flows. Effective December 31, 2019, the discount rate is determined by preparing an analysis of the respective plan’s expected future cash flows and high-quality fixed-income investments currently available and expected to be available during the period to maturity of the benefits.
LEASING	LEASING A right-of-use For real estate leases, lease components and non-lease non-interest
RECENT ACCOUNTING DEVELOPMENTS	RECENT ACCOUNTING DEVELOPMENTS Recently Adopted Accounting Standards Updates In August 2018, Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2018-15, Internal Use Software (Subtopic 350-40): internal-use In August 2018, FASB issued ASU 2018-13, In January 2017, the FASB issued ASU 2017-04, Securities and Exchange Commission (SEC) Ruling: In August 2018, the SEC issued a final rule that amends certain of the Commission’s disclosure requirements “that have become redundant, duplicative, overlapping, outdated, or superseded, in light of other Commission disclosure requirements, U.S. GAAP, or changes in the information environment.” The financial reporting implications of year-to-date year-to-date

Securities Available-for-Sale (

Securities Available-for-Sale (Tables)	12 Months Ended
	Dec. 31, 2020
Text Block [Abstract]
Summary of Securities Available-for-Sale	December 31, 2020 December 31, 2019 Amortized Gross Gross Estimated Amortized Gross Gross Estimated (dollars in thousands) SBA Backed Securities $ 44,328 $ — $ 289 $ 44,039 $ 54,331 $ 23 $ 143 $ 54,211 U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 177,239 819 317 177,741 184,580 139 532 184,187 Privately Issued Residential Mortgage-Backed Securities 330 2 4 328 397 1 2 396 Obligations Issued by States and Political Subdivisions 52,276 — — 52,276 18,016 60 — 18,076 Other Debt Securities 8,100 24 60 8,064 3,600 51 19 3,632 Total $ 282,273 $ 845 $ 670 $ 282,448 $ 260,924 $ 274 $ 696 $ 260,502
Estimated Maturity Distribution of Securities Available-for-Sale	The following table shows the estimated maturity distribution of the Company’s securities available-for-sale Amortized Fair Value (dollars in thousands) Within one year $ 52,501 $ 52,497 After one but within five years 114,877 115,236 After five but within ten years 97,911 97,880 More than ten years 16,984 16,835 Total $ 282,273 $ 282,448
Continuous Unrealized Loss Position for 12 Months or Less and 12 Months and Longer	The following table shows the temporarily impaired securities of the Company’s available-for-sale Temporarily Impaired Investments December 31, 2020 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) SBA Backed Securities $ 13,839 $ 42 $ 30,200 $ 247 $ 44,039 $ 289 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 18,188 50 33,617 267 51,805 317 Privately Issued Residential Mortgage-Backed Securities — — 210 4 210 4 Obligations Issued by States and Political Subdivisions — — — — — — Other Debt Securities 3,942 58 1,298 2 5,240 60 Total temporarily impaired securities $ 35,969 $ 150 $ 65,325 $ 520 $ 101,294 $ 670 Temporarily Impaired Investments December 31, 2019 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) SBA Backed Securities $ 14,560 $ 30 $ 22,092 $ 113 $ 36,652 $ 143 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 108,806 379 29,178 153 137,984 532 Privately Issued Residential Mortgage-Backed Securities 252 2 — — 252 2 Obligations Issued by States and Political Subdivisions — — — — — — Other Debt Securities 800 1 481 18 1,281 19 Total temporarily impaired securities $ 124,418 $ 412 $ 51,751 $ 284 $ 176,169 $ 696

Investment Securities Held-to_2

Investment Securities Held-to-Maturity (Tables)	12 Months Ended
	Dec. 31, 2020
Text Block [Abstract]
Summary of Held-to-Maturity Securities	December 31, 2020 December 31, 2019 Amortized Gross Gross Estimated Amortized Gross Gross Estimated (dollars in thousands) U.S. Government Sponsored Enterprises $ 244,220 $ 389 $ 866 $ 243,743 $ 98,867 $ 527 $ 96 $ 99,298 SBA Backed Securities 37,783 2,002 — 39,785 44,379 182 303 44,258 U.S. Government Sponsored Enterprises Mortgage-Backed Securities 2,227,085 69,522 1,032 2,295,575 2,207,874 20,720 10,846 2,217,748 Total $ 2,509,088 $ 71,913 $ 1,898 $ 2,579,103 $ 2,351,120 $ 21,429 $ 11,245 $ 2,361,304
Company's Securities Held-to-Maturity	The following table shows the maturity distribution of the Company’s securities held-to-maturity Amortized Fair Value (dollars in thousands) Within one year $ 79,572 $ 80,175 After one but within five years 1,870,589 1,927,961 After five but within ten years 555,932 567,577 More than ten years 2,995 3,390 Total $ 2,509,088 $ 2,579,103
Unrealized Market Loss of Securities	The following table shows the temporarily impaired securities of the Company’s held-to-maturity 53 and 0 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 600 holdings at December 31, 2020. Temporarily Impaired Investments December 31, 2020 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) U.S. Government Sponsored Enterprises $ 162,870 $ 866 $ — $ — $ 162,870 $ 866 SBA Backed Securities — — — — — — U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 302,401 1,032 — — 302,401 1,032 Total temporarily impaired securities $ 465,271 $ 1,898 $ — $ — $ 465,271 $ 1,898 The following table shows the temporarily impaired securities of the Company’s held-to-maturity portfolio at December , . This table shows the unrealized market loss of securities that have been in a continuous unrealized loss position for months or less and a continuous loss position for months and longer. Temporarily Impaired Investments December 31, 2019 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) U.S. Government Sponsored Enterprises $ 24,420 $ 72 $ 9,976 $ 24 $ 34,396 $ 96 SBA Backed Securities 25,251 303 — — 25,251 303 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 613,905 3,949 389,919 6,897 1,003,824 10,846 Total temporarily impaired securities $ 663,576 $ 4,324 $ 399,895 $ 6,921 $ 1,063,471 $ 11,245

Loans (Tables)

Loans (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Summary of Composition of Loan Portfolio	The following summary shows the composition of the loan portfolio at the dates indicated. December 31, 2020 2019 (dollars in thousands) Construction and land development $ 10,909 $ 8,992 Commercial and industrial 1,314,245 812,417 Municipal 137,607 120,455 Commercial real estate 789,836 786,102 Residential real estate 448,436 371,897 Consumer 20,007 21,071 Home equity 274,357 304,363 Overdrafts 432 822 Total $ 2,995,829 $ 2,426,119
Composition of Nonaccrual Loans and Impaired Loans	The composition of nonaccrual loans and impaired loans is as follows: December 31, 2020 2019 2018 (dollars in thousands) Loans on nonaccrual $ 3,996 $ 2,014 $ 1,313 Loans 90 days past due and still accruing 90 — — Impaired loans on nonaccrual included above 3,178 891 296 Total recorded investment in impaired loans 5,379 3,252 3,051 Average recorded investment of impaired loans 3,641 3,161 5,491 Accruing troubled debt restructures 2,202 2,361 2,559 Interest income not recorded on nonaccrual loans according to their original terms 95 67 64 Interest income on nonaccrual loans actually recorded — — — Interest income recognized on impaired loans 94 103 196
Aggregate Amount of Loans to Directors and Officers of Company and their Associates	The following table shows the aggregate amount of loans to directors and officers of the Company and their associates during 2020. Balance at Additions Repayments Balance at (dollars in thousands) $12,031 $6,340 $2,389 $15,982

Allowance for Loan Losses (Tabl

Allowance for Loan Losses (Tables)	12 Months Ended
	Dec. 31, 2020
Receivables [Abstract]
Analysis of the allowance for loan losses	An analysis of the allowance for loan losses for each of the three years ending December 31, 2020, 2019 and 2018 is as follows: 2020 2019 2018 (dollars in thousands) Allowance for loan losses, beginning of year $ 29,585 $ 28,543 $ 26,255 Loans charged-off (238 ) (454 ) (833 ) Recoveries on loans previously charged-off 314 246 1,771 Net recoveries (charge-offs ) 76 (208 ) 938 Provision charged to expense 5,825 1,250 1,350 Allowance for loan losses, end of year $ 35,486 $ 29,585 $ 28,543
Summary of Allowance for Loan Losses	Further information pertaining to the allowance for loan losses at December 31, 2020 follows: Construction Commercial Municipal Commercial Residential Consumer Home Unallocated Total (dollars in thousands) Allowance for Loan Losses: Ending balance at December 31, 2019 $ 331 $ 11,596 $ 2,566 $ 11,464 $ 2,194 $ 312 $ 1,065 $ 57 $ 29,585 Charge-offs — (29 ) — — — (209 ) — — (238 ) Recoveries — 197 — — — 112 5 — 314 Provision 98 4,949 238 287 (83 ) 26 138 172 5,825 Ending balance at December 31, 2020 $ 429 $ 16,713 $ 2,804 $ 11,751 $ 2,111 $ 241 $ 1,208 $ 229 $ 35,486 Amount of allowance for loan losses for loans deemed to be impaired $ — $ 140 $ — $ 449 $ — $ — $ — $ — $ 589 Amount of allowance for loan losses for loans not deemed to be impaired $ 429 $ 16,573 $ 2,804 $ 11,302 $ 2,111 $ 241 $ 1,208 $ 229 $ 34,897 Loans: Ending balance $ 10,909 $ 1,314,245 $ 137,607 $ 789,836 $ 448,436 $ 20,439 $ 274,357 $ — $ 2,995,829 Loans deemed to be impaired $ — $ 439 $ — $ 4,940 $ — $ — $ — $ — $ 5,379 Loans not deemed to be impaired $ 10,909 $ 1,313,806 $ 137,607 $ 784,896 $ 448,436 $ 20,439 $ 274,357 $ — $ 2,990,450 Further information pertaining to the allowance for loan losses at December 31, 2019 follows: Construction Commercial Municipal Commercial Residential Consumer Home Unallocated Total (dollars in thousands) Allowance for Loan Losses: Ending balance at December 31, 2018 $ 1,092 $ 10,998 $ 1,838 $ 10,663 $ 2,190 $ 365 $ 1,111 $ 286 $ 28,543 Charge-offs — (137 ) — — — (295 ) (22 ) — (454 ) Recoveries — 60 — — — 186 — — 246 Provision (761 ) 675 728 801 4 56 (24 ) (229 ) 1,250 Ending balance at December 31, 2019 $ 331 $ 11,596 $ 2,566 $ 11,464 $ 2,194 $ 312 $ 1,065 $ 57 $ 29,585 Amount of allowance for loan losses for loans deemed to be impaired $ — $ 15 $ — $ 87 $ — $ — $ — $ — $ 102 Amount of allowance for loan losses for loans not deemed to be impaired $ 331 $ 11,581 $ 2,566 $ 11,377 $ 2,194 $ 312 $ 1,065 $ 57 $ 29,483 Loans: Ending balance $ 8,992 $ 812,417 $ 120,455 $ 786,102 $ 371,897 $ 21,893 $ 304,363 $ — $ 2,426,119 Loans deemed to be impaired $ — $ 906 $ — $ 2,346 $ — $ — $ — $ — $ 3,252 Loans not deemed to be impaired $ 8,992 $ 811,511 $ 120,455 $ 783,756 $ 371,897 $ 21,893 $ 304,363 $ — $ 2,422,867
Loans by Risk Rating	The following table presents the Company’s loans by risk rating at December 31, 2020. Construction Commercial Municipal Commercial (dollars in thousands) Grade: 1-3 $10,909 $ 1,309,861 $ 137,607 $761,101 4 (Monitor) — 3,945 — 23,795 5 (Substandard) — — — — 6 (Doubtful) — — — — Impaired — 439 — 4,940 Total $10,909 $ 1,314,245 $ 137,607 $789,836 The following table presents the Company’s loans by risk rating at December 31, 2019. Construction Commercial Municipal Commercial (dollars in thousands) Grade: 1-3 $ 8,992 $ 807,486 $ 120,455 $ 759,402 4 (Monitor) — 4,025 — 24,354 5 (Substandard) — — — — 6 (Doubtful) — — — — Impaired — 906 — 2,346 Total $ 8,992 $ 812,417 $ 120,455 $ 786,102
Loans by Credit Rating	The following table presents the Company’s loans by credit rating at December 31, 2020. Commercial Municipal Commercial Total (dollars in thousands) Credit Rating: Aaa-Aa3 $ 710,955 $ 74,291 $ 38,035 $ 823,281 A1-A3 183,123 7,103 145,583 335,809 Baa1-Baa3 50,000 51,133 140,905 242,038 Ba2 — 5,080 — 5,080 Total $ 944,078 $ 137,607 $ 324,523 $ 1,406,208 The following table presents the Company’s loans by credit rating at December 31, 2019. Commercial Municipal Commercial Total (dollars in thousands) Credit Rating: Aaa-Aa3 $ 523,644 $ 53,273 $ 40,437 $ 617,354 A1-A3 186,044 7,354 148,346 341,744 Baa1-Baa3 — 51,133 144,711 195,844 Ba2 — 5,895 — 5,895 Total $ 709,688 $ 117,655 $ 333,494 $ 1,160,837
Aging of Past Due Loan Losses	At December 31, 2020, the aging of past due loans are as follows: Accruing 30-89 Days Non Accruing Total Current Total (dollars in thousands) Construction and land development $ — $ — $ — $ — $ 10,909 $ 10,909 Commercial and industrial 56 297 90 443 1,313,802 1,314,245 Municipal — — — — 137,607 137,607 Commercial real estate — 2,881 — 2,881 786,955 789,836 Residential real estate 390 527 — 917 447,519 448,436 Consumer and overdrafts 21 1 — 22 20,417 20,439 Home equity 1,001 290 — 1,291 273,066 274,357 Total $ 1,468 $ 3,996 $ 90 $ 5,554 $ 2,990,275 $ 2,995,829 At December 31, 2019 the aging of past due loans are as follows: Accruing 30-89 Days Non Accruing Total Current Total (dollars in thousands) Construction and land development $ — $ — $ — $ — $ 8,992 $ 8,992 Commercial and industrial 227 400 — 627 811,790 812,417 Municipal — — — — 120,455 120,455 Commercial real estate 840 492 — 1,332 784,770 786,102 Residential real estate 1,563 683 — 2,246 369,651 371,897 Consumer and overdrafts 18 4 — 22 21,871 21,893 Home equity 603 435 — 1,038 303,325 304,363 Total $ 3,251 $ 2,014 $ — $ 5,265 $ 2,420,854 $ 2,426,119
Information Pertaining to Impaired Loans	The following is information pertaining to impaired loans at December 31, 2020: Carrying Unpaid Required Average Interest (dollars in thousands) With no required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 4 5 — 633 — Municipal — — — — — Commercial real estate 272 306 — 437 — Residential real estate — — — 127 — Consumer — — — — — Home equity — — — — — Total $ 276 $ 311 $ — $ 1,197 $ — With required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 435 454 140 121 8 Municipal — — — — — Commercial real estate 4,668 4,797 449 2,323 86 Residential real estate — — — — — Consumer — — — — — Home equity — — — — — Total $ 5,103 $ 5,251 $ 589 $ 2,444 $ 94 Total Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 439 459 140 754 8 Municipal — — — — — Commercial real estate 4,940 5,103 449 2,760 86 Residential real estate — — — 127 — Consumer — — — — — Home equity — — — — — Total $ 5,379 $ 5,562 $ 589 $ 3,641 $ 94 The following is information pertaining to impaired loans at December 31, 2019: Carrying Unpaid Required Average Interest (dollars in thousands) With no required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 770 976 — 138 6 Municipal — — — — — Commercial real estate 160 189 — 445 — Residential real estate — — — — — Consumer — — — — — Home equity — — — — — Total $ 930 $ 1,165 $ — $ 583 $ 6 With required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 136 137 15 264 7 Municipal — — — — — Commercial real estate 2,186 2,306 87 2,314 90 Residential real estate — — — — — Consumer — — — — — Home equity — — — — — Total $ 2,322 $ 2,443 $ 102 $ 2,578 $ 97 Total Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 906 1,113 15 402 13 Municipal — — — — — Commercial real estate 2,346 2,495 87 2,759 90 Residential real estate — — — — — Consumer — — — — — Home equity — — — — — Total $ 3,252 $ 3,608 $ 102 $ 3,161 $ 103

Bank Premises and Equipment (Ta

Bank Premises and Equipment (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Schedule of Bank Premises and Equipment	December 31, 2020 2019 Estimated Useful Life (dollars in thousands) Land $ 7,905 $ 7,246 — Bank premises 34,556 28,175 30-39 Furniture and equipment 34,381 33,259 3-10 Leasehold improvements 12,830 12,674 30-39 89,672 81,354 Accumulated depreciation and amortization (50,610 ) (47,402 ) Total $ 39,062 $ 33,952

Goodwill and Identifiable Int_2

Goodwill and Identifiable Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Identifiable Intangible Assets	The changes in goodwill and identifiable intangible assets for the years ended December 31, 2020 and 2019 are shown in the table below. Carrying Amount of Goodwill and Intangibles Goodwill Mortgage Total (dollars in thousands) Balance at December 31, 2018 $ 2,714 $ 1,226 $ 3,940 Additions — 237 237 Amortization Expense — (261 ) (261 ) Balance at December 31, 2019 $ 2,714 $ 1,202 $ 3,916 Additions — — — Amortization Expense — (429 ) (429 ) Balance at December 31, 2020 $ 2,714 $ 773 $ 3,487

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Financial Instruments Measured at Fair Value on a Recurring and Non-recurring Basis	The results of the fair value hierarchy as of December 31, 2020, are as follows: Fair Value Measurements Using Carrying Quoted Prices Significant Significant Other (dollars in thousands) Financial Instruments Measured at Fair Value on a Recurring Basis Securities AFS SBA Backed Securities $ 44,039 $ — $ 44,039 $ — U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 177,741 — 177,741 — Privately Issued Residential Mortgage-Backed Securities 328 — 328 — Obligations Issued by States and Political Subdivisions 52,276 — — 52,276 Other Debt Securities 8,064 — 8,064 — Total $ 282,448 $ — $ 230,172 $ 52,276 Equity Securities $ 1,668 $ 303 $ 1,365 $ — Financial Instruments Measured at Fair Value on a Non-recurring Impaired Loans $ 3,178 $ — $ — $ 3,178 The results of the fair value hierarchy as of December 31, 2019, are as follows: Fair Value Measurements Using Carrying Quoted Prices Significant Significant Other (dollars in thousands) Financial Instruments Measured at Fair Value on a Recurring Basis Securities AFS SBA Backed Securities $ 54,211 $ — $ 54,211 $ — U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 184,187 — 184,187 — Privately Issued Residential Mortgage-Backed Securities 396 — 396 — Obligations Issued by States and Political Subdivisions 18,076 — 4,775 13,301 Other Debt Securities 3,632 — 3,632 — Total $ 260,502 $ — $ 247,201 $ 13,301 Equity Securities $ 1,688 $ 343 $ 1,345 $ — Financial Instruments Measured at Fair Value on a Non-recurring Impaired Loans $ 877 $ — $ — $ 877
Assets Measured at Fair Value	The following table presents additional information about assets measured at fair value on a recurring and nonrecurring basis for which the Company has utilized Level 3 inputs to determine fair value (dollars in Asset Fair Value Valuation Technique Unobservable Input Unobservable Input Securities AFS (1) $ 52,276 Discounted cash flow Discount rate 0%-1.0% (2) Impaired Loans 3,178 Appraisal of collateral (3) Appraisal adjustments (4) 0%-17% discount The following table presents additional information about assets measured at fair value on a recurring and nonrecurring basis for which the Company has utilized Level 3 inputs to determine fair value (dollars in thousands) at December 31, 2019. Management continues to monitor the assumptions used to value the assets listed below. Asset Fair Value Valuation Technique Unobservable Input Unobservable Input Securities AFS (1) $ 13,301 Discounted cash flow Discount rate 1.5%-3.2% (2) Impaired Loans 877 Appraisal of collateral (3) Appraisal adjustments (4) 0%-30% discount (1) Municipal securities generally have maturities of one year or less and, therefore, the amortized cost equates to the fair value. (2) Weighted averages. (3) Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various Level 3 inputs which are not identifiable. (4) Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated expenses.
Changes in Level 3 Securities	The changes in Level 3 securities for the year ended December 31, 2019 are as shown in the table below: Obligations (dollars in thousands) Balance at December 31, 2018 $ 88,728 Purchases 21,408 Maturities/redemptions (96,812 ) Transfer to Level 2 — Amortization (23 ) Change in fair value — Balance at December 31, 2019 $ 13,301

Deposits (Tables)

Deposits (Tables)	12 Months Ended
	Dec. 31, 2020
Deposits [Abstract]
Summary of Remaining Maturities or Re-pricing of Time Deposits	The following is a summary of remaining maturities or re-pricing 2020 Percent 2019 Percent (dollars in thousands) Within one year $ 465,080 85 % $ 383,497 69 % Over one year to two years 50,456 9 % 123,016 22 % Over two years to three years 19,104 4 % 27,223 5 % Over three years to five years 11,503 2 % 21,711 4 % Total $ 546,143 100 % $ 555,447 100 %

Securities Sold Under Agreeme_2

Securities Sold Under Agreements to Repurchase (Tables)	12 Months Ended
	Dec. 31, 2020
Brokers and Dealers [Abstract]
Summary of Securities Sold Under Agreements to Repurchase	The following is a summary of securities sold under agreements to repurchase as of December 31, 2020 2019 2018 (dollars in thousands) Amount outstanding at December 31 $ 232,090 $ 266,045 $ 154,240 Weighted average rate at December 31 0.34 % 0.96 % 0.82 % Maximum amount outstanding at any month end $ 253,980 $ 307,235 $ 174,150 Daily average balance outstanding during the year $ 221,609 $ 224,361 $ 147,944 Weighted average rate during the year 0.62 % 1.05 % 0.66 %

Other Borrowed Funds and Subo_2

Other Borrowed Funds and Subordinated Debentures (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Summary of Other Borrowed Funds and Subordinated Debentures	The following is a summary of other borrowed funds and subordinated debentures as of December 31, 2020 2019 2018 (dollars in thousands) Amount outstanding at December 31 $ 213,092 $ 407,038 $ 238,461 Weighted average rate at December 31 2.38 % 2.37 % 2.76 % Maximum amount outstanding at any month end $ 348,203 $ 487,502 $ 542,913 Daily average balance outstanding during the year $ 201,656 $ 231,926 $ 291,674 Weighted average rate during the year 2.66 % 2.95 % 2.61 %
Schedule of the Maturity Distribution of FHLBB Advances with the Weighted Average Interest Rates	A schedule of the maturity distribution of FHLBB advances with the weighted average interest rates is as follows: December 31, 2020 2019 2018 Amount Weighted Amount Weighted Amount Weighted (dollars in thousands) Within one year $ 67,500 1.78 % $ 218,000 1.86 % $ 63,000 2.17 % Over one year to two years $ 3,500 2.15 % $ 42,500 2.58 % $ 28,000 2.29 % Over two years to three years $ 30,000 3.38 % $ 3,500 2.15 % $ 25,000 3.34 % Over three years to five years $ 67,000 2.64 % $ 70,000 2.85 % $ 33,500 2.23 % Over five years $ 9,009 2.83 % $ 36,955 2.88 % $ 52,878 2.47 % Total $ 177,009 2.44 % $ 370,955 2.23 % $ 202,378 2.42 %

Reclassifications Out of Accu_2

Reclassifications Out of Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2020
Text Block [Abstract]
Reclassifications Out of Accumulated Other Comprehensive Income	Amount Reclassified from Accumulated Details About Accumulated Other Year Ended (a) Year Ended (a) Affected Line Item in the Statement Unrealized gains and losses on available-for-sale $ — $ 61 Net gains on sales of investments — (17 ) Provision for income taxes $ — $ 44 Net income Accretion of unrealized losses transferred $ (798 ) $ (1,022 ) Interest income securities held-to-maturity 207 269 Provision for income taxes $ (591 ) $ (753 ) Net income Amortization of defined benefit pension items Prior-service costs $ (114 ) $ (114 ) Salaries and employee benefits (b) Actuarial gains (losses) (1,889 ) (1,351 ) Salaries and employee benefits (b) Total before tax (2,003 ) (1,465 ) Income before taxes Tax (expense) or benefit 562 412 Provision for income taxes Net of tax $ (1,441 ) $ (1,053 ) Net income (a) Amounts in parentheses indicate decreases to profit/loss. (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see employee benefits footnote (Note 17) for additional details).

Earnings per share ("EPS") (Tab

Earnings per share ("EPS") (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Reconciliation of Basic EPS and Diluted EPS	The following table is a reconciliation of basic EPS and diluted EPS: Year Ended December 31, 2020 2019 2018 (in thousands except share and per share data) BASIC EPS COMPUTATION Numerator: Net income, Class A $ 33,449 $ 31,351 $ 28,479 Net income, Class B 8,760 8,348 7,734 Denominator: Weighted average shares outstanding, Class A 3,653,939 3,633,044 3,608,179 Weighted average shares outstanding, Class B 1,913,970 1,934,865 1,959,730 Basic EPS, Class A $ 9.15 $ 8.63 $ 7.89 Basic EPS, Class B $ 4.58 $ 4.31 $ 3.95 DILUTED EPS COMPUTATION Numerator: Net income, Class A $ 33,449 $ 31,351 $ 28,479 Net income, Class B 8,760 8,348 7,734 Total net income, for diluted EPS, Class A computation 42,209 39,699 36,213 Denominator: Weighted average shares outstanding, basic, Class A 3,653,939 3,633,044 3,608,179 Weighted average shares outstanding, Class B 1,913,970 1,934,865 1,959,730 Weighted average shares outstanding diluted, Class A 5,567,909 5,567,909 5,567,909 Weighted average shares outstanding, Class B 1,913,970 1,934,865 1,959,730 Diluted EPS, Class A $ 7.58 $ 7.13 $ 6.50 Diluted EPS, Class B $ 4.58 $ 4.31 $ 3.95

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Summary of the Bank's Actual Capital Amounts and Ratios	The Bank’s actual capital amounts and ratios are presented in the following table: Actual For Capital To Be Well Capitalized Amount Ratio Amount Ratio Amount Ratio As of December 31, 2020 (Basel III) Total Capital (to Risk-Weighted Assets) $ 448,695 13.03 % $ 275,403 8.00 % $ 344,254 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 413,209 12.00 % 206,553 6.00 % 275,403 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 413,209 12.00 % 154,914 4.50 % 223,765 6.50 % Tier 1 Capital (to 4th Quarter Average Assets) 413,209 6.41 % 257,763 4.00 % 322,204 5.00 % As of December 31, 2019 (Basel III) Total Capital (to Risk-Weighted Assets) $ 401,850 13.57 % $ 236,830 8.00 % $ 296,037 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 372,265 12.57 % 177,622 6.00 % 236,830 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 372,265 12.57 % 133,217 4.50 % 192,424 6.50 % Tier 1 Capital (to 4th Quarter Average Assets) 372,265 7.01 % 212,549 4.00 % 265,686 5.00 %
Summary of the Company's Actual Capital Amounts and Ratios	The Company’s actual capital amounts and ratios are presented in the following table: Actual For Capital To Be Well Capitalized Amount Ratio Amount Ratio Amount Ratio As of December 31, 2020 (Basel III) Total Capital (to Risk-Weighted Assets) $ 464,331 13.43 % $ 276,673 8.00 % $ 345,842 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 428,845 12.40 % 207,505 6.00 % 276,673 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 393,845 11.39 % 155,629 4.50 % 224,797 6.50 % Tier 1 Capital (to 4th Quarter Average Assets) 428,845 6.64 % 258,414 4.00 % 323,017 5.00 % As of December 31, 2019 (Basel III) Total Capital (to Risk-Weighted Assets) $ 415,863 13.97 % $ 238,132 8.00 % $ 297,665 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 386,308 12.98 % 178,599 6.00 % 238,132 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 351,308 11.80 % 133,949 4.50 % 193,482 6.50 % Tier 1 Capital (to 4th Quarter Average Assets) 386,308 7.25 % 213,222 4.00 % 266,528 5.00 %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Summary of Current and Deferred Components of Income Tax Expense (Benefit)	The current and deferred components of income tax expense (benefit) for the years ended December 31, are as follows: 2020 2019 2018 (dollars in thousands) Current expense: Federal $ 3,651 $ 2,548 $ 2,637 State 1,845 697 697 Total current expense 5,496 3,245 3,334 Deferred (benefit) expense: Federal (491 ) (1,660 ) (1,238 ) State (598 ) (367 ) (528 ) Valuation Allowance — (108 ) — Total deferred (benefit) expense (1,089 ) (2,135 ) (1,766 ) Provision for income taxes $ 4,407 $ 1,110 $ 1,568
Income Tax Accounts Included in Other Assets	Income tax accounts included in other assets at December 31, are as follows: 2020 2019 (dollars in thousands) Income tax receivable $ 2,250 $ 3,446 Deferred income tax asset, net 26,431 24,566 Total $ 28,681 $ 28,012
Summary of Differences between Income Tax (Benefit) Expense at the Statutory Federal Income Tax Rate and Total Income Tax Expense	Differences between income tax expense (benefit) at the statutory federal income tax rate and total income tax expense are summarized as follows: 2020 2019 2018 (dollars in thousands) Federal income tax expense at statutory rate $ 9,789 $ 8,570 $ 7,934 State income tax, net of federal income tax benefit 930 261 134 Life insurance income (834 ) (265 ) (176 ) Tax-exempt (5,490 ) (6,737 ) (6,510 ) Tax credits, net (231 ) (292 ) (349 ) Valuation allowance reversal — (108 ) — Sequestration (reversal) accrual — (438 ) 438 Other, net 243 119 97 Total $ 4,407 $ 1,110 $ 1,568 Effective tax rate 9.45 % 2.72 % 4.15 %
Gross Deferred Income Tax Assets and Gross Deferred Income Tax Liabilities	The following table sets forth the Company’s gross deferred income tax assets and gross deferred income tax liabilities at December 31: 2020 2019 (dollars in thousands) Deferred income tax assets: Allowance for loan losses $ 10,025 $ 8,354 Deferred compensation 8,833 8,910 Pension and SERP liability 9,911 8,770 Operating lease liabilities 3,917 3,567 Unrealized losses on securities transferred to held-to-maturity 436 643 Depreciation 1,214 1,060 QZAB credit — 812 PPP loan origination fees 919 — Accrued bonus — 708 Charitable contributions carryforward — 276 Nonaccrual interest 75 115 Other 305 206 Gross deferred income tax asset 35,635 33,421 Deferred income tax liabilities: Pension liability (4,142 ) (4,258 ) Operating lease right-of-use (3,855 ) (3,520 ) Deferred origination costs (584 ) (516 ) Prepaid expenses (361 ) (337 ) Mortgage servicing rights (217 ) (338 ) Unrealized (gains) losses on securities available-for-sale (45 ) 114 Gross deferred income tax liability (9,204 ) (8,855 ) Deferred income tax asset net $ 26,431 $ 24,566

Employee Benefits (Tables)

Employee Benefits (Tables)	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Fair Value of Plan Assets and Major Categories	The fair value of plan assets and major categories as of December 31, 2020, is as follows: Description Percent NAV Level 1 Level 2 Level 3 Total (dollars in thousands) Collective Funds 10.6 % $ — $ 5,999 $ — $ — $ 5,999 Equity Securities 26.6 % — 14,952 — — 14,952 Diversified Mutual Funds 12.3 % — 6,949 — — 6,949 Short Term Investments 0.2 % — 121 — — 121 Total investments measured in the fair value hierarchy 49.7 % — 28,021 — — 28,021 Investments measured at net asset value (1) 50.3 % 28,412 — — — 28,412 100.0 % $28,412 $28,021 $ $ $56,433 (1) In accordance with Subtopic 820-10, The fair value of plan assets and major categories as of December 31, 2019, is as follows: Description Percent NAV Level 1 Level 2 Level 3 Total (dollars in thousands) Collective Funds 8.3 % $ — $ 4,289 $ — $ — $ 4,289 Equity Securities 9.7 % — 5,016 — — 5,016 Diversified Mutual Funds 31.1 % — 16,081 — — 16,081 Total investments measured in the fair value hierarchy 49.1 % — 25,386 — — 25,386 Investments measured at net asset value (1) 50.9 % 26,274 — — — 26,274 100.0 % $ 26,274 $ 25,386 $ — $ — $ 51,660 (1) In accordance with Subtopic 820-10,
Schedule of Investments Measured Using Net Asset Value Per Share Practical Expedient	The investments measured using the net asset value per share practical expedient as of December 31, 2020, is as follows: Percent Fair Value (dollars in thousands) Collective Funds by Category: Equity 12.6 % $ 7,113 US debt securities 18.0 % 10,131 International equities 14.1 % 7,964 Limited Partnerships by Category: Emerging markets 4.1 % 2,308 Hedge Funds by Category: Global opportunities (1) 0.3 % 197 Private investment entities and/or separately managed accounts (2) 1.2 % 699 50.3 % $ 28,412 The investments measured using the net asset value per share practical expedient as of December 31, 2019, is as follows: Percent Fair Value (dollars in thousands) Collective Funds by Category: Equity 19.2 % $ 9,932 US debt securities 15.2 % 7,874 International equities 10.1 % 5,208 Limited Partnerships by Category: Emerging markets 3.2 % 1,635 Multi-strategy 1.2 % 644 Hedge Funds by Category: Global opportunities (1) 0.5 % 259 Private investment entities and/or separately managed accounts (2) 1.4 % 722 50.9 % $ 26,274 (1) This category has an investment strategy to pursue a hybrid absolute return via portfolio managers, secondaries, and co-investments (2) The Fund’s investment objective is to invest in highly attractive, select investment opportunities by maintaining investments through private investment entities and/or separately managed accounts (each, an Investment or a Portfolio and collectively, the Investments or the Portfolios) with investment management professionals (each a Manager and collectively, the Managers) specializing in various alternative investment strategies. The Managers have broad investment experience and the ability to leverage their existing relationships with corporate management teams, investment banks and other institutions to gain access to certain investment opportunities. As such, the Manager is presented with “best idea” investment opportunities, typically in asset classes where market dislocations or other events have created attractive investment opportunities. The Managers are not restricted in the investment strategies that they may employ across different asset classes and regions. The Manager anticipates that any number of strategies will be eligible for consideration for investment by the Fund and the Fund reserves the right to invest in any particular strategy or asset class it deems appropriate.
Components of Net Periodic Benefit Cost	Defined Benefit Pension Plan Supplemental Insurance/ Retirement Plan 2020 2019 2020 2019 (dollars in thousands) Change projected in benefit obligation Benefit obligation at beginning of year $ 49,434 $ 40,509 $ 49,976 $ 40,405 Service cost 1,377 1,103 1,412 1,024 Interest cost 1,801 1,892 1,862 1,926 Prior service cost — — 501 — Actuarial (gain)/loss 3,245 7,099 4,703 7,537 Benefits paid (1,418 ) (1,169 ) (4,511 ) (916 ) Projected benefit obligation at end of year $ 54,439 $ 49,434 $ 53,943 $ 49,976 Change in plan assets Fair value of plan assets at beginning of year $ 51,660 $ 44,437 Actual return on plan assets 6,191 8,392 Employer contributions — — Benefits paid (1,418 ) (1,169 ) Fair value of plan assets at end of year $ 56,433 $ 51,660 (Unfunded) Funded status $ 1,994 $ 2,226 $ (53,943 ) $ (49,976 ) Accumulated benefit obligation $ 54,439 $ 49,434 $ 48,767 $ 45,238 Weighted-average assumptions as of December 31 Discount rate—Liability 3.25 % 3.71 % 3.21 % 3.71 % Discount rate—Expense 3.71 % 4.76 % 3.71 % 4.79 % Expected return on plan assets 7.50 % 7.50 % NA NA Rate of compensation increase 4.00 % 4.00 % 4.00 % 4.00 % Components of net periodic benefit cost Service cost $ 1,377 $ 1,103 $ 1,412 $ 1,024 Interest cost 1,801 1,892 1,862 1,926 Expected return on plan assets (3,808 ) (3,275 ) — — Recognized prior service cost — — 114 114 Recognized net losses 1,041 916 848 435 Net periodic cost (benefit) $ 411 $ 636 $ 4,236 $ 3,499 Other changes in plan assets and benefit obligations recognized in other comprehensive income Amortization of prior service cost $ — $ — $ (114) $ (114 ) Net (gain) loss (179 ) 1,066 3,809 7,101 Total recognized in other comprehensive income (179 ) 1,066 3,695 6,987 Total recognized in net periodic benefit cost and other comprehensive income $ 232 $ 1,702 $ 7,931 $ 10,486
Summary of Defined Pension Plan and Supplemental Insurance Retirement Plan	Plan December 31, 2020 Plan Total Plan December 31, 2019 Total (dollars in thousands) Prior service cost $ — $ (739 ) $ (739 ) $ — $ (307 ) $ (307 ) Net actuarial loss (12,741 ) (21,780 ) (34,521 ) (12,920 ) (17,971 ) (30,891 ) Total $ (12,741 ) $ (22,519 ) $ (35,260 ) $ (12,920 ) $ (18,278 ) $ (31,198 )
Summary of Accumulated Other Comprehensive Loss Expected to be Recognized	The following table summarizes the amounts included in Accumulated Other Comprehensive Loss at December 31, 2020, expected to be recognized as components of net periodic benefit cost in the next year: Plan Supplemental Amortization of prior service cost to be recognized in 2021 $ — $ 214 Amortization of loss to be recognized in 2021 965 1,079

Financial Instruments with Of_2

Financial Instruments with Off-Balance-Sheet Risk (Tables)	12 Months Ended
	Dec. 31, 2020
Risks and Uncertainties [Abstract]
Summary of Financial Instruments with Off-Balance-Sheet Risk	Financial instruments with off-balance Contract or Notional Amount 2020 2019 (dollars in thousands) Financial instruments whose contract amount represents credit risk Commitments to originate 1–4 family mortgages $ 28,163 $ 13,806 Standby and commercial letters of credit 5,344 5,779 Unused lines of credit 787,206 625,524 Unadvanced portions of construction loans 54,553 11,062 Unadvanced portions of other loans 13,767 15,801

Other Operating Expenses (Table

Other Operating Expenses (Tables)	12 Months Ended
	Dec. 31, 2020
Other Income and Expenses [Abstract]
Summary of Other Operating Expenses	Year ended December 31, 2020 2019 2018 (dollars in thousands) Marketing $ 1,295 $ 2,132 $ 2,346 Software maintenance/amortization 2,580 2,409 2,002 Legal and audit 1,668 1,514 1,444 Contributions 870 813 1,077 Processing services 1,712 1,875 1,740 Consulting 1,151 1,552 1,464 Postage and delivery 896 1,002 1,021 Supplies 1,096 985 987 Telephone 1,012 956 946 Directors’ fees 500 456 478 Insurance 464 456 420 Pension 1,859 2,008 678 Other 2,373 1,744 1,685 Total $ 17,476 $ 17,902 $ 16,288

Fair Values of Financial Inst_2

Fair Values of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2020
Investments, All Other Investments [Abstract]
Carrying Amounts and Fair Values of Company's Financial Instruments	The following presents (in thousands) the carrying amount, estimated fair value, and placement in the fair value hierarchy of the Company’s financial instruments as of December 31, 2020 and December 31, 2019. This table excludes financial instruments for which the carrying amount approximates fair value. Financial assets for which the fair value approximates carrying value include cash and cash equivalents, FHLBB stock and accrued interest receivable. Financial liabilities for which the fair value approximates carrying value include deposits other than time deposits, securities sold under agreements to repurchase, and accrued interest payable. Fair Value Measurements Carrying Estimated Level 1 Level 2 Level 3 (dollars in thousands) December 31, 2020 Financial assets: Securities held-to-maturity $ 2,509,088 $ 2,579,103 $ — $ 2,579,103 $ — Loans (1) 2,960,343 2,902,390 — — 2,902,390 Financial liabilities: Time deposits 546,143 556,470 — 556,470 — Other borrowed funds 177,009 183,000 — 183,000 — Subordinated debentures 36,083 36,083 — 36,083 — December 31, 2019 Financial assets: Securities held-to-maturity $ 2,351,120 $ 2,361,304 $ — $ 2,361,304 $ — Loans (1) 2,396,534 2,424,770 — — 2,424,770 Financial liabilities: Time deposits 555,447 560,746 — 560,746 — Other borrowed funds 370,955 374,531 — 374,531 — Subordinated debentures 36,083 36,083 — 36,083 — (1) Comprised of loans (including collateral dependent impaired loans), net of deferred loan costs and the allowance for loan losses.

Revenue from Contracts with C_2

Revenue from Contracts with Customers (Tables)	12 Months Ended
	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Schedule of Revenue From Contracts	The following table presents total revenues as presented in the Consolidated Statements of Income and the related amounts which are from contracts with customers within the scope of Topic 606. As illustrated here, the vast majority of our revenues are specifically excluded from the scope of Topic 606. Year ended Revenue from Year ended Revenue from Year ended Revenue from (dollars in thousands) Total net interest income $ 106,829 $ — $ 95,789 $ — $ 92,576 $ — Noninterest income: Service charges on deposit accounts 8,818 8,818 9,220 9,220 8,560 8,560 Lockbox fees 3,745 3,745 3,973 3,973 3,274 3,274 Brokerage commissions 271 — 277 — 348 — Net gains on sales of securities — — 61 — 302 — Gains on sales of mortgage loans — — 412 — — — Other income 6,266 2,132 4,456 2,799 3,764 2,536 Total noninterest income 19,100 14,695 18,399 15,992 16,248 14,370 Total revenues $ 125,929 $ 14,695 $ 114,188 $ 15,992 $ 108,824 $ 14,370
Information about Receivables with Customers	The following table provides information about receivables with customers. December 31, 2020 2019 (dollars in thousands) Receivables, which are included in “Other assets” $ 1,397 $ 1,200

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Components of lease expense	The components of lease expense were as follows: Year Ended December 31, 2020 2019 (in thousands) Operating lease cost $ 2,179 $ 2,216 Variable lease cost 579 528 Total lease cost $ 2,758 $ 2,744
Supplemental cash flow information related to leases	Supplemental cash flow information related to leases was as follows: Year Ended December 31, 2020 2019 (in thousands) Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 2,085 $ 2,130 Right-of-use Operating leases $ 3,044 $ 138
Supplemental balance sheet information related to leases	Supplemental balance sheet information related to leases was as follows: Year Ended December 31, 2020 2019 (in thousands, except lease term Operating Leases: Operating lease right-of-use $ 13,713 $ 12,521 Operating lease $ 13,935 $ 12,690 Weighted Average Remaining Lease Term: Operating Leases 10 Years 11 Years Weighted Average Discount Rate: Operating Leases 3.1 % 3.5 %
Summary of Future Payments of Lease Liabilities	A summary of future payments of lease liabilities were as follows: Minimum Rental Year Ending December 31, 2020 2019 (in thousands) 2020 $ — $ 2,030 2021 2,156 1,754 2022 1,995 1,603 2023 1,962 1,545 2024 1,692 1,277 2025 1,471 — Thereafter 7,394 7,312 Total lease payments $ 16,670 $ 15,521 Less imputed interest (2,735 ) (2,831 ) Present value of lease liability $ 13,935 $ 12,690

Quarterly Results of Operatio_2

Quarterly Results of Operations (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results of Operations (Unaudited)	2020 Quarters Fourth Third Second First (in thousands, except share data) Interest income $ 36,617 $ 36,504 $ 36,120 $ 39,795 Interest expense 8,138 9,173 10,302 14,594 Net interest income 28,479 27,331 25,818 25,201 Provision for loan losses 2,150 900 1,700 1,075 Net interest income after provision for loan losses 26,329 26,431 24,118 24,126 Other operating income 6,580 4,169 4,041 4,310 Operating expenses 20,106 18,167 17,042 18,173 Income before income taxes 12,803 12,433 11,117 10,263 Provision for income taxes 1,203 1,546 1,061 597 Net income $ 11,600 $ 10,887 $ 10,056 $ 9,666 Share data: Average shares outstanding, basic Class A 3,655,469 3,655,469 3,652,469 3,652,349 Class B 1,912,440 1,912,440 1,915,440 1,915,560 Average shares outstanding, diluted Class A 5,567,909 5,567,909 5,567,909 5,567,909 Class B 1,912,440 1,912,440 1,915,440 1,915,560 Earnings per share, basic Class A $ 2.52 $ 2.36 $ 2.18 $ 2.10 Class B $ 1.26 $ 1.18 $ 1.09 $ 1.05 Earnings per share, diluted Class A $ 2.08 $ 1.96 $ 1.81 $ 1.74 Class B $ 1.26 $ 1.18 $ 1.09 $ 1.05 2019 Quarters Fourth Third Second First (in thousands, except share data) Interest income $ 40,518 $ 39,852 $ 39,692 $ 39,077 Interest expense 15,187 16,082 16,442 15,639 Net interest income 25,331 23,770 23,250 23,438 Provision for loan losses 550 75 250 375 Net interest income after provision for loan losses 24,781 23,695 23,000 23,063 Other operating income 4,689 4,286 4,997 4,427 Operating expenses 18,212 17,462 18,264 18,190 Income before income taxes 11,258 10,519 9,733 9,300 Provision for income taxes 526 435 267 (118 ) Net income $ 10,732 $ 10,084 $ 9,466 $ 9,418 Share data: Average shares outstanding, basic Class A 3,650,949 3,650,449 3,620,449 3,610,329 Class B 1,916,960 1,917,460 1,947,460 1,957,580 Average shares outstanding, diluted Class A 5,567,909 5,567,909 5,567,909 5,567,909 Class B 1,916,960 1,917,460 1,947,460 1,957,580 Earnings per share, basic Class A $ 2.33 $ 2.19 $ 2.06 $ 2.05 Class B $ 1.16 $ 1.09 $ 1.03 $ 1.03 Earnings per share, diluted Class A $ 1.93 $ 1.81 $ 1.70 $ 1.69 Class B $ 1.16 $ 1.09 $ 1.03 $ 1.03

Parent Company Financial Stat_2

Parent Company Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2020
Condensed Financial Information Disclosure [Abstract]
Balance Sheets of Parent Company	BALANCE SHEETS December 31, 2020 2019 (dollars in thousands) ASSETS: Cash $ 5,141 $ 3,177 Investment in subsidiary, at equity 389,724 353,489 Other assets 15,920 16,325 Total assets $ 410,785 $ 372,991 LIABILITIES AND STOCKHOLDERS’ EQUITY: Liabilities $ 4,293 $ 4,327 Subordinated debentures 36,083 36,083 Stockholders’ equity 370,409 332,581 Total liabilities and stockholders’ equity $ 410,785 $ 372,991
Statements of Income of Parent Company	STATEMENTS OF INCOME Year Ended December 31, 2020 2019 2018 (dollars in thousands) Income: Dividends from subsidiary $ 5,000 $ 5,000 $ 4,750 Other income 25 65 53 Total income 5,025 5,065 4,803 Interest expense 951 1,577 1,474 Operating expenses 235 215 225 Income before income taxes and equity in undistributed income of subsidiary 3,839 3,273 3,104 Benefit from income taxes (244 ) (363 ) (347 ) Income before equity in undistributed income of subsidiary 4,083 3,636 3,451 Equity in undistributed income of subsidiary 38,126 36,063 32,762 Net income $ 42,209 $ 39,699 $ 36,213
Statements of Cash Flows of Parent Company	STATEMENTS OF CASH FLOWS December 31, 2020 2019 2018 (dollars in thousands) CASH FLOWS FROM OPERATING ACTIVITIES: Net income $ 42,209 $ 39,699 $ 36,213 Adjustments to reconcile net income to net cash provided by operating activities Undistributed income of subsidiary (38,126 ) (36,063 ) (32,762 ) Decrease (increase) in other assets 405 665 (158 ) (Decrease) increase in liabilities (34 ) (180 ) (1,808 ) Net cash provided by (used in) operating activities 4,454 4,121 1,485 CASH FLOWS FROM FINANCING ACTIVITIES: Cash dividends paid (2,490 ) (2,207 ) (2,203 ) Net cash used in financing activities (2,490 ) (2,207 ) (2,203 ) Net (decrease) in cash 1,964 1,914 (718 ) Cash at beginning of year 3,177 1,263 1,981 Cash at end of year $ 5,141 $ 3,177 $ 1,263

Basis of Financial Statement Pr

Basis of Financial Statement Presentation - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020Segment
Number of Reportable Segments	1
Century Bancorp Capital Trust II [Member]
Equity ownership interest	100.00%
Number of Reportable Segments	1

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Dec. 21, 2021	Dec. 31, 2020USD ($)Segment
Summary Of Significant Accounting Policies And Other Information [Line Items]
Number of reportable segments | Segment		1
Securities held for trading | $		$ 0
Held-to-maturity substantial portion of principal outstanding		85.00%
Number of reporting units		1
Goodwill impairment description		Goodwill impairment is evaluated by first assessing qualitative factors (events and circumstances) to determine whether it is more likely than not (meaning a likelihood of more than 50 percent) that the fair value of a reporting unit is less than its carrying amount. If, after considering all relevant events and circumstances, an entity determines it is not morelikely than not that the fair value of a reporting unit is less than its carrying amount, then performing the quantitative impairment test will be unnecessary.​​​​​​​
Excess AMT credit carryforwards refundable rate		50.00%
Scenario, Forecast [Member]
Summary Of Significant Accounting Policies And Other Information [Line Items]
Held-to-maturity substantial portion of principal outstanding	100.00%
Minimum [Member]
Summary Of Significant Accounting Policies And Other Information [Line Items]
Class A common stock entitled dividend per share percent in comparison to Class B common stock		200.00%

Cash and Due from Banks - Addit

Cash and Due from Banks - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Cash and Due from Banks [Abstract]
Reserve balance of cash and due from banks	$ 0	$ 0

Securities Available-for-Sale -

Securities Available-for-Sale - Summary of Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 282,273	$ 260,924
Gross Unrealized Gains	845	274
Gross Unrealized Losses	670	696
Total, Fair Value	282,448	260,502
SBA Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	44,328	54,331
Gross Unrealized Gains		23
Gross Unrealized Losses	289	143
Total, Fair Value	44,039	54,211
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	177,239	184,580
Gross Unrealized Gains	819	139
Gross Unrealized Losses	317	532
Total, Fair Value	177,741	184,187
Privately Issued Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	330	397
Gross Unrealized Gains	2	1
Gross Unrealized Losses	4	2
Total, Fair Value	328	396
Obligations Issued by States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	52,276	18,016
Gross Unrealized Gains		60
Total, Fair Value	52,276	18,076
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,100	3,600
Gross Unrealized Gains	24	51
Gross Unrealized Losses	60	19
Total, Fair Value	$ 8,064	$ 3,632

Securities Available-for-Sale_2

Securities Available-for-Sale - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020USD ($)Security	Dec. 31, 2019USD ($)Security	Dec. 31, 2018USD ($)
Schedule of Available-for-sale Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	$ 1,866,989,000	$ 1,776,399,000
Securities available-for-sale are securities at fair value pledged for borrowing	29,885,000	32,297,000
Gross gains on sales of securities	0	61,000	$ 302,000
Proceeds from sales of securities available-for-sale		16,285,000	27,517,000
Securities at floating rate or adjustable rate	222,696,000
Net gains on sales of securities	$ 0	61,000	302,000
Federal Home Loan Bank [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross gains on sales of securities		13,000	302,000
Net gains on sales of securities		$ 13,000	$ 302,000
Securities AFS [Member]
Schedule of Available-for-sale Securities [Line Items]
Weighted average remaining life of investment securities available-for-sale	4 years 8 months 12 days
Number of securities, temporarily impaired for less than 12 months | Security	13	10
Number of securities, temporarily impaired for 12 months or longer | Security	21	30
Number of securities, temporarily impaired, total | Security	153	190
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	$ 183,269,000	$ 186,245,000

Securities Available-for-Sale_3

Securities Available-for-Sale - Estimated Maturity Distribution of Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$ 52,501
After one but within five years, Amortized Cost	114,877
After five but within ten years, Amortized Cost	97,911
More than ten years, Amortized Cost	16,984
Amortized Cost	282,273	$ 260,924
Within one year, Fair Value	52,497
After one but within five years, Fair Value	115,236
After five but within ten years, Fair Value	97,880
More than ten years, Fair Value	16,835
Total, Fair Value	$ 282,448	$ 260,502

Securities Available-for-Sale_4

Securities Available-for-Sale - Continuous Unrealized Loss Position for 12 Months or Less and 12 Months and Longer (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$ 35,969	$ 124,418
Less Than 12 Months, Unrealized Losses	150	412
12 Months or Longer, Fair Value	65,325	51,751
12 Months or Longer, Unrealized Losses	520	284
Total, Fair Value	101,294	176,169
Total, Unrealized Losses	670	696
SBA Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	13,839	14,560
Less Than 12 Months, Unrealized Losses	42	30
12 Months or Longer, Fair Value	30,200	22,092
12 Months or Longer, Unrealized Losses	247	113
Total, Fair Value	44,039	36,652
Total, Unrealized Losses	289	143
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	18,188	108,806
Less Than 12 Months, Unrealized Losses	50	379
12 Months or Longer, Fair Value	33,617	29,178
12 Months or Longer, Unrealized Losses	267	153
Total, Fair Value	51,805	137,984
Total, Unrealized Losses	317	532
Privately Issued Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		252
Less Than 12 Months, Unrealized Losses		2
12 Months or Longer, Fair Value	210
12 Months or Longer, Unrealized Losses	4
Total, Fair Value	210	252
Total, Unrealized Losses	4	2
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	3,942	800
Less Than 12 Months, Unrealized Losses	58	1
12 Months or Longer, Fair Value	1,298	481
12 Months or Longer, Unrealized Losses	2	18
Total, Fair Value	5,240	1,281
Total, Unrealized Losses	$ 60	$ 19

Investment Securities Held-to_3

Investment Securities Held-to-Maturity - Summary of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,509,088	$ 2,351,120
Gross Unrealized Gains	71,913	21,429
Gross Unrealized Losses	1,898	11,245
Estimated Fair Value	2,579,103	2,361,304
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	244,220	98,867
Gross Unrealized Gains	389	527
Gross Unrealized Losses	866	96
Estimated Fair Value	243,743	99,298
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37,783	44,379
Gross Unrealized Gains	2,002	182
Gross Unrealized Losses	0	303
Estimated Fair Value	39,785	44,258
Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,227,085	2,207,874
Gross Unrealized Gains	69,522	20,720
Gross Unrealized Losses	1,032	10,846
Estimated Fair Value	$ 2,295,575	$ 2,217,748

Investment Securities Held-to_4

Investment Securities Held-to-Maturity - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020USD ($)Security	Dec. 31, 2019USD ($)Security
Schedule of Held-to-maturity Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	$ 1,866,989,000	$ 1,776,399,000
Securities held to maturity at floating rate or adjustable rate	85,000
Sales of held-to-maturity securities	3,900,000	1,193,000
Commercial Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Realized gross gains from sales of securities held-to-maturity	$ 0	48,000
Sales of held-to-maturity securities		$ 1,193,000,000
Held-to-Maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life of investment securities held-to-maturity	3 years 10 months 24 days
Number of securities, temporarily impaired for less than 12 months | Security	53	56
Number of securities, temporarily impaired for 12 months or longer | Security	0	315
Number of securities, temporarily impaired, total | Security	600	475
Held-to-Maturity Securities [Member] | U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life	$ 218,730,000
Federal Home Loan Bank [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities pledged for borrowing at the Federal Home Loan Bank	$ 537,367,000	$ 399,646,000

Investment Securities Held-to_5

Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$ 79,572
After one but within five years, Amortized Cost	1,870,589
After five but within ten years, Amortized Cost	555,932
More than ten years, Amortized Cost	2,995
Amortized Cost	2,509,088	$ 2,351,120
Within one year, Fair Value	80,175
After one but within five years, Fair Value	1,927,961
After five but within ten years, Fair Value	567,577
More than ten years, Fair Value	3,390
Estimated Fair Value	$ 2,579,103	$ 2,361,304

Investment Securities Held-to_6

Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 465,271	$ 663,576
Less Than 12 Months, Unrealized Losses	1,898	4,324
12 Months or Longer, Fair Value	0	399,895
12 Months or Longer, Unrealized Losses	0	6,921
Total, Fair Value	465,271	1,063,471
Total, Unrealized Losses	1,898	11,245
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	162,870	24,420
Less Than 12 Months, Unrealized Losses	866	72
12 Months or Longer, Fair Value	0	9,976
12 Months or Longer, Unrealized Losses	0	24
Total, Fair Value	162,870	34,396
Total, Unrealized Losses	866	96
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	0	25,251
Less Than 12 Months, Unrealized Losses	0	303
Total, Fair Value	0	25,251
Total, Unrealized Losses	0	303
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	302,401	613,905
Less Than 12 Months, Unrealized Losses	1,032	3,949
12 Months or Longer, Fair Value	0	389,919
12 Months or Longer, Unrealized Losses	0	6,897
Total, Fair Value	302,401	1,003,824
Total, Unrealized Losses	$ 1,032	$ 10,846

Loans - Summary of Composition

Loans - Summary of Composition of Loan Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 2,995,829	$ 2,426,119
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	789,836	786,102
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	448,436	371,897
Consumer Portfolio Segment Other Than Overdrafts [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	20,007	21,071
Municipal [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	137,607	120,455
Construction and land development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	10,909	8,992
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,314,245	812,417
Home Equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	274,357	304,363
Overdrafts [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 432	$ 822

Loans - Additional Information

Loans - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Loans carried net of premiums	$ (74,000)	$ (292,000)
Net deferred fee	4,444,000	220,000
Mortgage loans sold to others	125,998,000	204,690,000
Total recorded investment in impaired loans	5,379,000	3,252,000	$ 3,051,000
Loans held for sale	0	0
Mortgage servicing rights, net	773,000	1,202,000
Impaired loans	5,103,000	2,322,000
Specific reserves	$ 589,000	$ 102,000

Loans - Composition of Nonaccru

Loans - Composition of Nonaccrual Loans and Impaired Loans (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Loans on nonaccrual	$ 3,996,000	$ 2,014,000	$ 1,313,000
Loans 90 days past due and still accruing	90,000	0	0
Impaired loans on nonaccrual included above	3,178,000	891,000	296,000
Total recorded investment in impaired loans	5,379,000	3,252,000	3,051,000
Average recorded investment of impaired loans	3,641,000	3,161,000	5,491,000
Accruing troubled debt restructures	2,202,000	2,361,000	2,559,000
Interest income not recorded on nonaccrual loans according to their original terms	95,000	67,000	64,000
Interest income on nonaccrual loans actually recorded	0	0	0
Interest income recognized on impaired loans	$ 94,000	$ 103,000	$ 196,000

Loans - Aggregate Amount of Loa

Loans - Aggregate Amount of Loans to Directors and Officers of Company and Their Associates (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Debt Disclosure [Abstract]
Beginning balance	$ 12,031
Additions	6,340
Repayments and Deletions	2,389
Ending balance	$ 15,982

Allowance for Loan Losses - Ana

Allowance for Loan Losses - Analysis of Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Receivables [Abstract]
Allowance for loan losses, beginning of year				$ 29,585				$ 28,543	$ 29,585	$ 28,543	$ 26,255
Loans charged-off									(238)	(454)	(833)
Recoveries on loans previously charged-off									314	246	1,771
Net recoveries (charge-offs)									76	(208)	938
Provision charged to expense	$ 2,150	$ 900	$ 1,700	$ 1,075	$ 550	$ 75	$ 250	$ 375	5,825	1,250	1,350
Allowance for loan losses, end of year	$ 35,486				$ 29,585				$ 35,486	$ 29,585	$ 28,543

Allowance for Loan Losses - Sum

Allowance for Loan Losses - Summary of Allowance for Loan Losses (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	$ 29,585,000	$ 28,543,000	$ 26,255,000
Charge-offs	(238,000)	(454,000)	(833,000)
Recoveries	314,000	246,000	1,771,000
Provision	5,825,000	1,250,000
Allowance for loan losses, end of year	35,486,000	29,585,000	28,543,000
Amount of allowance for loan losses for loans deemed to be impaired	589,000	102,000
Amount of allowance for loan losses for loans not deemed to be impaired	34,897,000	29,483,000
Loans:
Total loans, net	2,995,829,000	2,426,119,000
Loans deemed to be impaired	5,379,000	3,252,000	3,051,000
Loans not deemed to be impaired	2,990,450,000	2,422,867,000
Construction and Land Development [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	331,000	1,092,000
Provision	98,000	(761,000)
Allowance for loan losses, end of year	429,000	331,000	1,092,000
Amount of allowance for loan losses for loans not deemed to be impaired	429,000	331,000
Loans:
Total loans, net	10,909,000	8,992,000
Loans not deemed to be impaired	10,909,000	8,992,000
Commercial and Industrial [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	11,596,000	10,998,000
Charge-offs	(29,000)	(137,000)
Recoveries	197,000	60,000
Provision	4,949,000	675,000
Allowance for loan losses, end of year	16,713,000	11,596,000	10,998,000
Amount of allowance for loan losses for loans deemed to be impaired	140,000	15,000
Amount of allowance for loan losses for loans not deemed to be impaired	16,573,000	11,581,000
Loans:
Total loans, net	1,314,245,000	812,417,000
Loans deemed to be impaired	439,000	906,000
Loans not deemed to be impaired	1,313,806,000	811,511,000
Municipal [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	2,566,000	1,838,000
Provision	238,000	728,000
Allowance for loan losses, end of year	2,804,000	2,566,000	1,838,000
Amount of allowance for loan losses for loans not deemed to be impaired	2,804,000	2,566,000
Loans:
Total loans, net	137,607,000	120,455,000
Loans not deemed to be impaired	137,607,000	120,455,000
Commercial Real Estate [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	11,464,000	10,663,000
Provision	287,000	801,000
Allowance for loan losses, end of year	11,751,000	11,464,000	10,663,000
Amount of allowance for loan losses for loans deemed to be impaired	449,000	87,000
Amount of allowance for loan losses for loans not deemed to be impaired	11,302,000	11,377,000
Loans:
Total loans, net	789,836,000	786,102,000
Loans deemed to be impaired	4,940,000	2,346,000
Loans not deemed to be impaired	784,896,000	783,756,000
Residential Real Estate [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	2,194,000	2,190,000
Provision	(83,000)	4,000
Allowance for loan losses, end of year	2,111,000	2,194,000	2,190,000
Amount of allowance for loan losses for loans not deemed to be impaired	2,111,000	2,194,000
Loans:
Total loans, net	448,436,000	371,897,000
Loans not deemed to be impaired	448,436,000	371,897,000
Consumer [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	312,000	365,000
Charge-offs	(209,000)	(295,000)
Recoveries	112,000	186,000
Provision	26,000	56,000
Allowance for loan losses, end of year	241,000	312,000	365,000
Amount of allowance for loan losses for loans not deemed to be impaired	241,000	312,000
Loans:
Total loans, net	20,439,000	21,893,000
Loans not deemed to be impaired	20,439,000	21,893,000
Home Equity [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	1,065,000	1,111,000
Charge-offs		(22,000)
Recoveries	5,000
Provision	138,000	(24,000)
Allowance for loan losses, end of year	1,208,000	1,065,000	1,111,000
Amount of allowance for loan losses for loans not deemed to be impaired	1,208,000	1,065,000
Loans:
Total loans, net	274,357,000	304,363,000
Loans not deemed to be impaired	274,357,000	304,363,000
Unallocated [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	57,000	286,000
Provision	172,000	(229,000)
Allowance for loan losses, end of year	229,000	57,000	$ 286,000
Amount of allowance for loan losses for loans not deemed to be impaired	$ 229,000	$ 57,000

Allowance for Loan Losses - Loa

Allowance for Loan Losses - Loans by Risk Rating (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Loans by risk rating
Financing Receivable, Net	$ 2,995,829	$ 2,426,119
Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	10,909	8,992
Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	1,314,245	812,417
Municipal [Member]
Loans by risk rating
Financing Receivable, Net	137,607	120,455
Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	789,836	786,102
1-3 (Pass) [Member] | Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	10,909	8,992
1-3 (Pass) [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	1,309,861	807,486
1-3 (Pass) [Member] | Municipal [Member]
Loans by risk rating
Financing Receivable, Net	137,607	120,455
1-3 (Pass) [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	761,101	759,402
4 (Monitor) [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	3,945	4,025
4 (Monitor) [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	23,795	24,354
Impaired [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	439	906
Impaired [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	$ 4,940	$ 2,346

Allowance for Loan Losses - L_2

Allowance for Loan Losses - Loans by Credit Rating (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	$ 1,406,208	$ 1,160,837
Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	823,281	617,354
A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	335,809	341,744
Baa1 - Baa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	242,038	195,844
Ba2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	5,080	5,895
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	944,078	709,688
Commercial and Industrial [Member] | Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	710,955	523,644
Commercial and Industrial [Member] | A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	183,123	186,044
Commercial and Industrial [Member] | Baa1 - Baa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	50,000
Municipal [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	137,607	117,655
Municipal [Member] | Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	74,291	53,273
Municipal [Member] | A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	7,103	7,354
Municipal [Member] | Baa1 - Baa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	51,133	51,133
Municipal [Member] | Ba2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	5,080	5,895
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	324,523	333,494
Commercial Real Estate [Member] | Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	38,035	40,437
Commercial Real Estate [Member] | A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	145,583	148,346
Commercial Real Estate [Member] | Baa1 - Baa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	$ 140,905	$ 144,711

Allowance for Loan Losses - Agi

Allowance for Loan Losses - Aging of Past Due Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 1,468	$ 3,251
Non Accrual	3,996	2,014	$ 1,313
Accruing Greater Than 90 Days	90	0	$ 0
Total Past Due	5,554	5,265
Current Loans	2,990,275	2,420,854
Total loans, net	2,995,829	2,426,119
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	10,909	8,992
Total loans, net	10,909	8,992
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	56	227
Non Accrual	297	400
Accruing Greater Than 90 Days	90
Total Past Due	443	627
Current Loans	1,313,802	811,790
Total loans, net	1,314,245	812,417
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	137,607	120,455
Total loans, net	137,607	120,455
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due		840
Non Accrual	2,881	492
Total Past Due	2,881	1,332
Current Loans	786,955	784,770
Total loans, net	789,836	786,102
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	390	1,563
Non Accrual	527	683
Total Past Due	917	2,246
Current Loans	447,519	369,651
Total loans, net	448,436	371,897
Consumer and overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	21	18
Non Accrual	1	4
Total Past Due	22	22
Current Loans	20,417	21,871
Total loans, net	20,439	21,893
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,001	603
Non Accrual	290	435
Total Past Due	1,291	1,038
Current Loans	273,066	303,325
Total loans, net	$ 274,357	$ 304,363

Allowance for Loan Losses - Inf

Allowance for Loan Losses - Information Pertaining to Impaired Loans (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	$ 276,000	$ 930,000
With no required reserve recorded, Unpaid Balance Principal	311,000	1,165,000
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value Recognized	1,197,000	583,000
With no required reserve recorded, Interest Income	0	6,000
With required reserve recorded, Carrying Value	5,103,000	2,322,000
With required reserve recorded, Unpaid Balance Principal	5,251,000	2,443,000
With required reserve recorded, Required Reserve	589,000	102,000
With required reserve recorded, Average Carrying Value Recognized	2,444,000	2,578,000
With required reserve recorded, Interest Income	94,000	97,000
Carrying Value	5,379,000	3,252,000	$ 3,051,000
Unpaid Balance Principal	5,562,000	3,608,000
With required reserve recorded, Required Reserve	589,000	102,000
Average Carrying Value Recognized	3,641,000	3,161,000	$ 5,491,000
Interest Income	94,000	103,000
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Required Reserve	0	0
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	4,000	770,000
With no required reserve recorded, Unpaid Balance Principal	5,000	976,000
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value Recognized	633,000	138,000
With no required reserve recorded, Interest Income	0	6,000
With required reserve recorded, Carrying Value	435,000	136,000
With required reserve recorded, Unpaid Balance Principal	454,000	137,000
With required reserve recorded, Required Reserve	140,000	15,000
With required reserve recorded, Average Carrying Value Recognized	121,000	264,000
With required reserve recorded, Interest Income	8,000	7,000
Carrying Value	439,000	906,000
Unpaid Balance Principal	459,000	1,113,000
With required reserve recorded, Required Reserve	140,000	15,000
Average Carrying Value Recognized	754,000	402,000
Interest Income	8,000	13,000
Municipal [Member] | Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Required Reserve	0	0
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	272,000	160,000
With no required reserve recorded, Unpaid Balance Principal	306,000	189,000
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value Recognized	437,000	445,000
With required reserve recorded, Carrying Value	4,668,000	2,186,000
With required reserve recorded, Unpaid Balance Principal	4,797,000	2,306,000
With required reserve recorded, Required Reserve	449,000	87,000
With required reserve recorded, Average Carrying Value Recognized	2,323,000	2,314,000
With required reserve recorded, Interest Income	86,000	90,000
Carrying Value	4,940,000	2,346,000
Unpaid Balance Principal	5,103,000	2,495,000
With required reserve recorded, Required Reserve	449,000	87,000
Average Carrying Value Recognized	2,760,000	2,759,000
Interest Income	86,000	90,000
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value Recognized	127,000
Average Carrying Value Recognized	127,000	0
Interest Income		0
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Required Reserve	0	0
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Required Reserve	$ 0	$ 0

Allowance for Loan Losses - Add

Allowance for Loan Losses - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2020
Extended Maturity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Description Of Covid Nineteen Loan Modifications Aggregating Deferral Principal		As of December 31, 2020, and as a result of COVID-19 loan modifications, the Company has 20 loans operating under modified terms aggregating $25,022,000, primarily consisting of short-term payment deferrals. Of these modifications, $25,022,000, or 100%, were performing in accordance with their modified terms.
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Post-modification outstanding recorded investment	$ 39,000

Bank Premises and Equipment - S

Bank Premises and Equipment - Schedule of Bank Premises and Equipment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	$ 89,672	$ 81,354
Accumulated depreciation and amortization	(50,610)	(47,402)
Total	39,062	33,952
Land [Member]
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	7,905	7,246
Bank Premises [Member]
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	34,556	28,175
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	34,381	33,259
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	$ 12,830	$ 12,674
Minimum [Member] | Bank Premises [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	30 years
Minimum [Member] | Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	3 years
Minimum [Member] | Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	30 years
Maximum [Member] | Bank Premises [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	39 years
Maximum [Member] | Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	10 years
Maximum [Member] | Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	39 years

Bank Premises and Equipment - A

Bank Premises and Equipment - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Depreciation of leased property	$ 3,266,000	$ 3,235,000	$ 3,206,000

Goodwill and Identifiable Int_3

Goodwill and Identifiable Intangible Assets - Carrying Amount of Goodwill and Intangibles (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Intangible Assets And Goodwill [Line Items]
Goodwill, Beginning Balance	$ 2,714	$ 2,714	$ 2,714
Total, Beginning Balance	3,487	3,940
Additions		237
Amortization Expense	(429)	(261)
Total, Ending Balance	3,916	3,487
Mortgage Servicing Rights [Member]
Intangible Assets And Goodwill [Line Items]
Beginning Balance	1,202	1,226
Additions		237
Amortization Expense	(429)	(261)
Ending Balance	$ 773	$ 1,202

Fair Value Measurements - Finan

Fair Value Measurements - Financial Instruments Measured at Fair Value on a Recurring and Non-recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	$ 282,448	$ 260,502
Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	230,172	247,201
Fair Value Measurements, Level 3 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	52,276	13,301
Financial Instruments Measured at Fair Value on a Non-recurring Basis Impaired Loans	3,178	877
SBA Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	44,039	54,211
SBA Backed Securities [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	44,039	54,211
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	177,741	184,187
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	177,741	184,187
Privately Issued Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	328	396
Privately Issued Residential Mortgage-Backed Securities [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	328	396
Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	52,276	18,076
Obligations Issued by States and Political Subdivisions [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS		4,775
Obligations Issued by States and Political Subdivisions [Member] | Fair Value Measurements, Level 3 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	52,276	13,301
Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	8,064	3,632
Other Debt Securities [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	8,064	3,632
Equity Securities [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments Measured at Fair Value on a Non Recurring Basis Equity Securities	303	343
Equity Securities [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments Measured at Fair Value on a Non Recurring Basis Equity Securities	1,365	1,345
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	282,448	260,502
Financial Instruments Measured at Fair Value on a Non-recurring Basis Impaired Loans	3,178	877
Carrying Value [Member] | SBA Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	44,039	54,211
Carrying Value [Member] | U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	177,741	184,187
Carrying Value [Member] | Privately Issued Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	328	396
Carrying Value [Member] | Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	52,276	18,076
Carrying Value [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	8,064	3,632
Carrying Value [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instruments Measured at Fair Value on a Non Recurring Basis Equity Securities	$ 1,668	$ 1,688

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Specific adjustments to impaired loans recognized	$ 501,000	$ 79,000
Amortized cost of Level 3 securities	1,635,000	1,635,000
Liabilities measured at fair value on a recurring or nonrecurring basis	0	0
Transfers between level 1, 2 and 3	0	0
Fair Value Measurements, Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized cost of Level 3 securities	52,276,000	13,301,000
Unrealized loss	$ 0	$ 0

Fair Value Measurements - Asset

Fair Value Measurements - Assets Measured at Fair Value (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Securities AFS [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Fair Value	$ 52,276	$ 13,301
Valuation Technique	Discounted cash flow	Discounted cash flow
Unobservable Input	Discount rate	Discount rate
Impaired Loans [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Fair Value	$ 3,178	$ 877
Valuation Technique	Appraisal of collateral	Appraisal of collateral
Unobservable Input	Appraisal adjustments	Appraisal adjustments
Minimum [Member] | Securities AFS [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unobservable Input Value or Range	0.00%
Securities AFS, Unobservable Input Value or Range		1.5
Minimum [Member] | Impaired Loans [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unobservable Input Value or Range	0.00%
Impaired Loans, Unobservable Input Value or Range		0
Maximum [Member] | Securities AFS [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unobservable Input Value or Range	1.00%
Securities AFS, Unobservable Input Value or Range		3.2
Maximum [Member] | Impaired Loans [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unobservable Input Value or Range	17.00%
Impaired Loans, Unobservable Input Value or Range		30

Fair Value Measurements - Chang

Fair Value Measurements - Changes in Level 3 Securities (Detail) - Obligations Issued by States and Political Subdivisions [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 13,301	$ 88,728
Purchases	71,095	21,408
Maturities/redemptions	(32,073)	(96,812)
Amortization	(47)	(23)
Ending Balance	$ 52,276	$ 13,301

Fair Value Measurements - Ass_2

Fair Value Measurements - Assets Measured at Fair Value (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Municipal [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities maturity period	one year or less	one year or less

Deposits - Summary of Remaining

Deposits - Summary of Remaining Maturities or Re-pricing of Time Deposits (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Maturities of Time Deposits [Abstract]
Within one year	$ 465,080	$ 383,497
Over one year to two years	50,456	123,016
Over two years to three years	19,104	27,223
Over three years to five years	11,503	21,711
Time Deposits, Total	$ 546,143	$ 555,447
Within one year, Percent	85.00%	69.00%
Over one year to two years, Percent	9.00%	22.00%
Over two years to three years, Percent	4.00%	5.00%
Over three years to five years, Percent	2.00%	4.00%
Total, Percent	100.00%	100.00%

Deposits - Additional Informati

Deposits - Additional Information (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Deposits [Abstract]
Time deposits 250000 or more	$ 350,290,000	$ 342,809,000
Deposits to related parties	$ 34,622,000	$ 34,964,000

Securities Sold under Agreeme_3

Securities Sold under Agreements to Repurchase - Summary of Securities Sold Under Agreements to Repurchase (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Brokers and Dealers [Abstract]
Amount outstanding at December 31	$ 232,090,000	$ 266,045,000	$ 154,240,000
Weighted average rate at December 31	0.34%	0.96%	0.82%
Maximum amount outstanding at any month end	$ 253,980,000	$ 307,235,000	$ 174,150,000
Daily average balance outstanding during the year	$ 221,609,000	$ 224,361,000	$ 147,944,000
Weighted average rate during the year	0.62%	1.05%	0.66%

Securities Sold under Agreeme_4

Securities Sold under Agreements to Repurchase - Additional Information (Detail) - U.S. Government Sponsored Enterprises [Member] - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assets Sold under Agreements to Repurchase [Line Items]
U.S. Government Sponsored Enterprise securities with a total amortized cost	$ 226,942,000	$ 264,737,000	$ 160,576,000
Fair value of the collateral repurchase agreement	$ 234,125,000	$ 265,687,000	$ 156,369,000

Other Borrowed Funds and Subo_3

Other Borrowed Funds and Subordinated Debentures - Summary of Other Borrowed Funds and Subordinated Debentures (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Amount outstanding at December 31	$ 213,092,000	$ 407,038,000	$ 238,461,000
Weighted average rate at December 31	2.38%	2.37%	2.76%
Maximum amount outstanding at any month end	$ 348,203,000	$ 487,502,000	$ 542,913,000
Daily average balance outstanding during the year	$ 201,656,000	$ 231,926,000	$ 291,674,000
Weighted average rate during the year	2.66%	2.95%	2.61%

Other Borrowed Funds and Subo_4

Other Borrowed Funds and Subordinated Debentures - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Dec. 31, 2004	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
FHLBB advances		$ 40,000,000	$ 40,000,000	$ 40,000,000
Subordinated debt securities issued		36,083,000	36,083,000
Prior Restructuring FHLBB [Member]
Debt Instrument [Line Items]
FHLBB advances			$ 15,000,000
FHLBB Weighted average rate			3.33%
FHLBB Weighted average maturity			14 months
Post Restructuring FHLBB [Member]
Debt Instrument [Line Items]
FHLBB Weighted average rate			2.37%
FHLBB Weighted average maturity			60 months
Century Bancorp Capital Trust II [Member]
Debt Instrument [Line Items]
Subordinated debt securities issued	$ 36,083,000
Subordinated debt securities due year	2034
Federal Home Loan Bank Borrowings [Member]
Debt Instrument [Line Items]
Bank's remaining term borrowing capacity at the FHLBB		591,974,000
Line of credit with the FHLBB		$ 14,500,000
Subordinated Debt [Member] | Century Bancorp Capital Trust II [Member]
Debt Instrument [Line Items]
Liquidation value of shares of cumulative trust preferred securities		$ 1,000
LIBOR rate		LIBOR rate plus 1.87% for the remaining 20 years.
LIBOR rate trust preferred securities		1.87%
Duration of LIBOR rate		20 years
Investment coupon rate		2.09%	3.76%
Cumulative Preferred Stock Subject to Mandatory Redemption [Member] | Subordinated Debt [Member] | Century Bancorp Capital Trust II [Member]
Debt Instrument [Line Items]
Shares of cumulative trust preferred securities		35,000
Trust preferred securities annual dividend rate		6.65%

Other Borrowed Funds and Subo_5

Other Borrowed Funds and Subordinated Debentures - Schedule of the Maturity Distribution of FHLBB Advances with the Weighted Average Interest Rates (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Federal Home Loan Bank, Advances, Fiscal Year Maturity [Abstract]
Within one year	$ 67,500	$ 218,000	$ 63,000
Over one year to two years	3,500	42,500	28,000
Over two years to three years	30,000	3,500	25,000
Over three years to five years	67,000	70,000	33,500
Over five years	9,009	36,955	52,878
Federal Home Loan Bank, Advances, Total	$ 177,009	$ 370,955	$ 202,378
Within one year, Weighted Average Rate	1.78%	1.86%	2.17%
Over one year to two years, Weighted Average Rate	2.15%	2.58%	2.29%
Over two years to three years, Weighted Average Rate	3.38%	2.15%	3.34%
Over three years to five years, Weighted Average Rate	2.64%	2.85%	2.23%
Over five years, Weighted Average Rate	2.83%	2.88%	2.47%
Weighted Average Rate, Total	2.44%	2.23%	2.42%

Reclassifications Out of Accu_3

Reclassifications Out of Accumulated Other Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities									$ 0	$ 61	$ 302
Provision for income taxes	$ (1,203)	$ (1,546)	$ (1,061)	$ (597)	$ (526)	$ (435)	$ (267)	$ 118	(4,407)	(1,110)	(1,568)
Net income	$ 11,600	$ 10,887	$ 10,056	$ 9,666	$ 10,732	$ 10,084	$ 9,466	$ 9,418	42,209	39,699	36,213
Salaries and employee benefits									(45,303)	(44,014)	$ (42,710)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities										61
Provision for income taxes										(17)
Net income										44
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Accretion of Unrealized Losses Transferred [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities									(798)	(1,022)
Provision for income taxes									207	269
Net income									(591)	(753)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Prior-Service Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits									(114)	(114)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Actuarial Gains (Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits									(1,889)	(1,351)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Amortization of Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before taxes									(2,003)	(1,465)
Provision for income taxes									562	412
Net income									$ (1,441)	$ (1,053)

Earnings Per Share ("EPS") - Ad

Earnings Per Share ("EPS") - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Minimum [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Class A common stock entitled dividend per share percent in comparison to Class B common stock	200.00%
Class A Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Number of Stock options outstanding	0	0	0
Class A Common Stock [Member] | Minimum [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Class A common stock entitled dividend per share percent in comparison to Class B common stock	200.00%

Earnings Per Share ("EPS") - Re

Earnings Per Share ("EPS") - Reconciliation of Basic EPS and Diluted EPS (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 11,600	$ 10,887	$ 10,056	$ 9,666	$ 10,732	$ 10,084	$ 9,466	$ 9,418	$ 42,209	$ 39,699	$ 36,213
Class A Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income									$ 33,449	$ 31,351	$ 28,479
Weighted average shares outstanding, basic	3,655,469	3,655,469	3,652,469	3,652,349	3,650,949	3,650,449	3,620,449	3,610,329	3,653,939	3,633,044	3,608,179
Basic earnings per share	$ 2.52	$ 2.36	$ 2.18	$ 2.10	$ 2.33	$ 2.19	$ 2.06	$ 2.05	$ 9.15	$ 8.63	$ 7.89
Net income									$ 33,449	$ 31,351	$ 28,479
Weighted average shares outstanding, diluted	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909
Diluted earnings per share	$ 2.08	$ 1.96	$ 1.81	$ 1.74	$ 1.93	$ 1.81	$ 1.70	$ 1.69	$ 7.58	$ 7.13	$ 6.50
Class B Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income									$ 8,760	$ 8,348	$ 7,734
Weighted average shares outstanding, basic	1,912,440	1,912,440	1,915,440	1,915,560	1,916,960	1,917,460	1,947,460	1,957,580	1,913,970	1,934,865	1,959,730
Basic earnings per share	$ 1.26	$ 1.18	$ 1.09	$ 1.05	$ 1.16	$ 1.09	$ 1.03	$ 1.03	$ 4.58	$ 4.31	$ 3.95
Net income									$ 8,760	$ 8,348	$ 7,734
Weighted average shares outstanding, diluted	1,912,440	1,912,440	1,915,440	1,915,560	1,916,960	1,917,460	1,947,460	1,957,580	1,913,970	1,934,865	1,959,730
Diluted earnings per share	$ 1.26	$ 1.18	$ 1.09	$ 1.05	$ 1.16	$ 1.09	$ 1.03	$ 1.03	$ 4.58	$ 4.31	$ 3.95

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020
Minimum [Member]
Class of Stock [Line Items]
Class A common stock entitled dividend per share percent in comparison to Class B common stock	200.00%

Stockholders' Equity - Summary

Stockholders' Equity - Summary of the Bank's Actual Capital Amounts and Ratios (Detail) - Century Bancorp Capital Trust II [Member]	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 448,695	$ 401,850
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	413,209	372,265
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	413,209	372,265
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	$ 413,209	$ 372,265
Total Capital (to Risk-Weighted Assets), Ratio	0.1303	0.1357
Tier 1 Capital (to Risk-Weighted Assets), Ratio	0.1200	0.1257
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Ratio	12.00%	12.57%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	0.0641	0.0701
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	$ 275,403	$ 236,830
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	206,553	177,622
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	154,914	133,217
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	$ 257,763	$ 212,549
Total Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	0.0800	0.0800
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	0.0600	0.0600
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	4.50%	4.50%
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Ratio	0.0400	0.0400
Total Capital (to Risk-Weighted Assets), Well Capitalized Amount	$ 344,254	$ 296,037
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	275,403	236,830
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	223,765	192,424
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$ 322,204	$ 265,686
Total Capital (to Risk-Weighted Assets), Well Capitalized Ratio	0.1000	0.1000
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	0.0800	0.0800
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	6.50%	6.50%
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Ratio	0.0500	0.0500

Stockholders' Equity - Summar_2

Stockholders' Equity - Summary of the Company's Actual Capital Amounts and Ratios (Detail) - Company [Member]	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Compliance with Regulatory Capital Requirements for Mortgage Companies [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 464,331	$ 415,863
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	428,845	386,308
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	393,845	351,308
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	$ 428,845	$ 386,308
Total Capital (to Risk-Weighted Assets), Ratio	0.1343	0.1397
Tier 1 Capital (to Risk-Weighted Assets), Ratio	0.1240	0.1298
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Ratio	11.39%	11.80%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	0.0664	0.0725
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	$ 276,673	$ 238,132
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	207,505	178,599
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	155,629	133,949
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	$ 258,414	$ 213,222
Total Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	0.0800	0.0800
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	0.0600	0.0600
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	4.50%	4.50%
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Ratio	0.0400	0.0400
Total Capital (to Risk-Weighted Assets), Well Capitalized Amount	$ 345,842	$ 297,665
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	276,673	238,132
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	224,797	193,482
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$ 323,017	$ 266,528
Total Capital (to Risk-Weighted Assets), Well Capitalized Ratio	0.1000	0.1000
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	0.0800	0.0800
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	6.50%	6.50%
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Ratio	0.0500	0.0500

Income Taxes - Summary of Curre

Income Taxes - Summary of Current and Deferred Components of Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current expense:
Federal									$ 3,651	$ 2,548	$ 2,637
State									1,845	697	697
Total current expense									5,496	3,245	3,334
Deferred (benefit) expense:
Federal									(491)	(1,660)	(1,238)
State									(598)	(367)	(528)
Valuation Allowance										(108)
Total deferred (benefit) expense									(1,089)	(2,135)	(1,766)
Provision for income taxes	$ 1,203	$ 1,546	$ 1,061	$ 597	$ 526	$ 435	$ 267	$ (118)	$ 4,407	$ 1,110	$ 1,568

Income Taxes - Income Tax Accou

Income Taxes - Income Tax Accounts Included in Other Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income tax receivable	$ 2,250	$ 3,446
Deferred income tax asset, net	26,431	24,566
Total	$ 28,681	$ 28,012

Income Taxes - Summary of Diffe

Income Taxes - Summary of Differences between Income Tax Expense (Benefit) at the Statutory Federal Income Tax Rate and Total Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rates									$ 9,789	$ 8,570	$ 7,934
State income tax, net of federal income tax benefit									930	261	134
Insurance income									(834)	(265)	(176)
Effect of tax-exempt interest									(5,490)	(6,737)	(6,510)
Net tax credit									(231)	(292)	(349)
Valuation allowance reversal										(108)
Sequestration (reversal) accrual										(438)	438
Other									243	119	97
Provision for income taxes	$ 1,203	$ 1,546	$ 1,061	$ 597	$ 526	$ 435	$ 267	$ (118)	$ 4,407	$ 1,110	$ 1,568
Effective tax rate									9.45%	2.72%	4.15%

Income Taxes - Gross Deferred I

Income Taxes - Gross Deferred Income Tax Assets and Gross Deferred Income Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred income tax assets:
Allowance for loan losses	$ 10,025	$ 8,354
Deferred compensation	8,833	8,910
Pension and SERP liability	9,911	8,770
Operating lease liabilities	3,917	3,567
Unrealized losses on securities transferred to held-to-maturity	436	643
Depreciation	1,214	1,060
QZAB credit		812
PPP loan origination fees	919
Accrued bonus		708
Charitable contributions carryforward		276
Nonaccrual interest	75	115
Other	305	206
Gross deferred income tax asset	35,635	33,421
Deferred income tax liabilities:
Pension liability	(4,142)	(4,258)
Operating lease right-of-use assets	(3,855)	(3,520)
Deferred origination costs	(584)	(516)
Prepaid expenses	(361)	(337)
Mortgage servicing rights	(217)	(338)
Unrealized (gains) losses on securities available-for-sale	(45)	114
Gross deferred income tax liability	(9,204)	(8,855)
Deferred income tax asset net	$ 26,431	$ 24,566

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020
Income Taxes [Line Items]
Federal tax rate		21.00%	34.00%
Valuation allowance				$ 0
Reduction in sequestration reversal accrual	$ 438,000
Unrecognized tax benefits				0
Accrued interest and penalties related to uncertain tax positions				$ 0

Employee Benefits - Additional

Employee Benefits - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Benefits expected to be paid in 2021	$ 2,008,000
Benefits expected to be paid in 2022	2,188,000
Benefits expected to be paid in 2023	2,322,000
Benefits expected to be paid in 2024	2,521,000
Benefits expected to be paid in 2025	2,707,000
Aggregate benefits expected to paid	$ 16,087,000
Voluntary contribution of employees	33.30%
Contributions matched by compensation contribution	6.00%
Voluntary contribution of employees, amount	$ 533,000	$ 458,000	$ 454,000
Discretionary bonus expense	2,529,000	2,364,000	2,355,000
Expenses related to the insurance agreement	464,000	456,000	$ 420,000
Term Life Insurance [Member]
Defined Benefit Plan Disclosure [Line Items]
Aggregate insurance amount	$ 15,000,000
term of insurance policy	ten years
Expenses related to the insurance agreement	$ 155,000
Change in Assumptions for Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in the projected benefit obligations		$ 6,800,000
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefits expected to be paid in 2021	2,570,000
Benefits expected to be paid in 2022	2,507,000
Benefits expected to be paid in 2023	2,439,000
Benefits expected to be paid in 2024	2,595,000
Benefits expected to be paid in 2025	3,102,000
Aggregate benefits expected to paid	$ 17,913,000
Equity Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	47.00%
Equity Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	61.00%
Fixed Income Funds [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	24.00%
Fixed Income Funds [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	38.00%
Hedge Funds [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	9.00%
Hedge Funds [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	15.00%

Employee Benefits - Fair Value

Employee Benefits - Fair Value of Plan Assets and Major Categories (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	100.00%	100.00%
Total, fair value of plan assets	$ 56,433	$ 51,660
Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	49.70%	49.10%
Total, fair value of plan assets	$ 28,021	$ 25,386
Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	28,021	25,386
Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 28,021	$ 25,386
Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	50.30%	50.90%
Total, fair value of plan assets	$ 28,412	$ 26,274
Collective Funds [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	10.60%	8.30%
Total, fair value of plan assets	$ 5,999	$ 4,289
Collective Funds [Member] | Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 5,999	$ 4,289
Equity Securities [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	26.60%	9.70%
Total, fair value of plan assets	$ 14,952	$ 5,016
Equity Securities [Member] | Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 14,952	$ 5,016
Diversified [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	12.30%	31.10%
Total, fair value of plan assets	$ 6,949	$ 16,081
Diversified [Member] | Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 6,949	$ 16,081
Short-Term Investments [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	0.20%
Total, fair value of plan assets	$ 121
Short-Term Investments [Member] | Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 121

Employee Benefits - Schedule of

Employee Benefits - Schedule of Investments Measured Using Net Asset Value Per Share Practical Expedient (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	100.00%	100.00%
Investments, Fair Value	$ 56,433	$ 51,660
Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	50.30%	50.90%
Investments, Fair Value	$ 28,412	$ 26,274
Collective Funds [Member] | Equity [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	12.60%	19.20%
Investments, Fair Value	$ 7,113	$ 9,932
Collective Funds [Member] | US Debt Securities [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	18.00%	15.20%
Investments, Fair Value	$ 10,131	$ 7,874
Collective Funds [Member] | International Equities [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	14.10%	10.10%
Investments, Fair Value	$ 7,964	$ 5,208
Limited Partnerships [Member] | Emerging Markets [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	4.10%	3.20%
Investments, Fair Value	$ 2,308	$ 1,635
Limited Partnerships [Member] | Multi-strategy [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent		1.20%
Investments, Fair Value		$ 644
Hedge Funds [Member] | Global Opportunities [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	0.30%	0.50%
Investments, Fair Value	$ 197	$ 259
Hedge Funds [Member] | Private Investment Entities and/or Separately Managed Accounts [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	1.20%	1.40%
Investments, Fair Value	$ 699	$ 722

Employee Benefits - Components

Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Change in plan assets
Fair value of plan assets at beginning of year	$ 51,660
Fair value of plan assets at end of year	56,433	$ 51,660
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Total recognized in other comprehensive income	25,059	22,139
Defined Benefit Pension Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	49,434	40,509
Service cost	1,377	1,103
Interest cost	1,801	1,892
Actuarial (gain)/loss	3,245	7,099
Benefits paid	(1,418)	(1,169)
Projected benefit obligation at end of year	54,439	49,434
Change in plan assets
Fair value of plan assets at beginning of year	51,660	44,437
Actual return on plan assets	6,191	8,392
Benefits paid	(1,418)	(1,169)
Fair value of plan assets at end of year	56,433	51,660
(Unfunded) Funded status	1,994	2,226
Accumulated benefit obligation	$ 54,439	$ 49,434
Weighted-average assumptions as of December 31
Discount rate — Liability	3.25%	3.71%
Discount rate — Expense	3.71%	4.76%
Expected return on plan assets	7.50%	7.50%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	$ 1,377	$ 1,103
Interest cost	1,801	1,892
Expected return on plan assets	(3,808)	(3,275)
Recognized net losses	1,041	916
Net periodic benefit cost (credit)	411	636
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Net (gain) loss	(179)	1,066
Total recognized in other comprehensive income	(179)	1,066
Total recognized in net periodic benefit cost and other comprehensive income	232	1,702
Supplemental Insurance/ Retirement Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	49,976	40,405
Service cost	1,412	1,024
Interest cost	1,862	1,926
Prior sevice cost	501
Actuarial (gain)/loss	4,703	7,537
Benefits paid	(4,511)	(916)
Projected benefit obligation at end of year	53,943	49,976
Change in plan assets
Benefits paid	(4,511)	(916)
(Unfunded) Funded status	(53,943)	(49,976)
Accumulated benefit obligation	$ 48,767	$ 45,238
Weighted-average assumptions as of December 31
Discount rate — Liability	3.21%	3.71%
Discount rate — Expense	3.71%	4.79%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	$ 1,412	$ 1,024
Interest cost	1,862	1,926
Recognized prior service cost	114	114
Recognized net losses	848	435
Net periodic benefit cost (credit)	4,236	3,499
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost	(114)	(114)
Net (gain) loss	3,809	7,101
Total recognized in other comprehensive income	3,695	6,987
Total recognized in net periodic benefit cost and other comprehensive income	$ 7,931	$ 10,486

Employee Benefits - Summary of

Employee Benefits - Summary of Defined Pension Plan and Supplemental Insurance Retirement Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	$ (739)	$ (307)
Net actuarial loss	(34,521)	(30,891)
Total	(35,260)	(31,198)
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	(3,245)	(7,099)
Total	(54,439)	(49,434)	$ (40,509)
Defined Benefit Pension Plan [Member] | Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	(12,741)	(12,920)
Total	(12,741)	(12,920)
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	114	114
Net actuarial loss	(4,703)	(7,537)
Total	(53,943)	(49,976)	$ (40,405)
Supplemental Insurance/ Retirement Plan [Member] | Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(739)	(307)
Net actuarial loss	(21,780)	(17,971)
Total	$ (22,519)	$ (18,278)

Employee Benefits - Summary o_2

Employee Benefits - Summary of Accumulated Other Comprehensive Loss Expected to be Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Amortization of loss to be recognized in 2020	$ 4,361	$ 6,842	$ (3,770)
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of loss to be recognized in 2020	965
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost to be recognized in 2020	214
Amortization of loss to be recognized in 2020	$ 1,079

Financial Instruments with Of_3

Financial Instruments with Off-Balance-Sheet Risk - Summary of Financial Instruments with Off-Balance-Sheet Risk (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	$ 5,344	$ 5,779
Commitments to Originate 1-4 Family Mortgages [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	28,163	13,806
Unused Lines of Credit [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	787,206	625,524
Unadvanced Portions of Construction Loans [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	54,553	11,062
Unadvanced Portions of Other Loans [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	$ 13,767	$ 15,801

Other Operating Expenses - Summ

Other Operating Expenses - Summary of Other Operating Expenses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other Income and Expenses [Abstract]
Marketing	$ 1,295	$ 2,132	$ 2,346
Software maintenance/amortization	2,580	2,409	2,002
Legal and audit	1,668	1,514	1,444
Contributions	870	813	1,077
Processing services	1,712	1,875	1,740
Consulting	1,151	1,552	1,464
Postage and delivery	896	1,002	1,021
Supplies	1,096	985	987
Telephone	1,012	956	946
Directors' fees	500	456	478
Insurance	464	456	420
Pension	1,859	2,008	678
Other	2,373	1,744	1,685
Total	$ 17,476	$ 17,902	$ 16,288

Fair Values of Financial Inst_3

Fair Values of Financial Instruments - Carrying Amount and Fair Value of Company's Financial Instruments (Detail) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019
Financial assets:
Securities held-to-maturity		$ 2,509,088	$ 2,351,120
Loans		2,960,343	2,396,534
Financial liabilities:
Time deposits		546,143	555,447
Other borrowed funds		177,009	370,955
Subordinated debentures		36,083	36,083
Carrying Value [Member]
Financial assets:
Securities held-to-maturity		2,509,088	2,351,120
Loans	[1]	2,960,343	2,396,534
Financial liabilities:
Time deposits		546,143	555,447
Other borrowed funds		177,009	370,955
Subordinated debentures		36,083	36,083
Estimated Fair Value [Member]
Financial assets:
Securities held-to-maturity		2,579,103	2,361,304
Loans	[1]	2,902,390	2,424,770
Financial liabilities:
Time deposits		556,470	560,746
Other borrowed funds		183,000	374,531
Subordinated debentures		36,083	36,083
Fair Value Measurements, Level 2 Inputs [Member]
Financial assets:
Securities held-to-maturity		2,579,103	2,361,304
Financial liabilities:
Time deposits		556,470	560,746
Other borrowed funds		183,000	374,531
Subordinated debentures		36,083	36,083
Fair Value Measurements, Level 3 Inputs [Member]
Financial assets:
Loans	[1]	$ 2,902,390	$ 2,424,770
[1]	Comprised of loans (including collateral dependent impaired loans), net of deferred loan costs and the allowance for loan losses.

Revenue from Contracts with C_3

Revenue from Contracts with Customers - Schedule of Revenue from Contracts (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total net interest income	$ 28,479	$ 27,331	$ 25,818	$ 25,201	$ 25,331	$ 23,770	$ 23,250	$ 23,438	$ 106,829	$ 95,789	$ 92,576
Noninterest income:
Service charges on deposit accounts									8,818	9,220	8,560
Lockbox fees									3,745	3,973	3,274
Brokerage commissions									271	277	348
Net gains on sales of securities									0	61	302
Gains on sales of mortgage loans										412
Other income									6,266	4,456	3,764
Total noninterest income	$ 6,580	$ 4,169	$ 4,041	$ 4,310	$ 4,689	$ 4,286	$ 4,997	$ 4,427	19,100	18,399	16,248
ASU 2014-09 [Member]
Disaggregation of Revenue [Line Items]
Total net interest income									106,829	95,789	92,576
Noninterest income:
Service charges on deposit accounts									8,818	9,220	8,560
Lockbox fees									3,745	3,973	3,274
Brokerage commissions									271	277	348
Net gains on sales of securities										61	302
Gains on sales of mortgage loans										412
Other income									6,266	4,456	3,764
Total noninterest income									19,100	18,399	16,248
Total revenues									125,929	114,188	108,824
ASU 2014-09 [Member] | Revenue Guidance Contracts in Scope of Topic 606 [Member]
Noninterest income:
Service charges on deposit accounts									8,818	9,220	8,560
Lockbox fees									3,745	3,973	3,274
Other income									2,132	2,799	2,536
Total noninterest income									14,695	15,992	14,370
Total revenues									$ 14,695	$ 15,992	$ 14,370

Revenue from Contracts with C_4

Revenue from Contracts with Customers - Information about Receivables with Customers (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Other Assets [Member]
Disaggregation of Revenue [Line Items]
Receivables, which are included in "Other assets"	$ 1,397	$ 1,200

Leases - Additional Information

Leases - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating lease option to extend	options to extend the leases for up to 10 years, and some of which include options to terminate the leases within 1 year.
Sublease Income	$ 40,000
Sublease option to terminate	The sublease expires in 2022 with an option to terminate and no option to extend.
Lease expense	$ 2,758,000	$ 2,744,000	$ 2,601,000
Lease rental income	658,000	419,000	373,000
Barry R Sloane [Member]
Lease expense	$ 439,000	$ 458,000	$ 444,000
Maximum [Member]
Operating lease remaining lease term	31 years
Minimum [Member]
Operating lease remaining lease term	1 year

Leases - Components of lease ex

Leases - Components of lease expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating lease cost	$ 2,179	$ 2,216
Variable lease cost	579	528
Total lease cost	$ 2,758	$ 2,744

Leases - Supplemental cash flow

Leases - Supplemental cash flow information related to leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 2,085	$ 2,130
Right-of-use assets obtained in exchange for lease obligations:
Operating leases	$ 3,044	$ 138

Leases - Supplemental balance s

Leases - Supplemental balance sheet information related to leases (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Operating Leases:
Operating lease right-of-use assets	$ 13,713	$ 12,521
Operating lease liabilities	$ 13,935	$ 12,690
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OperatingLeaseLiability
Weighted Average Remaining Lease Term:
Operating Leases	10 years	11 years
Weighted Average Discount Rate:
Operating Leases	3.10%	3.50%

Leases - Summary of Future Paym

Leases - Summary of Future Payments of Lease Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
2020		$ 2,030
2021	$ 2,156	1,754
2022	1,995	1,603
2023	1,962	1,545
2024	1,692	1,277
2025	1,471
Thereafter	7,394	7,312
Total lease payments	16,670	15,521
Less imputed interest	2,735	2,831
Present value of lease liability	$ 13,935	$ 12,690

Quarterly Results of Operatio_3

Quarterly Results of Operations - Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information [Line Items]
Interest income	$ 36,617	$ 36,504	$ 36,120	$ 39,795	$ 40,518	$ 39,852	$ 39,692	$ 39,077	$ 149,036	$ 159,139	$ 137,056
Interest expense	8,138	9,173	10,302	14,594	15,187	16,082	16,442	15,639	42,207	63,350	44,480
Net interest income	28,479	27,331	25,818	25,201	25,331	23,770	23,250	23,438	106,829	95,789	92,576
Provision for loan losses	2,150	900	1,700	1,075	550	75	250	375	5,825	1,250	1,350
Net interest income after provision for loan losses	26,329	26,431	24,118	24,126	24,781	23,695	23,000	23,063	101,004	94,539	91,226
Other operating income	6,580	4,169	4,041	4,310	4,689	4,286	4,997	4,427	19,100	18,399	16,248
Operating expenses	20,106	18,167	17,042	18,173	18,212	17,462	18,264	18,190	73,488	72,129	69,693
Income before income taxes	12,803	12,433	11,117	10,263	11,258	10,519	9,733	9,300	46,616	40,809	37,781
Provision for income taxes	1,203	1,546	1,061	597	526	435	267	(118)	4,407	1,110	1,568
Net income	$ 11,600	$ 10,887	$ 10,056	$ 9,666	$ 10,732	$ 10,084	$ 9,466	$ 9,418	$ 42,209	$ 39,699	$ 36,213
Class A Common Stock [Member]
Share data:
Average shares outstanding, basic	3,655,469	3,655,469	3,652,469	3,652,349	3,650,949	3,650,449	3,620,449	3,610,329	3,653,939	3,633,044	3,608,179
Average shares outstanding, diluted	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909	5,567,909
Earnings per share, basic	$ 2.52	$ 2.36	$ 2.18	$ 2.10	$ 2.33	$ 2.19	$ 2.06	$ 2.05	$ 9.15	$ 8.63	$ 7.89
Earnings per share, diluted	$ 2.08	$ 1.96	$ 1.81	$ 1.74	$ 1.93	$ 1.81	$ 1.70	$ 1.69	$ 7.58	$ 7.13	$ 6.50
Class B Common Stock [Member]
Share data:
Average shares outstanding, basic	1,912,440	1,912,440	1,915,440	1,915,560	1,916,960	1,917,460	1,947,460	1,957,580	1,913,970	1,934,865	1,959,730
Average shares outstanding, diluted	1,912,440	1,912,440	1,915,440	1,915,560	1,916,960	1,917,460	1,947,460	1,957,580	1,913,970	1,934,865	1,959,730
Earnings per share, basic	$ 1.26	$ 1.18	$ 1.09	$ 1.05	$ 1.16	$ 1.09	$ 1.03	$ 1.03	$ 4.58	$ 4.31	$ 3.95
Earnings per share, diluted	$ 1.26	$ 1.18	$ 1.09	$ 1.05	$ 1.16	$ 1.09	$ 1.03	$ 1.03	$ 4.58	$ 4.31	$ 3.95

Parent Company Financial Stat_3

Parent Company Financial Statements - Balance Sheets of Parent Company (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
ASSETS:
Cash	$ 136,735	$ 44,420
Other assets	165,581	157,354
Total assets	6,358,834	5,492,424
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	5,988,425	5,159,843
Subordinated debentures	36,083	36,083
Stockholders' equity	370,409	332,581	$ 300,439	$ 260,297
Total liabilities and stockholders' equity	6,358,834	5,492,424
Century Bancorp, Inc. [Member]
ASSETS:
Cash	5,141	3,177
Investment in subsidiary, at equity	389,724	353,489
Other assets	15,920	16,325
Total assets	410,785	372,991
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	4,293	4,327
Subordinated debentures	36,083	36,083
Stockholders' equity	370,409	332,581
Total liabilities and stockholders' equity	$ 410,785	$ 372,991

Parent Company Financial Stat_4

Parent Company Financial Statements - Statements of Income of Parent Company (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income:
Other income									$ 6,266	$ 4,456	$ 3,764
Total interest income	$ 36,617	$ 36,504	$ 36,120	$ 39,795	$ 40,518	$ 39,852	$ 39,692	$ 39,077	149,036	159,139	137,056
Interest expense	8,138	9,173	10,302	14,594	15,187	16,082	16,442	15,639	42,207	63,350	44,480
Operating expenses	20,106	18,167	17,042	18,173	18,212	17,462	18,264	18,190	73,488	72,129	69,693
Income before income taxes	12,803	12,433	11,117	10,263	11,258	10,519	9,733	9,300	46,616	40,809	37,781
Benefit from income taxes	1,203	1,546	1,061	597	526	435	267	(118)	4,407	1,110	1,568
Net income	$ 11,600	$ 10,887	$ 10,056	$ 9,666	$ 10,732	$ 10,084	$ 9,466	$ 9,418	42,209	39,699	36,213
Century Bancorp, Inc. [Member]
Income:
Dividends from subsidiary									5,000	5,000	4,750
Other income									25	65	53
Total interest income									5,025	5,065	4,803
Interest expense									951	1,577	1,474
Operating expenses									235	215	225
Income before income taxes									3,839	3,273	3,104
Benefit from income taxes									(244)	(363)	(347)
Income before equity in undistributed income of subsidiary									4,083	3,636	3,451
Equity in undistributed income of subsidiary									38,126	36,063	32,762
Net income									$ 42,209	$ 39,699	$ 36,213

Parent Company Financial Stat_5

Parent Company Financial Statements - Statements of Cash Flows of Parent Company (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 11,600	$ 10,887	$ 10,056	$ 9,666	$ 10,732	$ 10,084	$ 9,466	$ 9,418	$ 42,209	$ 39,699	$ 36,213
Adjustments to reconcile net income to net cash provided by operating activities
Decrease (increase) in other assets									(413)	8,532	2,326
Net cash provided by operating activities									44,375	47,849	40,788
CASH FLOWS FROM FINANCING ACTIVITIES:
Cash dividends paid									(2,490)	(2,207)	(2,203)
Net cash provided by financing activities									821,719	271,322	337,644
Century Bancorp, Inc. [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									42,209	39,699	36,213
Adjustments to reconcile net income to net cash provided by operating activities
Undistributed income of subsidiary									(38,126)	(36,063)	(32,762)
Decrease (increase) in other assets									405	665	(158)
(Decrease) increase in liabilities									(34)	(180)	(1,808)
Net cash provided by operating activities									4,454	4,121	1,485
CASH FLOWS FROM FINANCING ACTIVITIES:
Cash dividends paid									(2,490)	(2,207)	(2,203)
Net cash provided by financing activities									(2,490)	(2,207)	(2,203)
Net (decrease) in cash									1,964	1,914	(718)
Cash and cash equivalents at beginning of year				$ 3,177				$ 1,263	3,177	1,263	1,981
Cash and cash equivalents at end of year	$ 5,141				$ 3,177				$ 5,141	$ 3,177	$ 1,263