Century Bancorp (CNBKA) 10-K 15 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 28, 2018	Jun. 30, 2017
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	CNBKA
Entity Registrant Name	CENTURY BANCORP INC
Entity Central Index Key	812,348
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 230,477,369
Class A Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		3,607,429
Class B Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		1,960,480

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Cash and due from banks (Note 2)	$ 77,199	$ 62,400
Federal funds sold and interest-bearing deposits in other banks	279,231	173,751
Total cash and cash equivalents	356,430	236,151
Short-term investments		3,183
Securities available-for-sale, amortized cost $397,563 in 2017 and $500,220 in 2016 (Notes 3, 9 and 11)	397,475	499,297
Securities held-to-maturity, fair value $1,668,827 in 2017 and $1,635,808 in 2016 (Notes 4 and 11)	1,701,233	1,653,986
Federal Home Loan Bank of Boston, stock at cost	21,779	21,042
Loans, net (Note 5)	2,175,944	1,923,933
Less: allowance for loan losses (Note 6)	26,255	24,406
Net loans	2,149,689	1,899,527
Bank premises and equipment (Note 7)	23,527	23,417
Accrued interest receivable	11,179	9,645
Other assets (Notes 5, 8 and 16)	124,260	116,360
Total assets	4,785,572	4,462,608
LIABILITIES AND STOCKHOLDERS' EQUITY
Demand deposits	736,020	689,286
Savings and NOW deposits	1,367,358	1,304,394
Money market accounts	1,188,228	1,181,179
Time deposits (Note 10)	625,361	478,359
Total deposits	3,916,967	3,653,218
Securities sold under agreements to repurchase (Note 11)	158,990	182,280
Other borrowed funds (Note 12)	347,778	293,000
Subordinated debentures (Note 12)	36,083	36,083
Other liabilities	65,457	57,986
Total liabilities	4,525,275	4,222,567
Commitments and contingencies (Notes 7, 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	263,666	243,565
Stockholders' equity before adjustment of accumulated other comprehensive income (loss)	281,526	261,425
Unrealized losses on securities available-for-sale, net of taxes	(62)	(567)
Unrealized losses on securities transferred to held-to-maturity, net of taxes	(3,050)	(4,084)
Pension liability, net of taxes	(18,117)	(16,733)
Total accumulated other comprehensive loss, net of taxes (Notes 3, 13 and 15)	(21,229)	(21,384)
Total stockholders' equity	260,297	240,041
Total liabilities and stockholders' equity	4,785,572	4,462,608
Class A Common Stock [Member]
Stockholders' equity (Note 15):
Common stock value	3,606	3,601
Total stockholders' equity	3,606	3,601
Class B Common Stock [Member]
Stockholders' equity (Note 15):
Common stock value	1,962	1,967
Total stockholders' equity	$ 1,962	$ 1,967

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized cost	$ 397,563	$ 500,220
Held-to-maturity securities, fair value	$ 1,668,827	$ 1,635,808
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	3,605,829	3,600,729
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	1,962,080	1,967,180

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
INTEREST INCOME
Loans, taxable	$ 39,103	$ 34,324	$ 32,136
Loans, non-taxable	26,910	23,440	19,992
Securities available-for-sale, taxable	4,987	3,003	1,900
Securities available-for-sale, non-taxable	1,119	1,051	583
Federal Home Loan Bank of Boston dividends	872	966	658
Securities held-to-maturity	38,348	32,679	34,388
Federal funds sold, interest-bearing deposits in other banks and short-term investments	2,097	1,236	436
Total interest income	113,436	96,699	90,093
INTEREST EXPENSE
Savings and NOW deposits	6,296	4,020	2,817
Money market accounts	5,626	3,542	3,038
Time deposits	7,919	5,706	4,887
Securities sold under agreements to repurchase	496	472	487
Other borrowed funds and subordinated debentures	7,483	8,877	8,905
Total interest expense	27,820	22,617	20,134
Net interest income	85,616	74,082	69,959
Provision for loan losses (Note 6)	1,790	1,375	200
Net interest income after provision for loan losses	83,826	72,707	69,759
OTHER OPERATING INCOME
Service charges on deposit accounts	8,586	7,907	7,732
Lockbox fees	3,290	3,164	3,211
Brokerage commissions	353	315	380
Net gains on sales of securities	47	64	594
Gains on sales of mortgage loans	370	1,331	1,034
Other income	3,906	3,441	3,042
Total other operating income	16,552	16,222	15,993
OPERATING EXPENSES
Salaries and employee benefits (Note 17)	41,913	40,048	38,596
Occupancy	6,140	6,147	6,116
Equipment	2,892	2,845	2,626
FDIC assessments	1,581	1,902	2,152
Other (Note 20)	14,593	13,815	12,708
Total operating expenses	67,119	64,757	62,198
Income before income taxes	33,259	24,172	23,554
Provision for income taxes (Note 16)	10,958	(362)	533
Net income	$ 22,301	$ 24,534	$ 23,021
Class A Common Stock [Member]
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	3,604,029	3,600,729	3,600,729
Weighted average number of shares outstanding, diluted	5,567,909	5,567,909	5,567,909
Basic earnings per share	$ 4.86	$ 5.35	$ 5.02
Diluted earnings per share	$ 4.01	$ 4.41	$ 4.13
Class B Common Stock [Member]
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	1,963,880	1,967,180	1,967,180
Weighted average number of shares outstanding, diluted	1,963,880	1,967,180	1,967,180
Basic earnings per share	$ 2.43	$ 2.68	$ 2.51
Diluted earnings per share	$ 2.43	$ 2.68	$ 2.51

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 22,301	$ 24,534	$ 23,021
Unrealized gains (losses) on securities:
Unrealized holding gains (losses) arising during period	533	(289)	38
Less: reclassification adjustment for gains included in net income	(28)	(32)	(361)
Total unrealized gains (losses) on securities	505	(321)	(323)
Accretion of net unrealized losses transferred during period	1,034	2,812	3,583
Pension liability adjustment:
Net (loss) gain	(2,315)	(297)	(2,890)
Amortization of prior service cost and loss included in net periodic benefit cost	931	970	853
Total pension liability adjustment	(1,384)	673	(2,037)
Other comprehensive income	155	3,164	1,223
Comprehensive income (loss)	$ 22,456	$ 27,698	$ 24,244

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, 2014	$ 192,500	$ 3,601	$ 1,967	$ 12,292	$ 200,411			$ (25,771)
Net income	23,021				23,021
Other comprehensive income, net of tax:
Unrealized holding gains arising during period, net of taxes	(323)							(323)
Accretion of net unrealized losses transferred during the period, net of taxes	3,583							3,583
Pension liability adjustment, net of taxes	(2,037)							(2,037)
Cash dividends						$ (1,728)	$ (472)
Ending balance at Dec. 31, 2015	214,544	3,601	1,967	12,292	221,232			(24,548)
Net income	24,534				24,534
Other comprehensive income, net of tax:
Unrealized holding gains arising during period, net of taxes	(321)							(321)
Accretion of net unrealized losses transferred during the period, net of taxes	2,812							2,812
Pension liability adjustment, net of taxes	673							673
Cash dividends						(1,729)	(472)
Ending balance at Dec. 31, 2016	240,041	3,601	1,967	12,292	243,565			(21,384)
Net income	22,301				22,301
Other comprehensive income, net of tax:
Unrealized holding gains arising during period, net of taxes	505							505
Accretion of net unrealized losses transferred during the period, net of taxes	1,034							1,034
Pension liability adjustment, net of taxes	(1,384)							(1,384)
Conversion of Class B Common Stock to Class A Common Stock		5	(5)
Cash dividends						$ (1,729)	$ (471)
Ending balance at Dec. 31, 2017	$ 260,297	$ 3,606	$ 1,962	$ 12,292	$ 263,666			$ (21,229)

Consolidated Statements of Cha7

Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrealized holding gains arising during period, taxes	$ 337	$ 248	$ 211
Realized net gains	47	52	594
Accretion of net unrealized losses transferred during the period, taxes	1,258	1,505	1,919
Pension liability adjustment, taxes	$ (286)	$ 448	$ 1,357
Class A Common Stock [Member]
Conversion of Class B Common Stock to Class A Common Stock, shares	5,100
Cash dividends, per share	$ 0.48	$ 0.48	$ 0.48
Class B Common Stock [Member]
Conversion of Class B Common Stock to Class A Common Stock, shares	5,100
Cash dividends, per share	$ 0.24	$ 0.24	$ 0.24

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 22,301	$ 24,534	$ 23,021
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on sales of portfolio loans	(370)	(1,331)	(1,034)
Gain on sale of fixed assets	(11)
Net gains on sales of securities	(47)	(64)	(594)
Provision for loan losses	1,790	1,375	200
Deferred tax benefit (expense)	6,918	(4,676)	(3,259)
Net depreciation and amortization	3,047	3,561	3,296
Increase in accrued interest receivable	(1,534)	(1,643)	(1,761)
Gain on sales of other real estate owned			(57)
Increase in other assets	(16,195)	(2,953)	(10,862)
Increase in other liabilities	5,802	3,203	2,103
Net cash provided by (used in) operating activities	21,701	22,006	11,053
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from maturities of short-term investments	5,284	3,233
Purchase of short-term investments	(2,101)	(3,183)	(1,102)
Proceeds from redemptions of Federal Home Loan Bank of Boston stock	10,127	10,381	891
Purchase of Federal Home Loan Bank of Boston stock	(10,864)	(2,616)	(4,782)
Proceeds from calls/maturities of securities available-for-sale	259,388	277,657	206,109
Proceeds from sales of securities available-for-sale	18,180	2,376	47,853
Purchase of securities available-for-sale	(175,147)	(375,608)	(210,302)
Proceeds from calls/maturities of securities held-to-maturity	293,221	416,599	414,786
Proceeds from sales of securities held-to-maturity		192	3,698
Purchase of securities held-to-maturity	(337,773)	(627,670)	(444,969)
Proceeds from sales of portfolio loans	26,701	74,668	66,600
Net increase in loans	(278,242)	(265,732)	(467,048)
Proceeds from sales of other real estate owned			1,973
Proceeds from sales of fixed assets	11
Capital expenditures	(3,244)	(2,263)	(2,652)
Net cash used in investing activities	(194,459)	(491,966)	(388,945)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net increase in time deposit accounts	147,002	4,933	90,281
Net increase in demand, savings, money market and NOW deposits	116,747	573,225	247,188
Cash dividends	(2,200)	(2,201)	(2,200)
Net decrease in securities sold under agreements to repurchase	(23,290)	(15,570)	(14,510)
Net increase (decrease) in other borrowed funds	54,778	(75,000)	(27,500)
Net cash provided by (used in) financing activities	293,037	485,387	293,259
Net increase (decrease) in cash and cash equivalents	120,279	15,427	(84,633)
Cash and cash equivalents at beginning of year	236,151	220,724	305,357
Cash and cash equivalents at end of year	356,430	236,151	220,724
Cash paid during the year for:
Interest	27,731	22,668	19,979
Income taxes	5,330	3,730	4,300
Change in unrealized gains on securities available-for-sale, net of taxes	505	(321)	(323)
Change in unrealized losses on securities transferred to held-to-maturity, net of taxes	1,034	2,812	3,583
Pension liability adjustment, net of taxes	$ (1,384)	$ 673	(2,037)
Transfer of loans to other real estate owned			$ 1,916

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
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 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 G-7 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 corporate bonds and loans, mortgage whole loans, municipal bonds and over the counter (“OTC”) derivatives. 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. SHORT-TERM INVESTMENTS As of December 31, 2017 and 2016, short-term investments include highly liquid certificates of deposit with original maturities of more than 90 days but less than one year. 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. 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, 2017, 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 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 and loans that are measured at fair value. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off 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, 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. There were no new loans acquired during the year ended December 31, 2017. 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 two-step The first step, in the two-step 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. STOCK OPTION ACCOUNTING The Company follows the fair value recognition provisions of FASB ASC 718, Compensation — Stock Compensation During 2000 and 2004, common stockholders of the Company approved stock option plans (the “Option Plans”) that provide for granting of options to purchase up to 150,000 shares of Class A common stock per plan. Under the Option Plans, all officers and key employees of the Company are eligible to receive nonqualified or incentive stock options to purchase shares of Class A common stock. The Option Plans are administered by the Compensation Committee of the Board of Directors, whose members are ineligible to participate in the Option Plans. Based on management’s recommendations, the Committee submits its recommendations to the Board of Directors as to persons to whom options are to be granted, the number of shares granted to each, the option price (which may not be less than 85% of the fair market value for nonqualified stock options, or the fair market value for incentive stock options, of the shares on the date of grant) and the time period over which the options are exercisable (not more than ten years from the date of grant). There were no options to purchase shares of Class A common stock outstanding at December 31, 2017. The Company uses the fair value method to account for stock options. There were no options granted during 2017 and 2016. 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. 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. Accordingly, the full amount of the AMT credit carryforward will be recovered in tax years beginning before 2022. As a result of the change, the Company has classified its AMT credit carryforward as currently receivable. 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 only common stock equivalents for the Company are stock options. The company utilizes the two class method for reporting EPS. The two-class TREASURY STOCK Effective July 1, 2004, companies incorporated in Massachusetts became subject to Chapter 156D of the Massachusetts Business Corporation Act, provisions of which eliminate the concept of treasury stock and provide that shares reacquired by a company are to be treated as authorized but unissued shares. PENSION The Company provides pension benefits to 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. The Company utilizes 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. RECENT ACCOUNTING DEVELOPMENTS In February 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. In July 2017, FASB issued ASU 2017-11, 2017-11 In May 2017, the FASB issued ASU 2017-09, In March 2017, the FASB issued ASU 2017-08, 310-20) In March 2017, the FASB issued ASU 2017-07, In February 2017, the FASB issued ASU 2017-05, 610-20). 610-20, In January 2017, the FASB issued ASU 2017-04, In June 2016, the FASB issued ASU 2016-13, In May 2016, the FASB issued ASU 2016-12, 2015-14 2014-09 2014-09, 2014-09, In July 2015, the FASB approved the deferral of the new standard’s effective date by one year. The new standard is effective for annual reporting periods beginning after December 15, 2017. The FASB will permit companies to adopt the new standard early, but not before the original effective date of annual reporting periods beginning after December 15, 2016. We monitored FASB activity related to the new standard. A significant amount of the Company’s revenues are derived from interest income on financial assets, which are excluded from the scope of the amended guidance. In 2017, we established a cross-functional implementation team consisting of representatives from across our business segments. We utilized a bottom-up During 2017, we completed our evaluation of the potential changes from adopting the new standard on our future financial reporting and disclosures. In the third quarter of 2017, we finalized our contract reviews. The Company did not identify any significant changes in the timing of revenue recognition when considering the amended accounting guidance. In February 2016, the FASB issued ASU 2016-02, In November 2016, the FASB issued ASU 2016-18, Restricted Cash. beginning-of-period end-of-period In August 2016, the FASB issued ASU 2016-15, Classification of Certain Cash Receipts and Cash Payments. In January 2016, FASB issued ASU 2016-1, 825-10) Recognition and Measurement of Financial Assets and Financial Liabilities.

Cash and Due from Banks

Cash and Due from Banks	12 Months Ended
	Dec. 31, 2017
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, 2017, and $0 at December 31, 2016.

Securities Available-for-Sale

Securities Available-for-Sale	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Securities Available-for-Sale	3. Securities Available-for-Sale December 31, 2017 December 31, 2016 Amortized Gross Gross Estimated Amortized Gross Gross Estimated (dollars in thousands) U.S. Treasury $ 1,999 $ — $ 15 $ 1,984 $ 2,000 $ — $ — $ 2,000 U.S. Government Sponsored Enterprises — — — — 25,000 — 48 24,952 SBA Backed Securities 81,065 46 161 80,950 57,899 14 146 57,767 U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 225,537 555 317 225,775 243,703 293 671 243,325 Privately Issued Residential Mortgage-Backed Securities 897 4 9 892 1,121 2 14 1,109 Obligations Issued by States and Political Subdivisions 82,849 — 249 82,600 165,281 — 405 164,876 Other Debt Securities 5,100 68 197 4,971 5,100 18 194 4,924 Equity Securities 116 187 — 303 116 228 — 344 Total $ 397,563 $ 860 $ 948 $ 397,475 $ 500,220 $ 555 $ 1,478 $ 499,297 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 $216,353,000 and 210,780,000 at December 31, 2017 and 2016, 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 $ 78,343 $ 78,338 After one but within five years 104,041 104,123 After five but within ten years 144,184 144,307 More than ten years 69,379 69,062 Nonmaturing 1,616 1,645 Total $ 397,563 $ 397,475 The weighted average remaining life of investment securities available-for-sale As of December 31, 2017 and December 31, 2016, 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 and because 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, 2017 and December 31, 2016. 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. In the case of marketable equity securities, the severity of the unrealized loss, the length of time the unrealized loss has existed, and the issuer’s financial performance are considered. The following table shows the temporarily impaired securities of the Company’s available-for-sale December 31, 2017 Less Than 12 Months 12 Months or Longer Total Temporarily Impaired Investments Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) U.S. Treasury $ 1,984 $ 15 $ — $ — $ 1,984 $ 15 U.S. Government Sponsored Enterprises — — — — — — SBA Backed Securities 18,378 54 40,911 107 59,289 161 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 40,394 123 59,336 194 99,730 317 Privately Issued Residential Mortgage-Backed Securities — — 633 9 633 9 Obligations Issued by States and Political Subdivisions — — 4,458 249 4,458 249 Other Debt Securities 400 1 1,803 196 2,203 197 Total temporarily impaired securities $ 61,156 $ 193 $ 107,141 $ 755 $ 168,297 $ 948 The following table shows the temporarily impaired securities of the Company’s available-for-sale December 31, 2016 Less Than 12 Months 12 Months or Longer Total Temporarily Impaired Investments Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) U.S. Treasury $ — $ — $ — $ — $ — $ — U.S. Government Sponsored Enterprises 24,952 48 — — 24,952 48 SBA Backed Securities 52,346 145 951 1 53,297 146 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 135,612 485 31,504 186 167,116 671 Privately Issued Residential Mortgage-Backed Securities — — 757 14 757 14 Obligations Issued by States and Political Subdivisions — — 4,298 405 4,298 405 Other Debt Securities 453 47 1,553 147 2,006 194 Total temporarily impaired securities $ 213,363 $ 725 $ 39,063 $ 753 $ 252,426 $ 1,478

Investment Securities Held-to-M

Investment Securities Held-to-Maturity	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Investment Securities Held-to-Maturity	4. Investment Securities Held-to-Maturity December 31, 2017 December 31, 2016 Amortized Gross Gross Estimated Amortized Gross Gross Estimated (dollars in U.S. Government Sponsored Enterprises $ 104,653 $ 341 $ 472 $ 104,522 $ 148,326 $ 1,066 $ 527 $ 148,865 SBA Backed Securities 57,235 20 1,271 55,984 46,140 — 1,088 45,052 U.S. Government Sponsored Enterprises Mortgage-Backed Securities 1,539,345 2,261 33,285 1,508,321 1,459,520 4,948 22,577 1,441,891 Total $ 1,701,233 $ 2,622 $ 35,028 $ 1,668,827 $ 1,653,986 $ 6,014 $ 24,192 $ 1,635,808 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 $1,262,708,000 and $1,147,207,000 at December 31, 2017, and 2016, respectively. Also included are securities pledged for borrowing at the Federal Home Loan Bank at fair value amounting to $382,120,000 and $424,353,000 at December 31, 2017, and 2016, respectively. The Company did not realize any gains of sales of securities for the year ending December 31, 2017. The Company realized gains on sales of securities of $12,000 from the proceeds of sales of held-to-maturity held-to-maturity held-to-maturity At December 31, 2017 and 2016, all mortgage-backed securities are obligations of U.S. Government Sponsored Enterprises. Government Sponsored Enterprises primarily refer to debt securities of Fannie Mae and Freddie Mac. The following table shows the maturity distribution of the Company’s securities held-to-maturity Amortized Fair Value (dollars in thousands) Within one year $ 28,752 $ 28,726 After one but within five years 1,257,279 1,234,931 After five but within ten years 411,916 401,947 More than ten years 3,286 3,223 Total $ 1,701,233 $ 1,668,827 The weighted average remaining life of investment securities held-to-maturity As of December 31, 2017 and December 31, 2016, 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 their remaining amortized costs. 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 unrealized loss on U.S. Government Sponsored Enterprises, SBA Backed Securities and U.S. Government Sponsored Enterprises Mortgage-Backed Securities related primarily to interest rates and not credit quality, and because the Company does not intend to sell any of these securities and it is not more likely than not that it will be required to sell these securities before the anticipated recovery of the remaining amortized cost, the Company does not consider these investments to be other-than-temporarily impaired at December 31, 2017 and December 31, 2016. 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. The following table shows the temporarily impaired securities of the Company’s held-to-maturity December 31, 2017 Less Than 12 Months 12 Months or Longer Total Temporarily Impaired Investments Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) U.S. Government Sponsored Enterprises $ 15,257 $ 239 $ 14,768 $ 233 $ 30,025 $ 472 SBA Backed Securities 19,457 142 33,750 1,129 53,207 1,271 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 519,481 5,920 814,712 27,365 1,334,193 33,285 Total temporarily impaired securities $ 554,195 $ 6,301 $ 863,230 $ 28,727 $ 1,417,425 $ 35,028 The following table shows the temporarily impaired securities of the Company’s held-to-maturity December 31, 2016 Less Than 12 Months 12 Months or Longer Total Temporarily Impaired Investments Fair Value Unrealized Fair Unrealized Fair Value Unrealized (dollars in thousands) U.S. Government Sponsored Enterprises $ 59,219 $ 527 $ — $ — $ 59,219 $ 527 SBA Backed Securities 45,052 1,088 — — 45,052 1,088 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 1,008,960 20,725 58,535 1,852 1,067,495 22,577 Total temporarily impaired securities $ 1,113,231 $ 22,340 $ 58,535 $ 1,852 $ 1,171,766 $ 24,192

Loans

Loans	12 Months Ended
	Dec. 31, 2017
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 and New York. 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, 2017 2016 (dollars in thousands) Construction and land development $ 18,931 $ 14,928 Commercial and industrial 763,807 612,503 Municipal 106,599 135,418 Commercial real estate 732,491 696,173 Residential real estate 287,731 241,357 Consumer 18,458 11,013 Home equity 247,345 211,857 Overdrafts 582 684 Total $ 2,175,944 $ 1,923,933 At December 31, 2017, and December 31, 2016, loans were carried net of discounts of $272,000 and $313,000, respectively. Net deferred fees included in loans at December 31, 2017, and December 31, 2016, were $362,000 and $641,000, respectively. The Company was servicing mortgage loans sold to others without recourse of approximately $229,533,000 and $229,730,000 at December 31, 2017, and December 31, 2016, respectively. The Company had no residential real estate loans held for sale at December 31, 2017 and December 31, 2016. The Company’s mortgage servicing rights totaled $1,525,000 and $1,629,000 at December 31, 2017 and December 31, 2016, respectively. As of December 31, 2017 and 2016, the Company’s recorded investment in impaired loans was $7,114,000 and $3,830,000, 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, 2017, there were $ 6,581,000 impaired loans with specific reserves of $164,000. At December 31, 2016, there were $3,105,000 of impaired loans with a specific reserve of $173,000. 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. Restructured loans are included in the impaired loan category. The composition of nonaccrual loans and impaired loans is as follows: December 31, 2017 2016 2015 (dollars in thousands) Loans on nonaccrual $ 1,684 $ 1,084 $ 2,336 Loans 90 days past due and still accruing — — — Impaired loans on nonaccrual included above 254 304 332 Total recorded investment in impaired loans 7,114 3,830 3,225 Average recorded investment of impaired loans 5,608 3,661 4,490 Accruing troubled debt restructures 2,749 3,526 2,893 Interest income not recorded on nonaccrual loans according to their original terms 51 37 91 Interest income on nonaccrual loans actually recorded — — — Interest income recognized on impaired loans 182 140 104 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 2017. Balance at December 31, 2016 Additions Repayments Balance at (dollars in thousands) $10,982 $ 572 $ 5,729 $ 5,825

Allowance for Loan Losses

Allowance for Loan Losses	12 Months Ended
	Dec. 31, 2017
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, 2017, 2016 and 2015 is as follows: 2017 2016 2015 (dollars in thousands) Allowance for loan losses, beginning of year $ 24,406 $ 23,075 $ 22,318 Loans charged-off (390 ) (389 ) (781 ) Recoveries on loans previously charged-off 449 434 1,338 Net recoveries (charge-offs) 59 45 557 Provision charged to expense 1,790 1,375 200 Reclassification to other liabilities* — (89 ) — Allowance for loan losses, end of year $ 26,255 $ 24,406 $ 23,075 * The reclassification relates to allowance for loan losses allocations on unused commitments that have been reclassified to other liabilities. Further information pertaining to the allowance for loan losses at December 31, 2017 follows: Construction Commercial and Industrial Municipal Commercial Residential Estate Consumer Home Unallocated Total (dollars in thousands) Allowance for Loan Losses: Balance at December 31, 2016 $ 1,012 $ 6,972 $ 1,612 $ 11,135 $ 1,698 $ 582 $ 1,102 $ 293 $ 24,406 Charge-offs — (49 ) — — — (341 ) — — (390 ) Recoveries — 110 — — 2 255 82 — 449 Provision 633 2,618 108 (1,407 ) 173 (123 ) (195 ) (17 ) 1,790 Ending balance at December 31, 2017 $ 1,645 $ 9,651 $ 1,720 $ 9,728 $ 1,873 $ 373 $ 989 $ 276 $ 26,255 Amount of allowance for loan losses for loans deemed to be impaired $ — $ 7 $ — $ 99 $ 58 $ — $ — $ — $ 164 Amount of allowance for loan losses for loans not deemed to be impaired $ 1,645 $ 9,644 $ 1,720 $ 9,629 $ 1,815 $ 373 $ 989 $ 276 $ 26,091 Loans: Ending balance $ 18,931 $ 763,807 $ 106,599 $ 732,491 $ 287,731 $ 19,040 $ 247,345 $ — $ 2,175,944 Loans deemed to be impaired $ — $ 348 $ — $ 2,554 $ 4,212 $ — $ — $ — $ 7,114 Loans not deemed to be impaired $ 18,931 $ 763,459 $ 106,599 $ 729,937 $ 283,519 $ 19,040 $ 247,345 $ — $ 2,168,830 Further information pertaining to the allowance for loan losses at December 31, 2016 follows: Construction Commercial Municipal Commercial Residential Consumer Home Unallocated Total (dollars in thousands) Allowance for Loan Losses: Balance at December 31, 2015 $ 2,041 $ 5,899 $ 994 $ 10,589 $ 1,320 $ 644 $ 1,077 $ 511 $ 23,075 Charge-offs — — — — — (362 ) (27 ) — (389 ) Recoveries — 132 — — 6 296 — — 434 Reclassification to other liabilities (5 ) (25 ) — (9 ) (3 ) (3 ) (44 ) — (89 ) Provision (1,024 ) 966 618 555 375 7 96 (218 ) 1,375 Ending balance at December 31, 2016 $ 1,012 $ 6,972 $ 1,612 $ 11,135 $ 1,698 $ 582 $ 1,102 $ 293 $ 24,406 Amount of allowance for loan losses for loans deemed to be impaired $ 3 $ 23 $ — $ 140 $ 7 $ — $ — $ — $ 173 Amount of allowance for loan losses for loans not deemed to be impaired $ 1,009 $ 6,949 $ 1,612 $ 10,995 $ 1,691 $ 582 $ 1,102 $ 293 $ 24,233 Loans: Ending balance $ 14,928 $ 612,503 $ 135,418 $ 696,173 $ 241,357 $ 11,697 $ 211,857 $ — $ 1,923,933 Loans deemed to be impaired $ 94 $ 389 $ — $ 3,149 $ 198 $ — $ — $ — $ 3,830 Loans not deemed to be impaired $ 14,834 $ 612,114 $ 135,418 $ 693,024 $ 241,159 $ 11,697 $ 211,857 $ — $ 1,920,103 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, 2017. 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, 2017. 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, 2017, 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, 2017. Construction Commercial Municipal Commercial (dollars in thousands) Grade: 1-3 $ 18,931 $ 758,093 $ 106,599 $ 705,235 4 (Monitor) — 5,366 — 24,702 5 (Substandard) — — — — 6 (Doubtful) — — — — Impaired — 348 — 2,554 Total $ 18,931 $ 763,807 $ 106,599 $ 732,491 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, 2017. Commercial Municipal Commercial Total (dollars in thousands) Credit Rating: Aaa-Aa3 $ 478,905 $ 62,029 $ 45,066 $ 586,000 A1-A3 195,599 7,635 128,554 331,788 Baa1-Baa3 — 26,970 122,000 148,970 Ba2 — 8,165 — 8,165 Total $ 674,504 $ 104,799 $ 295,620 $ 1,074,923 The following table presents the Company’s loans by risk rating at December 31, 2016. Construction Commercial Municipal Commercial (dollars in thousands) Grade: 1-3 $ 14,834 $ 612,114 $ 135,418 $ 661,271 4 (Monitor) — — — 31,753 5 (Substandard) — — — — 6 (Doubtful) — — — — Impaired 94 389 — 3,149 Total $ 14,928 $ 612,503 $ 135,418 $ 696,173 The following table presents the Company’s loans by credit rating at December 31, 2016. Commercial Municipal Commercial Total (dollars in thousands) Credit Rating: Aaa-Aa3 $ 334,674 $ 66,245 $ 6,596 $ 407,515 A1-A3 188,777 33,365 129,423 351,565 Baa1-Baa3 — 26,970 127,366 154,336 Ba2 — 3,610 — 3,610 Total $ 523,451 $ 130,190 $ 263,385 $ 917,026 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, 2017 the aging of past due loans are as follows: Accruing 30-89 Days Non Accruing 90 Days Total Current Total (dollars in thousands) Construction and land development $ — $ — $ — $ — $ 18,931 $ 18,931 Commercial and industrial 65 44 — 109 763,698 763,807 Municipal — — — — 106,599 106,599 Commercial real estate 672 215 — 887 731,604 732,491 Residential real estate 4,282 724 — 5,006 282,725 287,731 Consumer and overdrafts 5 6 — 11 19,029 19,040 Home equity 618 695 — 1,313 246,032 247,345 Total $ 5,642 $ 1,684 $ — $ 7,326 $ 2,168,618 $ 2,175,944 At December 31, 2016 the aging of past due loans are as follows: Accruing 30-89 Days Non Accruing 90 Days Total Current Total (dollars in thousands) Construction and land development $ — $ 94 $ — $ 94 $ 14,834 $ 14,928 Commercial and industrial 37 65 — 102 612,401 612,503 Municipal — — — — 135,418 135,418 Commercial real estate 597 150 — 747 695,426 696,173 Residential real estate 245 656 — 901 240,456 241,357 Consumer and overdrafts — 11 — 11 11,686 11,697 Home equity 735 108 — 843 211,014 211,857 Total $ 1,614 $ 1,084 $ — $ 2,698 $ 1,921,235 $ 1,923,933 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, 2017: Carrying Unpaid Required Average Interest (dollars in thousands) With no required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 113 325 — 54 4 Municipal — — — — — Commercial real estate 420 548 — 286 21 Residential real estate — — — 73 — Consumer — — — — — Home equity — — — — — Total $ 533 $ 873 $ — $ 413 $ 25 With required reserve recorded: Construction and land development $ — $ — $ — $ 43 $ — Commercial and industrial 235 235 7 318 12 Municipal — — — — — Commercial real estate 2,134 2,135 99 2,501 72 Residential real estate 4,212 4,212 58 2,333 73 Consumer — — — — — Home equity — — — — — Total $ 6,581 $ 6,582 $ 164 $ 5,195 $ 157 Total Construction and land development $ — $ — $ — $ 43 $ — Commercial and industrial 348 560 7 372 16 Municipal — — — — — Commercial real estate 2,554 2,683 99 2,787 93 Residential real estate 4,212 4,212 58 2,406 73 Consumer — — — — — Home equity — — — — — Total $ 7,114 $ 7,455 $ 164 $ 5,608 $ 182 The following is information pertaining to impaired loans at December 31, 2016: Carrying Unpaid Required Average Interest (dollars in thousands) With no required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 45 232 — 53 — Municipal — — — — — Commercial real estate 590 590 — 375 39 Residential real estate 90 179 — 102 7 Consumer — — — — — Home equity — — — — — Total $ 725 $ 1,001 $ — $ 530 $ 46 With required reserve recorded: Construction and land development $ 94 $ 108 $ 3 $ 96 $ — Commercial and industrial 344 360 23 360 18 Municipal — — — — — Commercial real estate 2,559 2,665 140 2,324 71 Residential real estate 108 108 7 323 5 Consumer — — — — — Home equity — — — 28 — Total $ 3,105 $ 3,241 $ 173 $ 3,131 $ 94 Total Construction and land development $ 94 $ 108 $ 3 $ 96 $ — Commercial and industrial 389 592 23 413 18 Municipal — — — — — Commercial real estate 3,149 3,255 140 2,699 110 Residential real estate 198 287 7 425 12 Consumer — — — — — Home equity — — — 28 — Total $ 3,830 $ 4,242 $ 173 $ 3,661 $ 140 Troubled Debt Restructurings 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 were no troubled debt restructurings occurring during the year ended December 31, 2017. There was one commercial real estate troubled debt restructuring during the year ended December 31, 2016. The pre-modification There were no commitments to lend additional funds to troubled debt restructuring borrowers. There were no troubled debt restructurings that subsequently defaulted during 2017 and 2016.

Bank Premises and Equipment

Bank Premises and Equipment	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Bank Premises and Equipment	7. Bank Premises and Equipment December 31, 2017 2016 Estimated Useful Life (dollars in thousands) Land $ 3,850 $ 3,478 — Bank premises 21,055 19,272 30-39 years Furniture and equipment 27,117 26,271 3-10 Leasehold improvements 12,674 12,802 30-39 years or lease term 64,696 61,823 Accumulated depreciation and amortization (41,169 ) (38,406 ) Total $ 23,527 $ 23,417 The Company is obligated under a number of non-cancelable Future minimum rental commitments for non-cancelable Year Amount (dollars in thousands) 2018 $ 2,309 2019 2,149 2020 1,856 2021 1,382 2022 1,022 Thereafter 1,942 $ 10,660

Goodwill and Identifiable Intan

Goodwill and Identifiable Intangible Assets	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Identifiable Intangible Assets	8. Goodwill and Identifiable Intangible Assets At December 31, 2017 and 2016, 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, 2017 and 2016 are shown in the table below. Carrying Amount of Goodwill and Intangibles Goodwill Mortgage Total (dollars in thousands) Balance at December 31, 2015 $ 2,714 $ 1,305 $ 4,019 Additions — 708 708 Amortization Expense — (384 ) (384 ) Balance at December 31, 2016 $ 2,714 $ 1,629 $ 4,343 Additions — 276 276 Amortization Expense — (380 ) (380 ) Balance at December 31, 2017 $ 2,714 $ 1,525 $ 4,239

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
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 G-7 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 corporate bonds and loans, mortgage whole loans, municipal bonds and OTC derivatives. Level III — These instruments have little to no pricing observability as of the reported date. These financial instruments do not have two-way non-investment The results of the fair value hierarchy as of December 31, 2017, are as follows: Fair Value Measurements Using Carrying Quoted Prices Significant Significant (dollars in thousands) Financial Instruments Measured at Fair Value on a Recurring Basis — Securities AFS U.S. Treasury $ 1,984 $ — $ 1,984 $ — U.S. Government Agency Sponsored Enterprises — — — — SBA Backed Securities 80,950 — 80,950 — U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 225,775 — 225,775 — Privately Issued Residential Mortgage-Backed Securities 892 — 892 — Obligations Issued by States and Political Subdivisions 82,600 — — 82,600 Other Debt Securities 4,971 — 4,971 — Equity Securities 303 303 — — Total $ 397,475 $ 303 $ 314,572 $ 82,600 Financial Instruments Measured at Fair Value on a Non-recurring Impaired Loans $ 246 $ — $ — $ 246 Impaired loan balances in the table above represent those collateral dependent loans where management has estimated the credit loss by comparing the loan’s carrying value against the expected realizable fair value of the collateral. Fair value is generally determined through a review process that includes independent appraisals, discounted cash flows, or other external assessments of the underlying collateral, which generally include various Level 3 inputs which are not identifiable. The Company discounts the fair values, as appropriate, based on management’s observations of the local real estate market for loans in this category. Appraisals, discounted cash flows and real estate tax assessments are reviewed quarterly. There is no specific policy regarding how frequently appraisals will be updated. Adjustments are made to appraisals and real estate tax assessments based on management’s estimate of changes in real estate values. Within the past twelve months there have been no updated appraisals, however, all impaired loans have been reviewed during the past quarter using either a discounted cash flow analysis or other type of real estate tax assessment. The types of adjustments that are made to specific provisions (credits) relate to impaired loans recognized for 2017 for the estimated credit loss amounted to $3,000. There were no transfers between level 1, 2 and 3 for the year ended December 31, 2017. There were no liabilities measured at fair value on a recurring or nonrecurring basis during the year ended December 31, 2017. 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, 2017. 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) $ 82,600 Discounted cash flow Discount rate 1.0%-3.5%(2) Impaired Loans 246 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. The changes in Level 3 securities for the year ended December 31, 2017 are as shown in the table below: Auction Rate Obligations Equity Total (dollars in thousands) Balance at December 31, 2016 $ 4,298 $ 160,578 $ — $ 164,876 Purchases — 99,136 — 99,136 Maturities/redemptions — (181,394 ) — (181,394 ) Amortization — (179 ) — (179 ) Change in fair value 161 — — 161 Balance at December 31, 2017 $ 4,459 $ 78,141 $ — $ 82,600 The amortized cost of Level 3 securities was $82,849,000 with an unrealized loss of $249,000 at December 31, 2017. The securities in this category are generally equity investments, 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. The results of the fair value hierarchy as of December 31, 2016, are as follows: Fair Value Measurements Using Carrying Quoted Prices Significant Significant (Level 3) (dollars in thousands) Financial Instruments Measured at Fair Value on a Recurring Basis — Securities AFS U.S. Treasury $ 2,000 $ — $ 2,000 $ — U.S. Government Agency Sponsored Enterprises 24,952 — 24,952 — BA Backed Securities 57,767 — 57,767 — U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 243,325 — 243,325 — Privately Issued Residential Mortgage-Backed Securities 1,109 — 1,109 — Obligations Issued by States and Political Subdivisions 164,876 — — 164,876 Other Debt Securities 4,924 — 4,924 — Equity Securities 344 344 — — Total $ 499,297 $ 344 $ 334,077 $ 164,876 Financial Instruments Measured at Fair Value on a Non-recurring Impaired Loans $ 260 $ — $ — $ 260 Appraisals, discounted cash flows and real estate tax assessments are reviewed quarterly. There is no specific policy regarding how frequently appraisals will be updated. Adjustments are made to appraisals and real estate tax assessments based on management’s estimate of changes in real estate values. Within the past twelve months there have been no updated appraisals, however, all impaired loans have been reviewed during the past quarter using either a discounted cash flow analysis or other type of real estate tax assessment. The types of adjustments that are made to specific provisions (credits) relate to impaired loans recognized for 2016 for the estimated credit loss amounted to ($135,000). There were no transfers between level 1 and 2 for the year ended December 31, 2016. There were no liabilities measured at fair value on a recurring or nonrecurring basis during the year ended December 31, 2016. 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, 2016. 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) $ 164,876 Discounted cash flow Discount rate 0%-1% (2) Impaired Loans 260 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. The changes in Level 3 securities for the year ended December 31, 2016 are as shown in the table below: Auction Rate Obligations Equity Total (dollars in thousands) Balance at December 31, 2015 $ 3,820 $ 153,140 $ 37 $ 156,997 Purchases — 216,646 — 216,646 Maturities/redemptions — (208,990 ) (37 ) (209,027 ) Amortization — (218 ) — (218 ) Change in fair value 478 — — 478 Balance at December 31, 2016 $ 4,298 $ 160,578 $ — $ 164,876 The amortized cost of Level 3 securities was $165,281,000 with an unrealized loss of $405,000 at December 31, 2016. The securities in this category are generally equity investments, 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, 2017
Banking and Thrift [Abstract]
Deposits	10. Deposits The following is a summary of remaining maturities or re-pricing 2017 Percent 2016 Percent (dollars in thousands) Within one year $ 436,911 70 % $ 262,406 55 % Over one year to two years 121,802 19 % 87,952 18 % Over two years to three years 30,098 5 % 83,067 17 % Over three years to five years 36,550 6 % 44,934 10 % Total $ 625,361 100 % $ 478,359 100 % Time deposits of more than $250,000 totaled $345,183,000 and $250,476,000 in 2017 and 2016, respectively. The increase was mainly attributable to competitive market rates for these types of deposits. Deposits totaling $35,486,000 and $26,191,000 were attributable to related parties at December 31, 2017 and December 31, 2016, respectively.

Securities Sold Under Agreement

Securities Sold Under Agreements to Repurchase	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 (dollars in thousands) Amount outstanding at December 31 $ 158,990 $ 182,280 $ 197,850 Weighted average rate at December 31 0.32 % 0.21 % 0.21 % Maximum amount outstanding at any month end $ 228,848 $ 241,110 $ 299,890 Daily average balance outstanding during the year $ 189,684 $ 222,956 $ 245,276 Weighted average rate during the year 0.26 % 0.21 % 0.20 % Amounts outstanding at December 31, 2017, 2016 and 2015 carried maturity dates of the next business day. U.S. Government Sponsored Enterprise securities with a total amortized cost of $162,927,000, $183,829,000, and $199,152,000 were pledged as collateral and held by custodians to secure the agreements at December 31, 2017, 2016 and 2015, respectively. The approximate fair value of the collateral at those dates was $ 159,051,000, $182,074,000, and $197,318,000, respectively.

Other Borrowed Funds and Subord

Other Borrowed Funds and Subordinated Debentures	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [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, 2017 2016 2015 (dollars in thousands) Amount outstanding at December 31 $ 383,861 $ 329,083 $ 404,083 Weighted average rate at December 31 2.26 % 2.39 % 2.29 % Maximum amount outstanding at any month end $ 491,583 $ 467,083 $ 521,583 Daily average balance outstanding during the year $ 309,102 $ 357,974 $ 374,109 Weighted average rate during the year 2.42 % 2.48 % 2.38 % 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, 2017, was approximately $127,631,000. 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: 2017 2016 2015 December 31, Amount Weighted Amount Weighted Amount Weighted (dollars in thousands) Within one year $ 164,500 1.82 % $ 77,500 2.21 % $ 100,000 1.89 % Over one year to two years $ 63,000 2.17 % $ 54,500 2.25 % $ 57,500 2.72 % Over two years to three years $ 28,000 2.29 % $ 58,000 1.87 % $ 54,500 2.25 % Over three years to five years $ 28,500 3.19 % $ 58,000 2.68 % $ 91,000 1.85 % Over five years $ 63,778 2.38 % $ 45,000 2.85 % $ 65,000 3.23 % Total $ 347,778 2.13 % $ 293,000 2.34 % $ 368,000 2.30 % Included in the table above are $20,000,000, $45,000,000 and $55,000,000, respectively, of FHLBB advances at December 31, 2017, 2016 and 2015, that are putable at the discretion of FHLBB. These put dates were not utilized in the table above. SUBORDINATED DEBENTURES Subordinated debentures totaled $36,083,000 at December 31, 2017 and 2016. In May 1998, the Company consummated the sale of a trust preferred securities offering, in which it issued $29,639,000 of subordinated debt securities due 2029 to its newly formed unconsolidated subsidiary Century Bancorp Capital trust. Century Bancorp Capital Trust the issued 2,875,000 shares of Cumulative Trust Preferred with a liquidation value of $10 per share. These securities pay dividends at an annualized rate of 8.30% . The Company redeemed through its subsidiary, Century Bancorp Capital Trust, its 8.30% Trust Preferred Securities, January 10, 2005. 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 3.46% at December 31, 2017 and 2.83% at December 31, 2016. OTHER BORROWED FUNDS There were no overnight federal funds purchased at December 31, 2017 and 2016.

Reclassifications Out of Accumu

Reclassifications Out of Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2017
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 Comprehensive Income Components Year ended (a) Year ended (a) Affected line item in the Statement Unrealized gains and losses on available-for-sale $ 47 $ 52 Net gains on sales of investments (19 ) (20 ) Provision for income taxes $ 28 $ 32 Net income Accretion of unrealized losses transferred $ (2,292 ) $ (4,317 ) Securities held-to-maturity 1,258 1,505 Provision for income taxes $ (1,034 ) $ (2,812 ) Net income Amortization of defined benefit pension items Prior-service costs $ (10 ) $ (10 ) Salaries and employee benefits (b) Actuarial gains (losses) (1,540 ) (1,606 ) Salaries and employee benefits (b) Total before tax (1,550 ) (1,616 ) Income before taxes Tax (expense) or benefit 619 646 Provision for income taxes Net of tax $ (931 ) $ (970 ) 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, 2017
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 2017, 2016 and 2015, respectively. The following table is a reconciliation of basic EPS and diluted EPS: Year Ended December 31, 2017 2016 2015 (in thousands except share and per share data) BASIC EPS COMPUTATION Numerator: Net income, Class A $ 17,526 $ 19,270 $ 18,081 Net income, Class B 4,775 5,264 4,940 Denominator: Weighted average shares outstanding, Class A 3,604,029 3,600,729 3,600,729 Weighted average shares outstanding, Class B 1,963,880 1,967,180 1,967,180 Basic EPS, Class A $ 4.86 $ 5.35 $ 5.02 Basic EPS, Class B $ 2.43 $ 2.68 $ 2.51 DILUTED EPS COMPUTATION Numerator: Net income, Class A $ 17,526 $ 19,270 $ 18,081 Net income, Class B 4,775 5,264 4,940 Total net income, for diluted EPS, Class A computation 22,301 24,534 23,021 Denominator: Weighted average shares outstanding, basic, Class A 3,604,029 3,600,729 3,600,729 Weighted average shares outstanding, Class B 1,963,880 1,967,180 1,967,180 Weighted average shares outstanding diluted, Class A 5,567,909 5,567,909 5,567,909 Weighted average shares outstanding, Class B 1,963,880 1,967,180 1,967,180 Diluted EPS, Class A $ 4.01 $ 4.41 $ 4.13 Diluted EPS, Class B $ 2.43 $ 2.68 $ 2.51

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2017
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. STOCK OPTION PLAN During 2000 and 2004, common stockholders of the Company approved stock option plans (the “Option Plans”) that provide for granting of options for not more than 150,000 shares of Class A common stock per plan. Under the Option Plans, all officers and key employees of the Company are eligible to receive nonqualified and incentive stock options to purchase shares of Class A common stock. The Option Plans are administered by the Compensation Committee of the Board of Directors, whose members are ineligible to participate in the Option Plans. Based on management’s recommendations, the Committee submits its recommendations to the Board of Directors as to persons to whom options are to be granted, the number of shares granted to each, the option price (which may not be less than 85% of the fair market value for nonqualified stock options, or the fair market value for incentive stock options, of the shares on the date of grant) and the time period over which the options are exercisable (not more than ten years from the date of grant). There were no options outstanding at December 31, 2017 and December 31, 2016. 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, 2017, 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 Amount Ratio Amount Ratio Amount Ratio As of December 31, 2017 (Basel III) Total Capital (to Risk-Weighted Assets) $ 329,666 12.70 % $ 207,707 8.00 % $ 259,633 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 303,411 11.69 % 155,780 6.00 % 207,707 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 303,411 11.69 % 116,835 4.50 % 168,762 6.50 % Tier 1 Capital (to 4th Qtr. Average Assets) 303,411 6.55 % 185,199 4.00 % 231,499 5.00 % As of December 31, 2016 (Basel III) Total Capital (to Risk-Weighted Assets) $ 293,143 12.27 % $ 191,081 8.00 % $ 238,851 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 268,737 11.25 % 143,311 6.00 % 191,081 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 268,737 11.25 % 107,483 4.50 % 155,253 6.50 % Tier 1 Capital (to 4th Qtr. Average Assets) 268,737 6.02 % 178,469 4.00 % 223,086 5.00 % The Company’s actual capital amounts and ratios are presented in the following table: Actual For Capital To Be Well Amount Ratio Amount Ratio Amount Ratio As of December 31, 2017 (Basel III) Total Capital (to Risk-Weighted Assets) $ 341,033 13.05 % $ 209,049 8.00 % $ 261,312 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 314,778 12.05 % 156,787 6.00 % 209,049 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 279,778 10.71 % 117,590 4.50 % 169,853 6.50 % Tier 1 Capital (to 4th Qtr. Average Assets) 314,778 6.78 % 185,657 4.00 % 232,072 5.00 % As of December 31, 2016 (Basel III) Total Capital (to Risk-Weighted Assets) $ 305,065 12.72 % $ 191,904 8.00 % $ 239,880 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 280,659 11.70 % 143,928 6.00 % 191,904 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 249,753 10.41 % 107,946 4.50 % 155,922 6.50 % Tier 1 Capital (to 4th Qtr. Average Assets) 280,659 6.28 % 178,903 4.00 % 223,628 5.00 %

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	16. Income Taxes The current and deferred components of income tax (benefit) expense for the years ended December 31, are as follows: 2017 2016 2015 (dollars in thousands) Current expense: Federal $ 3,628 $ 3,875 $ 3,393 State 412 439 399 Total current expense 4,040 4,314 3,792 Deferred (benefit) expense: Federal 6,496 (4,450 ) (3,098 ) State 422 (334 ) (161 ) Valuation Allowance — 108 — Total deferred expense (benefit) 6,918 (4,676 ) (3,259 ) Provision for income taxes $ 10,958 $ (362 ) $ 533 Income tax accounts included in other assets at December 31, are as follows: 2017 2016 (dollars in thousands) Currently receivable $ 15,940 $ 633 Deferred income tax asset, net 20,892 43,129 Total $ 36,832 $ 43,762 Differences between income tax (benefit) expense at the statutory federal income tax rate and total income tax expense are summarized as follows: 2017 2016 2015 (dollars in thousands) Federal income tax expense at statutory rates $ 11,308 $ 8,218 $ 8,008 State income tax, net of federal income tax benefit 550 69 157 Insurance income (371 ) (406 ) (375 ) Effect of tax-exempt (8,683 ) (8,259 ) (6,915 ) Net tax credit (341 ) (395 ) (460 ) Valuation allowance — 108 — Deferred tax remeasurement 8,448 — — Other 47 303 118 Total $ 10,958 $ (362 ) $ 533 Effective tax rate 32.95 % (1.50 )% 2.30 % The following table sets forth the Company’s gross deferred income tax assets and gross deferred income tax liabilities at December 31: 2017 2016 (dollars in thousands) Deferred income tax assets: Allowance for loan losses $ 7,855 $ 10,419 AMT credit — 10,234 Deferred compensation 7,555 9,684 Pension and SERP liability 8,436 11,320 Unrealized losses on securities transferred to held-to-maturity 1,303 3,161 Depreciation 631 968 Accrued bonus — 612 Unrealized (gains) losses on securities available-for-sale 14 357 Charitable contributions carryforward 442 266 Acquisition premium 17 128 Nonaccrual interest 97 125 Limited partnerships 21 30 Investments write down 17 26 Other 173 220 Gross deferred income tax asset 26,561 47,550 Valuation allowance (108 ) (108 ) Gross deferred income tax asset, net of valuation allowance 26,453 47,442 Deferred income tax liabilities: Pension asset (liability) (4,403 ) (3,662 ) Deferred origination costs (481 ) — Prepaid expenses (248 ) — Mortgage servicing rights (429 ) (651 ) Gross deferred income tax liability (5,561 ) (4,313 ) Deferred income tax asset, net $ 20,892 $ 43,129 Based on the Company’s historical and current pre-tax 3-4 On December 22, 2017, the Tax Cuts and Jobs Act (the “Tax Act”) was enacted. The majority of the provisions of the Tax Act takes effect on January 1, 2018. The Tax Act lowers the Company’s federal tax rate from 34% to 21%. The Company evaluated its deferred taxes at 21% as of the enactment date and recorded additional tax expense of $8,448,000. 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. Accordingly, the full amount of the alternative minimum tax credit carryforward will be recovered in tax years beginning before 2022. 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. The Company is in an Alternative Minimum Tax (“AMT”) credit position. As the AMT has been repealed and the existing credit is refundable, the AMT credit, totalling $14,001,000, has been reclassified to currently receivable. The Company and its subsidiaries file a consolidated federal tax return. The Company is subject to federal and state examinations for tax years after December 31, 2013.

Employee Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2017
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 40% to 64% of total portfolio assets. The remainder of the portfolio is allocated to fixed income securities with target range of 15% to 25% and other investments including global asset allocation and hedge funds from 25% to 41%. 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 2018 to 2022 are $1,530,000, $1,569,000, $1,732,000, $1,832,000, and $1,988,000, respectively. The aggregate benefits expected to be paid in the five years from 2023 to 2027 are $11,531,000. 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, 2017, is as follows: Description Percent Level 1 Level 2 Level 3 Total (dollars in thousands) Collective Funds 3.6 % $ 1,741 $ — $ — $ 1,741 Equity Securities 10.7 % 5,195 — — 5,195 Diversified Mutual Funds 17.8 % 8,615 — — 8,615 Short-term investments 7.9 % 3,836 — — 3,836 Total investments measured in the fair value hierarchy 40.0 % 19,387 — — 19,387 Investments measured at net asset value(1) 60.0 % — — — 29,035 100.0 % $ 19,387 $ — $ — $ 48,422 (1) In accordance with Subtopic 820-10, The fair value of plan assets and major categories as of December 31, 2016, is as follows: Description Percent Level 1 Level 2 Level 3 Total (dollars in thousands) Collective Funds 6.9 % $ 2,600 $ — $ — $ 2,600 Equity Securities 19.7 % 7,363 — — 7,363 Diversified Mutual Funds 12.3 % 4,615 — — 4,615 Short-term investments 1.3 % 475 — — 475 Total investments measured in the fair value hierarchy 40.2 % 15,053 — — 15,053 Investments measured at net asset value(1) 59.8 % — — — 22,394 100.0 % $ 15,053 $ — $ — $ 37,447 (1) In accordance with Subtopic 820-10, There were no transfers to or from Level 1, 2, and 3 during the period. 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, 2017, is as follows: Percent Fair (dollars in thousands) Collective Funds by Category: Equity 31.6 % $ 15,304 Diversified 0.7 % 344 US debt securities 9.4 % 4,569 International equities 9.1 % 4,419 Limited Partnerships by Category: Emerging markets 2.8 % 1,353 Multi-strategy 1.5 % 705 Hedge Funds by Category: Multi-strategy(1) 3.5 % 1,674 Global opportunities(2) 0.7 % 345 Private investment entities and/or separately managed accounts (3) 0.7 % 322 60.0 % $ 29,035 The investments measured using the net asset value per share practical expedient as of December 31, 2016, is as follows: Percent Fair Value (dollars in thousands) Collective Funds by Category: Equity 24.1 % $ 9,013 Diversified 0.1 % 47 US debt securities 11.3 % 4,241 International equities 9.8 % 3,684 Limited Partnerships by Category: Emerging markets 0.0 % — Multi-strategy 7.0 % 2,623 Hedge Funds by Category: Multi-strategy(1) 5.6 % 2,082 Global opportunities(2) 1.1 % 422 Private investment entities and/or separately managed accounts(3) 0.8 % 282 59.8 % $ 22,394 (1) This category includes investments in hedge funds that pursue multiple strategies to diversify risks and reduce volatility. Fund objectives are to seek above-average rates of return and long-term capital growth through investments, which are fund of funds with a diversified portfolio of private investment entities and/or separately managed accounts managed by investment managers or achieve superior risk-adjusted capital appreciation over the long-term, generally through an investment, which invests in private investment funds and discretional managed accounts, structured notes, swaps or other similar products. The fair values of the investments in this category have been determined using the net asset value per share of the fund(s). (2) This category has an investment strategy to pursue a hybrid absolute return via portfolio managers, secondaries, and co-investments (3) 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. ASSET ALLOCATION 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 40% to 64% of total portfolio assets. The remainder of the portfolio is allocated to fixed income securities with a target range of 15% to 25% and other investments including global asset allocation and hedge funds from 25% to 41%. The Trustees of SBERA, through the Association’s 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 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 2018 to 2022 are $2,096,000, $2,068,000, $2,002,000, $1,939,000 and $1,966,000, respectively. The aggregate benefits expected to be paid in the five years from 2023 to 2027 are $13,107,000. Defined Benefit Supplemental Insurance/ 2017 2016 2017 2016 (dollars in thousands) Change projected in benefit obligation Benefit obligation at beginning of year $ 42,255 $ 38,597 $ 38,610 $ 38,204 Service cost 1,241 1,273 1,582 1,820 Interest cost 1,450 1,358 1,382 1,334 Actuarial (gain)/loss 3,456 2,593 2,087 (1,653 ) Benefits paid (1,337 ) (1,566 ) (1,082 ) (1,095 ) Projected benefit obligation at end of year $ 47,065 $ 42,255 $ 42,579 $ 38,610 Change in plan assets Fair value of plan assets at beginning of year $ 37,447 $ 33,717 Actual return on plan assets 5,312 3,221 Employer contributions 7,000 2,075 Benefits paid (1,337 ) (1,566 ) Fair value of plan assets at end of year $ 48,422 $ 37,447 (Unfunded) Funded status $ 1,357 $ (4,808 ) $ (42,579 ) $ (38,610 ) Accumulated benefit obligation $ 47,065 $ 42,255 $ 40,375 $ 36,392 Weighted-average assumptions as of December 31 Discount rate — Liability 3.49 % 3.99 % 3.42 % 3.85 % Discount rate — Expense 3.99 % 4.18 % 3.85 % 4.01 % Expected return on plan assets 8.00 % 8.00 % NA NA Rate of compensation increase 4.00 % 4.00 % 4.00 % 4.00 % Components of net periodic benefit cost Service cost $ 1,241 $ 1,273 $ 1,582 $ 1,820 Interest cost 1,450 1,358 1,382 1,334 Expected return on plan assets (2,985 ) (2,776 ) — — Recognized prior service cost (104 ) (104 ) 114 114 Recognized net losses 903 801 636 805 Net periodic cost (benefit) $ 505 $ 552 $ 3,714 $ 4,073 Other changes in plan assets and benefit obligations recognized in other comprehensive income Amortization of prior service cost $ 104 $ 104 $ (114 ) $ (114 ) Net (gain) loss 409 1,347 1,752 (2,458 ) Total recognized in other comprehensive income 513 1,451 1,638 (2,572 ) Total recognized in net periodic benefit cost and other comprehensive income $ 1,018 $ 2,003 $ 5,352 $ 1,501 December 31, 2017 December 31, 2016 Plan Supplemental Total Plan Supplemental Total (dollars in thousands) Prior service cost $ 100 $ (535 ) $ (435 ) $ 204 $ (649 ) $ (445 ) Net actuarial loss (14,408 ) (15,168 ) (29,576 ) (13,999 ) (13,416 ) (27,415 ) Total $ (14,308 ) $ (15,703 ) $ (30,011 ) $ (13,795 ) $ (14,065 ) $ (27,860 ) The following table summarizes the amounts included in Accumulated Other Comprehensive Loss at December 31, 2017, 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 2018 $ (100 ) $ 114 Amortization of loss to be recognized in 2018 904 707 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. Effective January 1, 2016, the Company changed its estimate of the service and interest components of the net periodic benefit cost. Previously, the Company estimated the service and interest cost components utilizing a single weighted-average discount rate derived from the yield curve used to measure the benefit obligation. The new estimate utilizes a full yield curve approach in the estimation of these components by applying the specific spot rates along the yield curve used in the determination of the benefit obligation to their underlying projected cash flows. The new estimate provided a more precise measurement of service and interests costs by improving the correlation between projected benefit cash flows and their corresponding spot rates. The change does not affect the measurement of the Company’s benefit obligations and it is accounted for as a change in accounting estimate, which is applied prospectively. For 2016, the change in estimate reduced periodic plan cost by $859,000 compared to the prior estimate. Mortality assumptions are based on the RP 2015 Mortality Table projected with Scale MP 2016. 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 $445,000 for 2017, $418,000 for 2016 and $403,000 for 2015. Administrative costs associated with the plan are absorbed by the Company. 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 $1,859,000, $1,418,000 and $1,178,000 in 2017, 2016, and 2015, respectively. The Company does not offer any postretirement programs other than pensions.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
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, 2017. 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. On September 7, 2017, Crimson Galeria Limited Partnership, Raj & Raj, LLC, Harvard Square Holdings LLC, and Charles River Holdings LLC (collectively, the “Plaintiffs”) filed suit in the United States District Court for the District of Massachusetts against the Attorney General of the Commonwealth of Massachusetts, the Massachusetts Department of Public Health, the City of Cambridge, the Town of Georgetown, as well as against the Bank, Healthy Pharms, Inc., (“Healthy Pharms”), Timbuktu Real Estate, LLC, Paul Overgaag, Nathaniel Averill, 4Front Advisors, LLC, 4Front Holdings LLC, Kristopher T. Krane, 3 Brothers Real Estate, LLC, Red Line Management, LLC, unspecified insurance providers to certain Plaintiffs, Tomolly, Inc., and (collectively, the “Defendants”). The Plaintiffs allege that they own property in Cambridge, MA, and claim that the value and use of their property will be impaired by Healthy Pharms decision to open a registered medicinal marijuana dispensary in abutting or nearby situated property. The Plaintiffs further allege that the Bank has a banking relationship with Healthy Pharms and that, by entering into such relationship, the Bank conspired with Healthy Pharms to violate the Racketeer Influenced and Corrupt Organizations Act, 18 U.S.C. § 1961 et seq. The Plaintiffs seek unspecified treble damages, and attorney’s costs and fees, as well as injunctive and declaratory relief. The Company believes that the claims and allegations against the Bank set forth in the complaint are without merit, and the Company and the Bank intend to vigorously defend against them. On December 15, 2017, the Company filed a motion to dismiss the complaint; the plaintiffs filed an opposition brief, and the Company filed a reply in further support of its motion.

Financial Instruments with Off-

Financial Instruments with Off-Balance-Sheet Risk	12 Months Ended
	Dec. 31, 2017
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-sheet These financial instruments primarily include commitments to originate and sell loans, standby letters of credit, unused lines of credit and unadvanced portions of construction loans. The instruments involve, to varying degrees, elements of credit and interest rate risk in excess of the amount recognized in the consolidated balance sheet. The contract or notional amounts of those instruments 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 nonperformance by the other party to the financial instrument for loan commitments, standby letters of credit and unadvanced portions of construction loans is represented by the contractual amount of those instruments. The Company uses the same credit policies in making commitments and conditional obligations as it does for on-balance-sheet off-balance-sheet Contract or Notional Amount 2017 2016 (dollars in thousands) Financial instruments whose contract amount represents credit risk: Commitments to originate 1–4 family mortgages $ 5,748 $ 13,877 Standby and commercial letters of credit 5,520 6,796 Unused lines of credit 434,618 362,357 Unadvanced portions of construction loans 15,152 22,049 Unadvanced portions of other loans 35,602 52,224 Commitments to originate loans, unadvanced portions of construction loans, unused lines of credit 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 creditworthiness on a case-by-case Standby letters of credit are conditional commitments issued by the Company to guarantee the performance by a customer to a third party. The credit risk involved in issuing letters of credit is essentially the same as that involved in extending loan facilities to customers.

Other Operating Expenses

Other Operating Expenses	12 Months Ended
	Dec. 31, 2017
Other Income and Expenses [Abstract]
Other Operating Expenses	20. Other Operating Expenses Year ended December 31, 2017 2016 2015 (dollars in thousands) Marketing $ 2,315 $ 2,185 $ 1,849 Software maintenance/amortization 1,859 1,863 1,670 Legal and audit 1,543 1,255 1,269 Contributions 993 789 690 Processing services 1,160 1,040 1,002 Consulting 1,199 1,168 1,050 Postage and delivery 966 987 905 Supplies 945 948 965 Telephone 1,020 1,032 804 Directors’ fees 440 413 377 Insurance 308 323 301 Other 1,845 1,812 1,826 Total $ 14,593 $ 13,815 $ 12,708

Fair Values of Financial Instru

Fair Values of Financial Instruments	12 Months Ended
	Dec. 31, 2017
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 For variable-rate loans, that reprice frequently and with no significant change in credit risk, fair values are based on carrying amounts. The fair value of other loans is estimated using discounted cash flow analysis, based on interest rates currently being offered for loans with similar terms to borrowers of similar credit quality. Incremental credit risk for nonperforming loans has been considered. 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. 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, 2017 and December 31, 2016. 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, short-term investments, FHLBB stock and accrued interest receivable. Financial liabilities for which the fair value approximates carrying value include non-maturity Carrying Amount Estimated Level 1 Inputs Fair Value Level 3 Inputs (dollars in thousands) December 31, 2017 Financial assets: Securities held-to-maturity $ 1,701,233 $ 1,668,827 $ — $ 1,668,827 $ — Loans(1) 2,149,689 2,094,517 — — 2,094,517 Financial liabilities: Time deposits 625,361 627,517 — 627,517 — Other borrowed funds 347,778 349,364 — 349,364 — Subordinated debentures 36,083 36,083 — — 36,083 December 31, 2016 Financial assets: Securities held-to-maturity $ 1,653,986 $ 1,635,808 $ — $ 1,635,808 $ — Loans(1) 1,899,527 1,873,703 — — 1,873,703 Financial liabilities: Time deposits 478,359 480,133 — 480,133 — Other borrowed funds 293,000 294,940 — 294,940 — 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 future expected loss experience, cash flows, current economic conditions, risk characteristics and other factors. These estimates are subjective in nature and involve uncertainties and matters of significant judgment and therefore cannot be determined with precision. Changes in assumptions and changes in the loan, debt and interest rate markets could significantly affect the estimates. Further, the income tax ramifications related to the realization of the unrealized gains and losses can have a significant effect on the fair value estimates and have not been considered.

Quarterly Results of Operations

Quarterly Results of Operations (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results of Operations (Unaudited)	22. Quarterly Results of Operations (unaudited) 2017 Quarters Fourth Third Second First (in thousands, except share data) Interest income $ 29,470 $ 28,521 $ 28,806 $ 26,639 Interest expense 7,768 7,168 6,701 6,183 Net interest income 21,702 21,353 22,105 20,456 Provision for loan losses 450 450 490 400 Net interest income after provision for loan losses 21,252 20,903 21,615 20,056 Other operating income 4,410 3,942 4,291 3,909 Operating expenses 15,992 16,205 17,197 17,725 Income before income taxes 9,670 8,640 8,709 6,240 Provision for income taxes 9,645 617 552 144 Net income $ 25 $ 8,023 $ 8,157 $ 6,096 Share data: Average shares outstanding, basic Class A 3,605,829 3,605,829 3,603,729 3,600,729 Class B 1,962,080 1,962,080 1,964,180 1,967,180 Average shares outstanding, diluted Class A 5,567,909 5,567,909 5,567,909 5,567,909 Class B 1,962,080 1,962,080 1,964,180 1,967,180 Earnings per share, basic Class A $ 0.01 $ 1.75 $ 1.78 $ 1.33 Class B $ — $ 0.87 $ 0.89 $ 0.66 Earnings per share, diluted Class A $ — $ 1.44 $ 1.47 $ 1.09 Class B $ — $ 0.87 $ 0.89 $ 0.66 2016 Quarters Fourth Third Second First (in thousands, except share data) Interest income $ 24,689 $ 25,005 $ 23,742 $ 23,263 Interest expense 5,927 5,791 5,486 5,413 Net interest income 18,762 19,214 18,256 17,850 Provision for loan losses 200 375 350 450 Net interest income after provision for loan losses 18,562 18,839 17,906 17,400 Other operating income 3,700 4,225 4,643 3,654 Operating expenses 16,156 16,630 16,288 15,683 Income before income taxes 6,106 6,434 6,261 5,371 Provision for income taxes (394 ) (52 ) 20 64 Net income $ 6,500 $ 6,486 $ 6,241 $ 5,307 Share data: Average shares outstanding, basic Class A 3,600,729 3,600,729 3,600,729 3,600,729 Class B 1,967,180 1,967,180 1,967,180 1,967,180 Average shares outstanding, diluted Class A 5,567,909 5,567,909 5,567,909 5,567,909 Class B 1,967,180 1,967,180 1,967,180 1,967,180 Earnings per share, basic Class A $ 1.42 $ 1.41 $ 1.36 $ 1.16 Class B $ 0.71 $ 0.71 $ 0.68 $ 0.58 Earnings per share, diluted Class A $ 1.17 $ 1.16 $ 1.12 $ 0.95 Class B $ 0.71 $ 0.71 $ 0.68 $ 0.58

Parent Company Financial Statem

Parent Company Financial Statements	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Parent Company Financial Statements	23. Parent Company Financial Statements The balance sheets of Century Bancorp, Inc. (“Parent Company”) as of December 31, 2017 and 2016 and the statements of income and cash flows for each of the years in the three-year period ended December 31, 2017, 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, 2017 2016 (dollars in thousands) ASSETS: Cash $ 1,981 $ 2,768 Investment in subsidiary, at equity 283,881 263,070 Other assets 16,833 10,335 Total assets $ 302,695 $ 276,173 LIABILITIES AND STOCKHOLDERS’ EQUITY: Liabilities $ 6,315 $ 49 Subordinated debentures 36,083 36,083 Stockholders’ equity 260,297 240,041 Total liabilities and stockholders’ equity $ 302,695 $ 276,173 STATEMENTS OF INCOME Year Ended December 31, 2017 2016 2015 (dollars in thousands) Income: Dividends from subsidiary $ 2,500 $ 2,000 $ 1,500 Interest income from deposits in bank 1 3 13 Other income 34 28 24 Total income 2,535 2,031 1,537 Interest expense 1,121 937 792 Operating expenses 209 220 212 Income before income taxes and equity in undistributed income of subsidiary 1,205 874 533 Benefit from income taxes (440 ) (383 ) (328 ) Income before equity in undistributed income of subsidiary 1,645 1,257 861 Equity in undistributed income of subsidiary 20,656 23,277 22,160 Net income $ 22,301 $ 24,534 $ 23,021 STATEMENTS OF CASH FLOWS December 31, 2017 2016 2015 (dollars in thousands) CASH FLOWS FROM OPERATING ACTIVITIES: Net income $ 22,301 $ 24,534 $ 23,021 Adjustments to reconcile net income to net cash provided by operating activities Undistributed income of subsidiary (20,656 ) (23,277 ) (22,160 ) Depreciation and amortization — — 3 Increase in other assets (6,498 ) (1,527 ) (1,112 ) Decrease in liabilities 6,266 9 4 Net cash (used in) operating activities 1,413 (261 ) (244 ) CASH FLOWS FROM FINANCING ACTIVITIES: Net proceeds from the exercise of stock options — — — Cash dividends paid (2,200 ) (2,201 ) (2,200 ) Net cash used in financing activities (2,200 ) (2,201 ) (2,200 ) Net (decrease) in cash (787 ) (2,462 ) (2,444 ) Cash at beginning of year 2,768 5,230 7,674 Cash at end of year $ 1,981 $ 2,768 $ 5,230

Summary of Significant Accoun32

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
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 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 G-7 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 corporate bonds and loans, mortgage whole loans, municipal bonds and over the counter (“OTC”) derivatives. 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.
SHORT-TERM INVESTMENTS	SHORT-TERM INVESTMENTS As of December 31, 2017 and 2016, short-term investments include highly liquid certificates of deposit with original maturities of more than 90 days but less than one year.
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. 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, 2017, 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 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 and loans that are measured at fair value. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off 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 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, 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. There were no new loans acquired during the year ended December 31, 2017.
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. 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	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 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 two-step The first step, in the two-step
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.
STOCK OPTION ACCOUNTING	STOCK OPTION ACCOUNTING The Company follows the fair value recognition provisions of FASB ASC 718, Compensation — Stock Compensation During 2000 and 2004, common stockholders of the Company approved stock option plans (the “Option Plans”) that provide for granting of options to purchase up to 150,000 shares of Class A common stock per plan. Under the Option Plans, all officers and key employees of the Company are eligible to receive nonqualified or incentive stock options to purchase shares of Class A common stock. The Option Plans are administered by the Compensation Committee of the Board of Directors, whose members are ineligible to participate in the Option Plans. Based on management’s recommendations, the Committee submits its recommendations to the Board of Directors as to persons to whom options are to be granted, the number of shares granted to each, the option price (which may not be less than 85% of the fair market value for nonqualified stock options, or the fair market value for incentive stock options, of the shares on the date of grant) and the time period over which the options are exercisable (not more than ten years from the date of grant). There were no options to purchase shares of Class A common stock outstanding at December 31, 2017. The Company uses the fair value method to account for stock options. There were no options granted during 2017 and 2016.
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. 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. Accordingly, the full amount of the AMT credit carryforward will be recovered in tax years beginning before 2022. As a result of the change, the Company has classified its AMT credit carryforward as currently receivable.
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 only common stock equivalents for the Company are stock options. The company utilizes the two class method for reporting EPS. The two-class
TREASURY STOCK	TREASURY STOCK Effective July 1, 2004, companies incorporated in Massachusetts became subject to Chapter 156D of the Massachusetts Business Corporation Act, provisions of which eliminate the concept of treasury stock and provide that shares reacquired by a company are to be treated as authorized but unissued shares.
PENSION	PENSION The Company provides pension benefits to 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. The Company utilizes 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.
RECENT ACCOUNTING DEVELOPMENTS	RECENT ACCOUNTING DEVELOPMENTS In February 2018, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2018-02, Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. In July 2017, FASB issued ASU 2017-11, 2017-11 In May 2017, the FASB issued ASU 2017-09, In March 2017, the FASB issued ASU 2017-08, 310-20) In March 2017, the FASB issued ASU 2017-07, In February 2017, the FASB issued ASU 2017-05, 610-20). 610-20, In January 2017, the FASB issued ASU 2017-04, In June 2016, the FASB issued ASU 2016-13, In May 2016, the FASB issued ASU 2016-12, 2015-14 2014-09 2014-09, 2014-09, In July 2015, the FASB approved the deferral of the new standard’s effective date by one year. The new standard is effective for annual reporting periods beginning after December 15, 2017. The FASB will permit companies to adopt the new standard early, but not before the original effective date of annual reporting periods beginning after December 15, 2016. We monitored FASB activity related to the new standard. A significant amount of the Company’s revenues are derived from interest income on financial assets, which are excluded from the scope of the amended guidance. In 2017, we established a cross-functional implementation team consisting of representatives from across our business segments. We utilized a bottom-up During 2017, we completed our evaluation of the potential changes from adopting the new standard on our future financial reporting and disclosures. In the third quarter of 2017, we finalized our contract reviews. The Company did not identify any significant changes in the timing of revenue recognition when considering the amended accounting guidance. In February 2016, the FASB issued ASU 2016-02, In November 2016, the FASB issued ASU 2016-18, Restricted Cash. beginning-of-period end-of-period In August 2016, the FASB issued ASU 2016-15, Classification of Certain Cash Receipts and Cash Payments. In January 2016, FASB issued ASU 2016-1, 825-10) Recognition and Measurement of Financial Assets and Financial Liabilities.

Securities Available-for-Sale (

Securities Available-for-Sale (Tables)	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Summary of Securities Available-for-Sale	December 31, 2017 December 31, 2016 Amortized Gross Gross Estimated Amortized Gross Gross Estimated (dollars in thousands) U.S. Treasury $ 1,999 $ — $ 15 $ 1,984 $ 2,000 $ — $ — $ 2,000 U.S. Government Sponsored Enterprises — — — — 25,000 — 48 24,952 SBA Backed Securities 81,065 46 161 80,950 57,899 14 146 57,767 U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 225,537 555 317 225,775 243,703 293 671 243,325 Privately Issued Residential Mortgage-Backed Securities 897 4 9 892 1,121 2 14 1,109 Obligations Issued by States and Political Subdivisions 82,849 — 249 82,600 165,281 — 405 164,876 Other Debt Securities 5,100 68 197 4,971 5,100 18 194 4,924 Equity Securities 116 187 — 303 116 228 — 344 Total $ 397,563 $ 860 $ 948 $ 397,475 $ 500,220 $ 555 $ 1,478 $ 499,297
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 $ 78,343 $ 78,338 After one but within five years 104,041 104,123 After five but within ten years 144,184 144,307 More than ten years 69,379 69,062 Nonmaturing 1,616 1,645 Total $ 397,563 $ 397,475
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 December 31, 2017 Less Than 12 Months 12 Months or Longer Total Temporarily Impaired Investments Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) U.S. Treasury $ 1,984 $ 15 $ — $ — $ 1,984 $ 15 U.S. Government Sponsored Enterprises — — — — — — SBA Backed Securities 18,378 54 40,911 107 59,289 161 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 40,394 123 59,336 194 99,730 317 Privately Issued Residential Mortgage-Backed Securities — — 633 9 633 9 Obligations Issued by States and Political Subdivisions — — 4,458 249 4,458 249 Other Debt Securities 400 1 1,803 196 2,203 197 Total temporarily impaired securities $ 61,156 $ 193 $ 107,141 $ 755 $ 168,297 $ 948 The following table shows the temporarily impaired securities of the Company’s available-for-sale December 31, 2016 Less Than 12 Months 12 Months or Longer Total Temporarily Impaired Investments Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) U.S. Treasury $ — $ — $ — $ — $ — $ — U.S. Government Sponsored Enterprises 24,952 48 — — 24,952 48 SBA Backed Securities 52,346 145 951 1 53,297 146 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 135,612 485 31,504 186 167,116 671 Privately Issued Residential Mortgage-Backed Securities — — 757 14 757 14 Obligations Issued by States and Political Subdivisions — — 4,298 405 4,298 405 Other Debt Securities 453 47 1,553 147 2,006 194 Total temporarily impaired securities $ 213,363 $ 725 $ 39,063 $ 753 $ 252,426 $ 1,478

Investment Securities Held-to34

Investment Securities Held-to-Maturity (Tables)	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Summary of Held-to-Maturity Securities	December 31, 2017 December 31, 2016 Amortized Gross Gross Estimated Amortized Gross Gross Estimated (dollars in U.S. Government Sponsored Enterprises $ 104,653 $ 341 $ 472 $ 104,522 $ 148,326 $ 1,066 $ 527 $ 148,865 SBA Backed Securities 57,235 20 1,271 55,984 46,140 — 1,088 45,052 U.S. Government Sponsored Enterprises Mortgage-Backed Securities 1,539,345 2,261 33,285 1,508,321 1,459,520 4,948 22,577 1,441,891 Total $ 1,701,233 $ 2,622 $ 35,028 $ 1,668,827 $ 1,653,986 $ 6,014 $ 24,192 $ 1,635,808
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 $ 28,752 $ 28,726 After one but within five years 1,257,279 1,234,931 After five but within ten years 411,916 401,947 More than ten years 3,286 3,223 Total $ 1,701,233 $ 1,668,827
Unrealized Market Loss of Securities	The following table shows the temporarily impaired securities of the Company’s held-to-maturity December 31, 2017 Less Than 12 Months 12 Months or Longer Total Temporarily Impaired Investments Fair Value Unrealized Fair Value Unrealized Fair Value Unrealized (dollars in thousands) U.S. Government Sponsored Enterprises $ 15,257 $ 239 $ 14,768 $ 233 $ 30,025 $ 472 SBA Backed Securities 19,457 142 33,750 1,129 53,207 1,271 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 519,481 5,920 814,712 27,365 1,334,193 33,285 Total temporarily impaired securities $ 554,195 $ 6,301 $ 863,230 $ 28,727 $ 1,417,425 $ 35,028 The following table shows the temporarily impaired securities of the Company’s held-to-maturity December 31, 2016 Less Than 12 Months 12 Months or Longer Total Temporarily Impaired Investments Fair Value Unrealized Fair Unrealized Fair Value Unrealized (dollars in thousands) U.S. Government Sponsored Enterprises $ 59,219 $ 527 $ — $ — $ 59,219 $ 527 SBA Backed Securities 45,052 1,088 — — 45,052 1,088 U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities 1,008,960 20,725 58,535 1,852 1,067,495 22,577 Total temporarily impaired securities $ 1,113,231 $ 22,340 $ 58,535 $ 1,852 $ 1,171,766 $ 24,192

Loans (Tables)

Loans (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 (dollars in thousands) Construction and land development $ 18,931 $ 14,928 Commercial and industrial 763,807 612,503 Municipal 106,599 135,418 Commercial real estate 732,491 696,173 Residential real estate 287,731 241,357 Consumer 18,458 11,013 Home equity 247,345 211,857 Overdrafts 582 684 Total $ 2,175,944 $ 1,923,933
Composition of Nonaccrual Loans and Impaired Loans	The composition of nonaccrual loans and impaired loans is as follows: December 31, 2017 2016 2015 (dollars in thousands) Loans on nonaccrual $ 1,684 $ 1,084 $ 2,336 Loans 90 days past due and still accruing — — — Impaired loans on nonaccrual included above 254 304 332 Total recorded investment in impaired loans 7,114 3,830 3,225 Average recorded investment of impaired loans 5,608 3,661 4,490 Accruing troubled debt restructures 2,749 3,526 2,893 Interest income not recorded on nonaccrual loans according to their original terms 51 37 91 Interest income on nonaccrual loans actually recorded — — — Interest income recognized on impaired loans 182 140 104
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 2017. Balance at December 31, 2016 Additions Repayments Balance at (dollars in thousands) $10,982 $ 572 $ 5,729 $ 5,825

Allowance for Loan Losses (Tabl

Allowance for Loan Losses (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017, 2016 and 2015 is as follows: 2017 2016 2015 (dollars in thousands) Allowance for loan losses, beginning of year $ 24,406 $ 23,075 $ 22,318 Loans charged-off (390 ) (389 ) (781 ) Recoveries on loans previously charged-off 449 434 1,338 Net recoveries (charge-offs) 59 45 557 Provision charged to expense 1,790 1,375 200 Reclassification to other liabilities* — (89 ) — Allowance for loan losses, end of year $ 26,255 $ 24,406 $ 23,075 * The reclassification relates to allowance for loan losses allocations on unused commitments that have been reclassified to other liabilities.
Summary of Allowance for Loan Losses	Further information pertaining to the allowance for loan losses at December 31, 2017 follows: Construction Commercial and Industrial Municipal Commercial Residential Estate Consumer Home Unallocated Total (dollars in thousands) Allowance for Loan Losses: Balance at December 31, 2016 $ 1,012 $ 6,972 $ 1,612 $ 11,135 $ 1,698 $ 582 $ 1,102 $ 293 $ 24,406 Charge-offs — (49 ) — — — (341 ) — — (390 ) Recoveries — 110 — — 2 255 82 — 449 Provision 633 2,618 108 (1,407 ) 173 (123 ) (195 ) (17 ) 1,790 Ending balance at December 31, 2017 $ 1,645 $ 9,651 $ 1,720 $ 9,728 $ 1,873 $ 373 $ 989 $ 276 $ 26,255 Amount of allowance for loan losses for loans deemed to be impaired $ — $ 7 $ — $ 99 $ 58 $ — $ — $ — $ 164 Amount of allowance for loan losses for loans not deemed to be impaired $ 1,645 $ 9,644 $ 1,720 $ 9,629 $ 1,815 $ 373 $ 989 $ 276 $ 26,091 Loans: Ending balance $ 18,931 $ 763,807 $ 106,599 $ 732,491 $ 287,731 $ 19,040 $ 247,345 $ — $ 2,175,944 Loans deemed to be impaired $ — $ 348 $ — $ 2,554 $ 4,212 $ — $ — $ — $ 7,114 Loans not deemed to be impaired $ 18,931 $ 763,459 $ 106,599 $ 729,937 $ 283,519 $ 19,040 $ 247,345 $ — $ 2,168,830 Further information pertaining to the allowance for loan losses at December 31, 2016 follows: Construction Commercial Municipal Commercial Residential Consumer Home Unallocated Total (dollars in thousands) Allowance for Loan Losses: Balance at December 31, 2015 $ 2,041 $ 5,899 $ 994 $ 10,589 $ 1,320 $ 644 $ 1,077 $ 511 $ 23,075 Charge-offs — — — — — (362 ) (27 ) — (389 ) Recoveries — 132 — — 6 296 — — 434 Reclassification to other liabilities (5 ) (25 ) — (9 ) (3 ) (3 ) (44 ) — (89 ) Provision (1,024 ) 966 618 555 375 7 96 (218 ) 1,375 Ending balance at December 31, 2016 $ 1,012 $ 6,972 $ 1,612 $ 11,135 $ 1,698 $ 582 $ 1,102 $ 293 $ 24,406 Amount of allowance for loan losses for loans deemed to be impaired $ 3 $ 23 $ — $ 140 $ 7 $ — $ — $ — $ 173 Amount of allowance for loan losses for loans not deemed to be impaired $ 1,009 $ 6,949 $ 1,612 $ 10,995 $ 1,691 $ 582 $ 1,102 $ 293 $ 24,233 Loans: Ending balance $ 14,928 $ 612,503 $ 135,418 $ 696,173 $ 241,357 $ 11,697 $ 211,857 $ — $ 1,923,933 Loans deemed to be impaired $ 94 $ 389 $ — $ 3,149 $ 198 $ — $ — $ — $ 3,830 Loans not deemed to be impaired $ 14,834 $ 612,114 $ 135,418 $ 693,024 $ 241,159 $ 11,697 $ 211,857 $ — $ 1,920,103
Loans by Risk Rating	The following table presents the Company’s loans by risk rating at December 31, 2017. Construction Commercial Municipal Commercial (dollars in thousands) Grade: 1-3 $ 18,931 $ 758,093 $ 106,599 $ 705,235 4 (Monitor) — 5,366 — 24,702 5 (Substandard) — — — — 6 (Doubtful) — — — — Impaired — 348 — 2,554 Total $ 18,931 $ 763,807 $ 106,599 $ 732,491 The following table presents the Company’s loans by risk rating at December 31, 2016. Construction Commercial Municipal Commercial (dollars in thousands) Grade: 1-3 $ 14,834 $ 612,114 $ 135,418 $ 661,271 4 (Monitor) — — — 31,753 5 (Substandard) — — — — 6 (Doubtful) — — — — Impaired 94 389 — 3,149 Total $ 14,928 $ 612,503 $ 135,418 $ 696,173
Loans by Credit Rating	The following table presents the Company’s loans by credit rating at December 31, 2017. Commercial Municipal Commercial Total (dollars in thousands) Credit Rating: Aaa-Aa3 $ 478,905 $ 62,029 $ 45,066 $ 586,000 A1-A3 195,599 7,635 128,554 331,788 Baa1-Baa3 — 26,970 122,000 148,970 Ba2 — 8,165 — 8,165 Total $ 674,504 $ 104,799 $ 295,620 $ 1,074,923 The following table presents the Company’s loans by credit rating at December 31, 2016. Commercial Municipal Commercial Total (dollars in thousands) Credit Rating: Aaa-Aa3 $ 334,674 $ 66,245 $ 6,596 $ 407,515 A1-A3 188,777 33,365 129,423 351,565 Baa1-Baa3 — 26,970 127,366 154,336 Ba2 — 3,610 — 3,610 Total $ 523,451 $ 130,190 $ 263,385 $ 917,026
Aging of Past Due Loan Losses	At December 31, 2017 the aging of past due loans are as follows: Accruing 30-89 Days Non Accruing 90 Days Total Current Total (dollars in thousands) Construction and land development $ — $ — $ — $ — $ 18,931 $ 18,931 Commercial and industrial 65 44 — 109 763,698 763,807 Municipal — — — — 106,599 106,599 Commercial real estate 672 215 — 887 731,604 732,491 Residential real estate 4,282 724 — 5,006 282,725 287,731 Consumer and overdrafts 5 6 — 11 19,029 19,040 Home equity 618 695 — 1,313 246,032 247,345 Total $ 5,642 $ 1,684 $ — $ 7,326 $ 2,168,618 $ 2,175,944 At December 31, 2016 the aging of past due loans are as follows: Accruing 30-89 Days Non Accruing 90 Days Total Current Total (dollars in thousands) Construction and land development $ — $ 94 $ — $ 94 $ 14,834 $ 14,928 Commercial and industrial 37 65 — 102 612,401 612,503 Municipal — — — — 135,418 135,418 Commercial real estate 597 150 — 747 695,426 696,173 Residential real estate 245 656 — 901 240,456 241,357 Consumer and overdrafts — 11 — 11 11,686 11,697 Home equity 735 108 — 843 211,014 211,857 Total $ 1,614 $ 1,084 $ — $ 2,698 $ 1,921,235 $ 1,923,933
Information Pertaining to Impaired Loans	The following is information pertaining to impaired loans at December 31, 2017: Carrying Unpaid Required Average Interest (dollars in thousands) With no required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 113 325 — 54 4 Municipal — — — — — Commercial real estate 420 548 — 286 21 Residential real estate — — — 73 — Consumer — — — — — Home equity — — — — — Total $ 533 $ 873 $ — $ 413 $ 25 With required reserve recorded: Construction and land development $ — $ — $ — $ 43 $ — Commercial and industrial 235 235 7 318 12 Municipal — — — — — Commercial real estate 2,134 2,135 99 2,501 72 Residential real estate 4,212 4,212 58 2,333 73 Consumer — — — — — Home equity — — — — — Total $ 6,581 $ 6,582 $ 164 $ 5,195 $ 157 Total Construction and land development $ — $ — $ — $ 43 $ — Commercial and industrial 348 560 7 372 16 Municipal — — — — — Commercial real estate 2,554 2,683 99 2,787 93 Residential real estate 4,212 4,212 58 2,406 73 Consumer — — — — — Home equity — — — — — Total $ 7,114 $ 7,455 $ 164 $ 5,608 $ 182 The following is information pertaining to impaired loans at December 31, 2016: Carrying Unpaid Required Average Interest (dollars in thousands) With no required reserve recorded: Construction and land development $ — $ — $ — $ — $ — Commercial and industrial 45 232 — 53 — Municipal — — — — — Commercial real estate 590 590 — 375 39 Residential real estate 90 179 — 102 7 Consumer — — — — — Home equity — — — — — Total $ 725 $ 1,001 $ — $ 530 $ 46 With required reserve recorded: Construction and land development $ 94 $ 108 $ 3 $ 96 $ — Commercial and industrial 344 360 23 360 18 Municipal — — — — — Commercial real estate 2,559 2,665 140 2,324 71 Residential real estate 108 108 7 323 5 Consumer — — — — — Home equity — — — 28 — Total $ 3,105 $ 3,241 $ 173 $ 3,131 $ 94 Total Construction and land development $ 94 $ 108 $ 3 $ 96 $ — Commercial and industrial 389 592 23 413 18 Municipal — — — — — Commercial real estate 3,149 3,255 140 2,699 110 Residential real estate 198 287 7 425 12 Consumer — — — — — Home equity — — — 28 — Total $ 3,830 $ 4,242 $ 173 $ 3,661 $ 140

Bank Premises and Equipment (Ta

Bank Premises and Equipment (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Schedule of Bank Premises and Equipment	December 31, 2017 2016 Estimated Useful Life (dollars in thousands) Land $ 3,850 $ 3,478 — Bank premises 21,055 19,272 30-39 years Furniture and equipment 27,117 26,271 3-10 Leasehold improvements 12,674 12,802 30-39 years or lease term 64,696 61,823 Accumulated depreciation and amortization (41,169 ) (38,406 ) Total $ 23,527 $ 23,417
Summary of Future Minimum Rental Commitments for Non-Cancelable Operating Leases	Future minimum rental commitments for non-cancelable Year Amount (dollars in thousands) 2018 $ 2,309 2019 2,149 2020 1,856 2021 1,382 2022 1,022 Thereafter 1,942 $ 10,660

Goodwill and Identifiable Int38

Goodwill and Identifiable Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 are shown in the table below. Carrying Amount of Goodwill and Intangibles Goodwill Mortgage Total (dollars in thousands) Balance at December 31, 2015 $ 2,714 $ 1,305 $ 4,019 Additions — 708 708 Amortization Expense — (384 ) (384 ) Balance at December 31, 2016 $ 2,714 $ 1,629 $ 4,343 Additions — 276 276 Amortization Expense — (380 ) (380 ) Balance at December 31, 2017 $ 2,714 $ 1,525 $ 4,239

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017, are as follows: Fair Value Measurements Using Carrying Quoted Prices Significant Significant (dollars in thousands) Financial Instruments Measured at Fair Value on a Recurring Basis — Securities AFS U.S. Treasury $ 1,984 $ — $ 1,984 $ — U.S. Government Agency Sponsored Enterprises — — — — SBA Backed Securities 80,950 — 80,950 — U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 225,775 — 225,775 — Privately Issued Residential Mortgage-Backed Securities 892 — 892 — Obligations Issued by States and Political Subdivisions 82,600 — — 82,600 Other Debt Securities 4,971 — 4,971 — Equity Securities 303 303 — — Total $ 397,475 $ 303 $ 314,572 $ 82,600 Financial Instruments Measured at Fair Value on a Non-recurring Impaired Loans $ 246 $ — $ — $ 246 The results of the fair value hierarchy as of December 31, 2016, are as follows: Fair Value Measurements Using Carrying Quoted Prices Significant Significant (Level 3) (dollars in thousands) Financial Instruments Measured at Fair Value on a Recurring Basis — Securities AFS U.S. Treasury $ 2,000 $ — $ 2,000 $ — U.S. Government Agency Sponsored Enterprises 24,952 — 24,952 — BA Backed Securities 57,767 — 57,767 — U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities 243,325 — 243,325 — Privately Issued Residential Mortgage-Backed Securities 1,109 — 1,109 — Obligations Issued by States and Political Subdivisions 164,876 — — 164,876 Other Debt Securities 4,924 — 4,924 — Equity Securities 344 344 — — Total $ 499,297 $ 344 $ 334,077 $ 164,876 Financial Instruments Measured at Fair Value on a Non-recurring Impaired Loans $ 260 $ — $ — $ 260
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 thousands) at December 31, 2017. 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) $ 82,600 Discounted cash flow Discount rate 1.0%-3.5%(2) Impaired Loans 246 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. The changes in Level 3 securities for the year ended December 31, 2017 are as shown in the table below: 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, 2016. 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) $ 164,876 Discounted cash flow Discount rate 0%-1% (2) Impaired Loans 260 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, 2017 are as shown in the table below: Auction Rate Obligations Equity Total (dollars in thousands) Balance at December 31, 2016 $ 4,298 $ 160,578 $ — $ 164,876 Purchases — 99,136 — 99,136 Maturities/redemptions — (181,394 ) — (181,394 ) Amortization — (179 ) — (179 ) Change in fair value 161 — — 161 Balance at December 31, 2017 $ 4,459 $ 78,141 $ — $ 82,600 The changes in Level 3 securities for the year ended December 31, 2016 are as shown in the table below: Auction Rate Obligations Equity Total (dollars in thousands) Balance at December 31, 2015 $ 3,820 $ 153,140 $ 37 $ 156,997 Purchases — 216,646 — 216,646 Maturities/redemptions — (208,990 ) (37 ) (209,027 ) Amortization — (218 ) — (218 ) Change in fair value 478 — — 478 Balance at December 31, 2016 $ 4,298 $ 160,578 $ — $ 164,876

Deposits (Tables)

Deposits (Tables)	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [Abstract]
Summary of Remaining Maturities or Re-pricing of Time Deposits	The following is a summary of remaining maturities or re-pricing 2017 Percent 2016 Percent (dollars in thousands) Within one year $ 436,911 70 % $ 262,406 55 % Over one year to two years 121,802 19 % 87,952 18 % Over two years to three years 30,098 5 % 83,067 17 % Over three years to five years 36,550 6 % 44,934 10 % Total $ 625,361 100 % $ 478,359 100 %

Securities Sold Under Agreeme41

Securities Sold Under Agreements to Repurchase (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 (dollars in thousands) Amount outstanding at December 31 $ 158,990 $ 182,280 $ 197,850 Weighted average rate at December 31 0.32 % 0.21 % 0.21 % Maximum amount outstanding at any month end $ 228,848 $ 241,110 $ 299,890 Daily average balance outstanding during the year $ 189,684 $ 222,956 $ 245,276 Weighted average rate during the year 0.26 % 0.21 % 0.20 %

Other Borrowed Funds and Subo42

Other Borrowed Funds and Subordinated Debentures (Tables)	12 Months Ended
	Dec. 31, 2017
Banking and Thrift [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, 2017 2016 2015 (dollars in thousands) Amount outstanding at December 31 $ 383,861 $ 329,083 $ 404,083 Weighted average rate at December 31 2.26 % 2.39 % 2.29 % Maximum amount outstanding at any month end $ 491,583 $ 467,083 $ 521,583 Daily average balance outstanding during the year $ 309,102 $ 357,974 $ 374,109 Weighted average rate during the year 2.42 % 2.48 % 2.38 %
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: 2017 2016 2015 December 31, Amount Weighted Amount Weighted Amount Weighted (dollars in thousands) Within one year $ 164,500 1.82 % $ 77,500 2.21 % $ 100,000 1.89 % Over one year to two years $ 63,000 2.17 % $ 54,500 2.25 % $ 57,500 2.72 % Over two years to three years $ 28,000 2.29 % $ 58,000 1.87 % $ 54,500 2.25 % Over three years to five years $ 28,500 3.19 % $ 58,000 2.68 % $ 91,000 1.85 % Over five years $ 63,778 2.38 % $ 45,000 2.85 % $ 65,000 3.23 % Total $ 347,778 2.13 % $ 293,000 2.34 % $ 368,000 2.30 %

Reclassifications Out of Accu43

Reclassifications Out of Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
Reclassifications Out of Accumulated Other Comprehensive Income	Amount Reclassified from Accumulated Details about Accumulated Other Comprehensive Income Components Year ended (a) Year ended (a) Affected line item in the Statement Unrealized gains and losses on available-for-sale $ 47 $ 52 Net gains on sales of investments (19 ) (20 ) Provision for income taxes $ 28 $ 32 Net income Accretion of unrealized losses transferred $ (2,292 ) $ (4,317 ) Securities held-to-maturity 1,258 1,505 Provision for income taxes $ (1,034 ) $ (2,812 ) Net income Amortization of defined benefit pension items Prior-service costs $ (10 ) $ (10 ) Salaries and employee benefits (b) Actuarial gains (losses) (1,540 ) (1,606 ) Salaries and employee benefits (b) Total before tax (1,550 ) (1,616 ) Income before taxes Tax (expense) or benefit 619 646 Provision for income taxes Net of tax $ (931 ) $ (970 ) 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, 2017
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, 2017 2016 2015 (in thousands except share and per share data) BASIC EPS COMPUTATION Numerator: Net income, Class A $ 17,526 $ 19,270 $ 18,081 Net income, Class B 4,775 5,264 4,940 Denominator: Weighted average shares outstanding, Class A 3,604,029 3,600,729 3,600,729 Weighted average shares outstanding, Class B 1,963,880 1,967,180 1,967,180 Basic EPS, Class A $ 4.86 $ 5.35 $ 5.02 Basic EPS, Class B $ 2.43 $ 2.68 $ 2.51 DILUTED EPS COMPUTATION Numerator: Net income, Class A $ 17,526 $ 19,270 $ 18,081 Net income, Class B 4,775 5,264 4,940 Total net income, for diluted EPS, Class A computation 22,301 24,534 23,021 Denominator: Weighted average shares outstanding, basic, Class A 3,604,029 3,600,729 3,600,729 Weighted average shares outstanding, Class B 1,963,880 1,967,180 1,967,180 Weighted average shares outstanding diluted, Class A 5,567,909 5,567,909 5,567,909 Weighted average shares outstanding, Class B 1,963,880 1,967,180 1,967,180 Diluted EPS, Class A $ 4.01 $ 4.41 $ 4.13 Diluted EPS, Class B $ 2.43 $ 2.68 $ 2.51

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2017
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 Amount Ratio Amount Ratio Amount Ratio As of December 31, 2017 (Basel III) Total Capital (to Risk-Weighted Assets) $ 329,666 12.70 % $ 207,707 8.00 % $ 259,633 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 303,411 11.69 % 155,780 6.00 % 207,707 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 303,411 11.69 % 116,835 4.50 % 168,762 6.50 % Tier 1 Capital (to 4th Qtr. Average Assets) 303,411 6.55 % 185,199 4.00 % 231,499 5.00 % As of December 31, 2016 (Basel III) Total Capital (to Risk-Weighted Assets) $ 293,143 12.27 % $ 191,081 8.00 % $ 238,851 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 268,737 11.25 % 143,311 6.00 % 191,081 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 268,737 11.25 % 107,483 4.50 % 155,253 6.50 % Tier 1 Capital (to 4th Qtr. Average Assets) 268,737 6.02 % 178,469 4.00 % 223,086 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 Amount Ratio Amount Ratio Amount Ratio As of December 31, 2017 (Basel III) Total Capital (to Risk-Weighted Assets) $ 341,033 13.05 % $ 209,049 8.00 % $ 261,312 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 314,778 12.05 % 156,787 6.00 % 209,049 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 279,778 10.71 % 117,590 4.50 % 169,853 6.50 % Tier 1 Capital (to 4th Qtr. Average Assets) 314,778 6.78 % 185,657 4.00 % 232,072 5.00 % As of December 31, 2016 (Basel III) Total Capital (to Risk-Weighted Assets) $ 305,065 12.72 % $ 191,904 8.00 % $ 239,880 10.00 % Tier 1 Capital (to Risk-Weighted Assets) 280,659 11.70 % 143,928 6.00 % 191,904 8.00 % Common Equity Tier 1 Capital (to Risk-Weighted Assets) 249,753 10.41 % 107,946 4.50 % 155,922 6.50 % Tier 1 Capital (to 4th Qtr. Average Assets) 280,659 6.28 % 178,903 4.00 % 223,628 5.00 %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Summary of Current and Deferred Components of Income Tax (Benefit) Expense	The current and deferred components of income tax (benefit) expense for the years ended December 31, are as follows: 2017 2016 2015 (dollars in thousands) Current expense: Federal $ 3,628 $ 3,875 $ 3,393 State 412 439 399 Total current expense 4,040 4,314 3,792 Deferred (benefit) expense: Federal 6,496 (4,450 ) (3,098 ) State 422 (334 ) (161 ) Valuation Allowance — 108 — Total deferred expense (benefit) 6,918 (4,676 ) (3,259 ) Provision for income taxes $ 10,958 $ (362 ) $ 533
Income Tax Accounts Included in Other Assets	Income tax accounts included in other assets at December 31, are as follows: 2017 2016 (dollars in thousands) Currently receivable $ 15,940 $ 633 Deferred income tax asset, net 20,892 43,129 Total $ 36,832 $ 43,762
Summary of Differences between Income Tax (Benefit) Expense at the Statutory Federal Income Tax Rate and Total Income Tax Expense	Differences between income tax (benefit) expense at the statutory federal income tax rate and total income tax expense are summarized as follows: 2017 2016 2015 (dollars in thousands) Federal income tax expense at statutory rates $ 11,308 $ 8,218 $ 8,008 State income tax, net of federal income tax benefit 550 69 157 Insurance income (371 ) (406 ) (375 ) Effect of tax-exempt (8,683 ) (8,259 ) (6,915 ) Net tax credit (341 ) (395 ) (460 ) Valuation allowance — 108 — Deferred tax remeasurement 8,448 — — Other 47 303 118 Total $ 10,958 $ (362 ) $ 533 Effective tax rate 32.95 % (1.50 )% 2.30 %
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: 2017 2016 (dollars in thousands) Deferred income tax assets: Allowance for loan losses $ 7,855 $ 10,419 AMT credit — 10,234 Deferred compensation 7,555 9,684 Pension and SERP liability 8,436 11,320 Unrealized losses on securities transferred to held-to-maturity 1,303 3,161 Depreciation 631 968 Accrued bonus — 612 Unrealized (gains) losses on securities available-for-sale 14 357 Charitable contributions carryforward 442 266 Acquisition premium 17 128 Nonaccrual interest 97 125 Limited partnerships 21 30 Investments write down 17 26 Other 173 220 Gross deferred income tax asset 26,561 47,550 Valuation allowance (108 ) (108 ) Gross deferred income tax asset, net of valuation allowance 26,453 47,442 Deferred income tax liabilities: Pension asset (liability) (4,403 ) (3,662 ) Deferred origination costs (481 ) — Prepaid expenses (248 ) — Mortgage servicing rights (429 ) (651 ) Gross deferred income tax liability (5,561 ) (4,313 ) Deferred income tax asset, net $ 20,892 $ 43,129

Employee Benefits (Tables)

Employee Benefits (Tables)	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
Fair Value of Plan Assets and Major Categories	The fair value of plan assets and major categories as of December 31, 2017, is as follows: Description Percent Level 1 Level 2 Level 3 Total (dollars in thousands) Collective Funds 3.6 % $ 1,741 $ — $ — $ 1,741 Equity Securities 10.7 % 5,195 — — 5,195 Diversified Mutual Funds 17.8 % 8,615 — — 8,615 Short-term investments 7.9 % 3,836 — — 3,836 Total investments measured in the fair value hierarchy 40.0 % 19,387 — — 19,387 Investments measured at net asset value(1) 60.0 % — — — 29,035 100.0 % $ 19,387 $ — $ — $ 48,422 (1) In accordance with Subtopic 820-10, The fair value of plan assets and major categories as of December 31, 2016, is as follows: Description Percent Level 1 Level 2 Level 3 Total (dollars in thousands) Collective Funds 6.9 % $ 2,600 $ — $ — $ 2,600 Equity Securities 19.7 % 7,363 — — 7,363 Diversified Mutual Funds 12.3 % 4,615 — — 4,615 Short-term investments 1.3 % 475 — — 475 Total investments measured in the fair value hierarchy 40.2 % 15,053 — — 15,053 Investments measured at net asset value(1) 59.8 % — — — 22,394 100.0 % $ 15,053 $ — $ — $ 37,447 (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, 2017, is as follows: Percent Fair (dollars in thousands) Collective Funds by Category: Equity 31.6 % $ 15,304 Diversified 0.7 % 344 US debt securities 9.4 % 4,569 International equities 9.1 % 4,419 Limited Partnerships by Category: Emerging markets 2.8 % 1,353 Multi-strategy 1.5 % 705 Hedge Funds by Category: Multi-strategy(1) 3.5 % 1,674 Global opportunities(2) 0.7 % 345 Private investment entities and/or separately managed accounts (3) 0.7 % 322 60.0 % $ 29,035 The investments measured using the net asset value per share practical expedient as of December 31, 2016, is as follows: Percent Fair Value (dollars in thousands) Collective Funds by Category: Equity 24.1 % $ 9,013 Diversified 0.1 % 47 US debt securities 11.3 % 4,241 International equities 9.8 % 3,684 Limited Partnerships by Category: Emerging markets 0.0 % — Multi-strategy 7.0 % 2,623 Hedge Funds by Category: Multi-strategy(1) 5.6 % 2,082 Global opportunities(2) 1.1 % 422 Private investment entities and/or separately managed accounts(3) 0.8 % 282 59.8 % $ 22,394 (1) This category includes investments in hedge funds that pursue multiple strategies to diversify risks and reduce volatility. Fund objectives are to seek above-average rates of return and long-term capital growth through investments, which are fund of funds with a diversified portfolio of private investment entities and/or separately managed accounts managed by investment managers or achieve superior risk-adjusted capital appreciation over the long-term, generally through an investment, which invests in private investment funds and discretional managed accounts, structured notes, swaps or other similar products. The fair values of the investments in this category have been determined using the net asset value per share of the fund(s). (2) This category has an investment strategy to pursue a hybrid absolute return via portfolio managers, secondaries, and co-investments (3) 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 Supplemental Insurance/ 2017 2016 2017 2016 (dollars in thousands) Change projected in benefit obligation Benefit obligation at beginning of year $ 42,255 $ 38,597 $ 38,610 $ 38,204 Service cost 1,241 1,273 1,582 1,820 Interest cost 1,450 1,358 1,382 1,334 Actuarial (gain)/loss 3,456 2,593 2,087 (1,653 ) Benefits paid (1,337 ) (1,566 ) (1,082 ) (1,095 ) Projected benefit obligation at end of year $ 47,065 $ 42,255 $ 42,579 $ 38,610 Change in plan assets Fair value of plan assets at beginning of year $ 37,447 $ 33,717 Actual return on plan assets 5,312 3,221 Employer contributions 7,000 2,075 Benefits paid (1,337 ) (1,566 ) Fair value of plan assets at end of year $ 48,422 $ 37,447 (Unfunded) Funded status $ 1,357 $ (4,808 ) $ (42,579 ) $ (38,610 ) Accumulated benefit obligation $ 47,065 $ 42,255 $ 40,375 $ 36,392 Weighted-average assumptions as of December 31 Discount rate — Liability 3.49 % 3.99 % 3.42 % 3.85 % Discount rate — Expense 3.99 % 4.18 % 3.85 % 4.01 % Expected return on plan assets 8.00 % 8.00 % NA NA Rate of compensation increase 4.00 % 4.00 % 4.00 % 4.00 % Components of net periodic benefit cost Service cost $ 1,241 $ 1,273 $ 1,582 $ 1,820 Interest cost 1,450 1,358 1,382 1,334 Expected return on plan assets (2,985 ) (2,776 ) — — Recognized prior service cost (104 ) (104 ) 114 114 Recognized net losses 903 801 636 805 Net periodic cost (benefit) $ 505 $ 552 $ 3,714 $ 4,073 Other changes in plan assets and benefit obligations recognized in other comprehensive income Amortization of prior service cost $ 104 $ 104 $ (114 ) $ (114 ) Net (gain) loss 409 1,347 1,752 (2,458 ) Total recognized in other comprehensive income 513 1,451 1,638 (2,572 ) Total recognized in net periodic benefit cost and other comprehensive income $ 1,018 $ 2,003 $ 5,352 $ 1,501
Summary of Defined Pension Plan and Supplemental Insurance Retirement Plan	December 31, 2017 December 31, 2016 Plan Supplemental Total Plan Supplemental Total (dollars in thousands) Prior service cost $ 100 $ (535 ) $ (435 ) $ 204 $ (649 ) $ (445 ) Net actuarial loss (14,408 ) (15,168 ) (29,576 ) (13,999 ) (13,416 ) (27,415 ) Total $ (14,308 ) $ (15,703 ) $ (30,011 ) $ (13,795 ) $ (14,065 ) $ (27,860 )
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, 2017, 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 2018 $ (100 ) $ 114 Amortization of loss to be recognized in 2018 904 707

Financial Instruments with Of48

Financial Instruments with Off-Balance-Sheet Risk (Tables)	12 Months Ended
	Dec. 31, 2017
Risks and Uncertainties [Abstract]
Summary of Financial Instruments with Off-Balance-Sheet Risk	Financial instruments with off-balance-sheet Contract or Notional Amount 2017 2016 (dollars in thousands) Financial instruments whose contract amount represents credit risk: Commitments to originate 1–4 family mortgages $ 5,748 $ 13,877 Standby and commercial letters of credit 5,520 6,796 Unused lines of credit 434,618 362,357 Unadvanced portions of construction loans 15,152 22,049 Unadvanced portions of other loans 35,602 52,224

Other Operating Expenses (Table

Other Operating Expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Other Income and Expenses [Abstract]
Summary of Other Operating Expenses	Year ended December 31, 2017 2016 2015 (dollars in thousands) Marketing $ 2,315 $ 2,185 $ 1,849 Software maintenance/amortization 1,859 1,863 1,670 Legal and audit 1,543 1,255 1,269 Contributions 993 789 690 Processing services 1,160 1,040 1,002 Consulting 1,199 1,168 1,050 Postage and delivery 966 987 905 Supplies 945 948 965 Telephone 1,020 1,032 804 Directors’ fees 440 413 377 Insurance 308 323 301 Other 1,845 1,812 1,826 Total $ 14,593 $ 13,815 $ 12,708

Fair Values of Financial Inst50

Fair Values of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and December 31, 2016. 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, short-term investments, FHLBB stock and accrued interest receivable. Financial liabilities for which the fair value approximates carrying value include non-maturity Carrying Amount Estimated Level 1 Inputs Fair Value Level 3 Inputs (dollars in thousands) December 31, 2017 Financial assets: Securities held-to-maturity $ 1,701,233 $ 1,668,827 $ — $ 1,668,827 $ — Loans(1) 2,149,689 2,094,517 — — 2,094,517 Financial liabilities: Time deposits 625,361 627,517 — 627,517 — Other borrowed funds 347,778 349,364 — 349,364 — Subordinated debentures 36,083 36,083 — — 36,083 December 31, 2016 Financial assets: Securities held-to-maturity $ 1,653,986 $ 1,635,808 $ — $ 1,635,808 $ — Loans(1) 1,899,527 1,873,703 — — 1,873,703 Financial liabilities: Time deposits 478,359 480,133 — 480,133 — Other borrowed funds 293,000 294,940 — 294,940 — 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.

Quarterly Results of Operatio51

Quarterly Results of Operations (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results of Operations (Unaudited)	2017 Quarters Fourth Third Second First (in thousands, except share data) Interest income $ 29,470 $ 28,521 $ 28,806 $ 26,639 Interest expense 7,768 7,168 6,701 6,183 Net interest income 21,702 21,353 22,105 20,456 Provision for loan losses 450 450 490 400 Net interest income after provision for loan losses 21,252 20,903 21,615 20,056 Other operating income 4,410 3,942 4,291 3,909 Operating expenses 15,992 16,205 17,197 17,725 Income before income taxes 9,670 8,640 8,709 6,240 Provision for income taxes 9,645 617 552 144 Net income $ 25 $ 8,023 $ 8,157 $ 6,096 Share data: Average shares outstanding, basic Class A 3,605,829 3,605,829 3,603,729 3,600,729 Class B 1,962,080 1,962,080 1,964,180 1,967,180 Average shares outstanding, diluted Class A 5,567,909 5,567,909 5,567,909 5,567,909 Class B 1,962,080 1,962,080 1,964,180 1,967,180 Earnings per share, basic Class A $ 0.01 $ 1.75 $ 1.78 $ 1.33 Class B $ — $ 0.87 $ 0.89 $ 0.66 Earnings per share, diluted Class A $ — $ 1.44 $ 1.47 $ 1.09 Class B $ — $ 0.87 $ 0.89 $ 0.66 2016 Quarters Fourth Third Second First (in thousands, except share data) Interest income $ 24,689 $ 25,005 $ 23,742 $ 23,263 Interest expense 5,927 5,791 5,486 5,413 Net interest income 18,762 19,214 18,256 17,850 Provision for loan losses 200 375 350 450 Net interest income after provision for loan losses 18,562 18,839 17,906 17,400 Other operating income 3,700 4,225 4,643 3,654 Operating expenses 16,156 16,630 16,288 15,683 Income before income taxes 6,106 6,434 6,261 5,371 Provision for income taxes (394 ) (52 ) 20 64 Net income $ 6,500 $ 6,486 $ 6,241 $ 5,307 Share data: Average shares outstanding, basic Class A 3,600,729 3,600,729 3,600,729 3,600,729 Class B 1,967,180 1,967,180 1,967,180 1,967,180 Average shares outstanding, diluted Class A 5,567,909 5,567,909 5,567,909 5,567,909 Class B 1,967,180 1,967,180 1,967,180 1,967,180 Earnings per share, basic Class A $ 1.42 $ 1.41 $ 1.36 $ 1.16 Class B $ 0.71 $ 0.71 $ 0.68 $ 0.58 Earnings per share, diluted Class A $ 1.17 $ 1.16 $ 1.12 $ 0.95 Class B $ 0.71 $ 0.71 $ 0.68 $ 0.58

Parent Company Financial Stat52

Parent Company Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Balance Sheets of Parent Company	BALANCE SHEETS December 31, 2017 2016 (dollars in thousands) ASSETS: Cash $ 1,981 $ 2,768 Investment in subsidiary, at equity 283,881 263,070 Other assets 16,833 10,335 Total assets $ 302,695 $ 276,173 LIABILITIES AND STOCKHOLDERS’ EQUITY: Liabilities $ 6,315 $ 49 Subordinated debentures 36,083 36,083 Stockholders’ equity 260,297 240,041 Total liabilities and stockholders’ equity $ 302,695 $ 276,173
Statements of Income of Parent Company	STATEMENTS OF INCOME Year Ended December 31, 2017 2016 2015 (dollars in thousands) Income: Dividends from subsidiary $ 2,500 $ 2,000 $ 1,500 Interest income from deposits in bank 1 3 13 Other income 34 28 24 Total income 2,535 2,031 1,537 Interest expense 1,121 937 792 Operating expenses 209 220 212 Income before income taxes and equity in undistributed income of subsidiary 1,205 874 533 Benefit from income taxes (440 ) (383 ) (328 ) Income before equity in undistributed income of subsidiary 1,645 1,257 861 Equity in undistributed income of subsidiary 20,656 23,277 22,160 Net income $ 22,301 $ 24,534 $ 23,021
Statements of Cash Flows of Parent Company	STATEMENTS OF CASH FLOWS December 31, 2017 2016 2015 (dollars in thousands) CASH FLOWS FROM OPERATING ACTIVITIES: Net income $ 22,301 $ 24,534 $ 23,021 Adjustments to reconcile net income to net cash provided by operating activities Undistributed income of subsidiary (20,656 ) (23,277 ) (22,160 ) Depreciation and amortization — — 3 Increase in other assets (6,498 ) (1,527 ) (1,112 ) Decrease in liabilities 6,266 9 4 Net cash (used in) operating activities 1,413 (261 ) (244 ) CASH FLOWS FROM FINANCING ACTIVITIES: Net proceeds from the exercise of stock options — — — Cash dividends paid (2,200 ) (2,201 ) (2,200 ) Net cash used in financing activities (2,200 ) (2,201 ) (2,200 ) Net (decrease) in cash (787 ) (2,462 ) (2,444 ) Cash at beginning of year 2,768 5,230 7,674 Cash at end of year $ 1,981 $ 2,768 $ 5,230

Summary of Significant Accoun53

Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2018USD ($)	Dec. 31, 2021	Dec. 31, 2017USD ($)SegmentReporting_Unitshares	Dec. 31, 2016shares
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%
Loans discontinued delinquency period			90 days
Number of reporting units | Reporting_Unit			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 more likely than not that the fair value of a reporting unit is less than its carrying amount, then performing the two-step impairment test will be unnecessary.
Nonqualified stock options			85.00%
Stock option granted | shares			0	0
Excess AMT credit carryforwards refundable rate			50.00%
Net tax benefit from the re-measurement of deferred taxes	$ 3,800,000		$ 8,448,000
Federal Home Loan Bank [Member]
Summary Of Significant Accounting Policies And Other Information [Line Items]
Impairment recognized			$ 0
Scenario, Forecast [Member]
Summary Of Significant Accounting Policies And Other Information [Line Items]
Excess AMT credit carryforwards refundable rate		100.00%
Maximum [Member]
Summary Of Significant Accounting Policies And Other Information [Line Items]
Time period over which stock option exercisable			10 years
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%
Supplemental plan			1 year
Loans Acquired with Deteriorated Credit Quality [Member]
Summary Of Significant Accounting Policies And Other Information [Line Items]
Non performing loans acquired			$ 0
Class A Common Stock [Member]
Summary Of Significant Accounting Policies And Other Information [Line Items]
Approved stock option plan for purchase of common stock | shares			150,000
Options to purchase an aggregate of shares | shares			0
Class A Common Stock [Member] | 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%
Century Bancorp Capital Trust II [Member]
Summary Of Significant Accounting Policies And Other Information [Line Items]
Equity ownership interest			100.00%

Cash and Due from Banks - Addit

Cash and Due from Banks - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 397,563	$ 500,220
Gross Unrealized Gains	860	555
Gross Unrealized Losses	948	1,478
Total, Fair Value	397,475	499,297
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,999	2,000
Gross Unrealized Losses	15
Total, Fair Value	1,984	2,000
U.S. Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		25,000
Gross Unrealized Losses		48
Total, Fair Value		24,952
SBA Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	81,065	57,899
Gross Unrealized Gains	46	14
Gross Unrealized Losses	161	146
Total, Fair Value	80,950	57,767
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	225,537	243,703
Gross Unrealized Gains	555	293
Gross Unrealized Losses	317	671
Total, Fair Value	225,775	243,325
Privately Issued Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	897	1,121
Gross Unrealized Gains	4	2
Gross Unrealized Losses	9	14
Total, Fair Value	892	1,109
Obligations Issued by States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	82,849	165,281
Gross Unrealized Losses	249	405
Total, Fair Value	82,600	164,876
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,100	5,100
Gross Unrealized Gains	68	18
Gross Unrealized Losses	197	194
Total, Fair Value	4,971	4,924
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	116	116
Gross Unrealized Gains	187	228
Total, Fair Value	$ 303	$ 344

Securities Available-for-Sale56

Securities Available-for-Sale - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)Security	Dec. 31, 2016USD ($)Security	Dec. 31, 2015USD ($)
Schedule of Available-for-sale Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	$ 1,262,708,000	$ 1,147,207,000
Securities available-for-sale are securities at fair value pledged for borrowing	67,780,000	53,396,000
Gross gains on sales of securities	47,000	64,000	$ 594,000
Proceeds from sales of securities available-for-sale	18,180,000	2,376,000	47,853,000
Securities at floating rate or adjustable rate	313,037,000
Federal Home Loan Bank [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross gains on sales of securities	$ 47,000	$ 52,000	$ 289,000
Securities AFS [Member]
Schedule of Available-for-sale Securities [Line Items]
Weighted average remaining life of investment securities available-for-sale	5 years 8 months 12 days
Number of securities, temporarily impaired for less than 12 months | Security	16	49
Number of securities, temporarily impaired for 12 months or longer | Security	28	15
Number of securities, temporarily impaired, total | Security	255	270
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	$ 216,353,000	$ 210,780,000

Securities Available-for-Sale57

Securities Available-for-Sale - Estimated Maturity Distribution of Securities Available-for-Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$ 78,343
After one but within five years, Amortized Cost	104,041
After five but within ten years, Amortized Cost	144,184
More than ten years, Amortized Cost	69,379
Non-maturing, Amortized Cost	1,616
Amortized Cost	397,563	$ 500,220
Within one year, Fair Value	78,338
After one but within five years, Fair Value	104,123
After five but within ten years, Fair Value	144,307
More than ten years, Fair Value	69,062
Non-maturing, Fair Value	1,645
Total, Fair Value	$ 397,475	$ 499,297

Securities Available-for-Sale58

Securities Available-for-Sale - Continuous Unrealized Loss Position for 12 Months or Less and 12 Months and Longer (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$ 61,156	$ 213,363
Less Than 12 Months, Unrealized Losses	193	725
12 Months or Longer, Fair Value	107,141	39,063
12 Months or Longer, Unrealized Losses	755	753
Total, Fair Value	168,297	252,426
Total, Unrealized Losses	948	1,478
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	1,984
Less Than 12 Months, Unrealized Losses	15
Total, Fair Value	1,984
Total, Unrealized Losses	15
U.S. Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		24,952
Less Than 12 Months, Unrealized Losses		48
Total, Fair Value		24,952
Total, Unrealized Losses		48
SBA Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	18,378	52,346
Less Than 12 Months, Unrealized Losses	54	145
12 Months or Longer, Fair Value	40,911	951
12 Months or Longer, Unrealized Losses	107	1
Total, Fair Value	59,289	53,297
Total, Unrealized Losses	161	146
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	40,394	135,612
Less Than 12 Months, Unrealized Losses	123	485
12 Months or Longer, Fair Value	59,336	31,504
12 Months or Longer, Unrealized Losses	194	186
Total, Fair Value	99,730	167,116
Total, Unrealized Losses	317	671
Privately Issued Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 Months or Longer, Fair Value	633	757
12 Months or Longer, Unrealized Losses	9	14
Total, Fair Value	633	757
Total, Unrealized Losses	9	14
Obligations Issued by States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
12 Months or Longer, Fair Value	4,458	4,298
12 Months or Longer, Unrealized Losses	249	405
Total, Fair Value	4,458	4,298
Total, Unrealized Losses	249	405
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	400	453
Less Than 12 Months, Unrealized Losses	1	47
12 Months or Longer, Fair Value	1,803	1,553
12 Months or Longer, Unrealized Losses	196	147
Total, Fair Value	2,203	2,006
Total, Unrealized Losses	$ 197	$ 194

Investment Securities Held-to59

Investment Securities Held-to-Maturity - Summary of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,701,233	$ 1,653,986
Gross Unrealized Gains	2,622	6,014
Gross Unrealized Losses	35,028	24,192
Estimated Fair Value	1,668,827	1,635,808
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	104,653	148,326
Gross Unrealized Gains	341	1,066
Gross Unrealized Losses	472	527
Estimated Fair Value	104,522	148,865
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	57,235	46,140
Gross Unrealized Gains	20
Gross Unrealized Losses	1,271	1,088
Estimated Fair Value	55,984	45,052
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,539,345	1,459,520
Gross Unrealized Gains	2,261	4,948
Gross Unrealized Losses	33,285	22,577
Estimated Fair Value	$ 1,508,321	$ 1,441,891

Investment Securities Held-to60

Investment Securities Held-to-Maturity - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)Security	Dec. 31, 2016USD ($)Security	Dec. 31, 2015USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	$ 1,262,708,000	$ 1,147,207,000
Realized gross gains from sales of securities held-to-maturity		12,000	$ 305,000
Proceeds from sales of securities held-to-maturity		$ 192,000	$ 3,698,000
Securities held to maturity at floating rate or adjustable rate	$ 159,000
Held-to-Maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life of investment securities held-to-maturity	4 years 3 months 19 days
Number of securities, temporarily impaired for less than 12 months | Security	117	194
Number of securities, temporarily impaired for 12 months or longer | Security	168	16
Number of securities, temporarily impaired, total | Security	404	375
Held-to-Maturity Securities [Member] | U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life	$ 20,496,000
Federal Home Loan Bank [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities pledged for borrowing at the Federal Home Loan Bank	$ 382,120,000	$ 424,353,000

Investment Securities Held-to61

Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$ 28,752
After one but within five years, Amortized Cost	1,257,279
After five but within ten years, Amortized Cost	411,916
More than ten years, Amortized Cost	3,286
Amortized Cost	1,701,233	$ 1,653,986
Within one year, Fair Value	28,726
After one but within five years, Fair Value	1,234,931
After five but within ten years, Fair Value	401,947
More than ten years, Fair Value	3,223
Estimated Fair Value	$ 1,668,827	$ 1,635,808

Investment Securities Held-to62

Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 554,195	$ 1,113,231
Less Than 12 Months, Unrealized Losses	6,301	22,340
12 Months or Longer, Fair Value	863,230	58,535
12 Months or Longer, Unrealized Losses	28,727	1,852
Total, Fair Value	1,417,425	1,171,766
Total, Unrealized Losses	35,028	24,192
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	15,257	59,219
Less Than 12 Months, Unrealized Losses	239	527
12 Months or Longer, Fair Value	14,768
12 Months or Longer, Unrealized Losses	233
Total, Fair Value	30,025	59,219
Total, Unrealized Losses	472	527
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	19,457	45,052
Less Than 12 Months, Unrealized Losses	142	1,088
12 Months or Longer, Fair Value	33,750
12 Months or Longer, Unrealized Losses	1,129
Total, Fair Value	53,207	45,052
Total, Unrealized Losses	1,271	1,088
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	519,481	1,008,960
Less Than 12 Months, Unrealized Losses	5,920	20,725
12 Months or Longer, Fair Value	814,712	58,535
12 Months or Longer, Unrealized Losses	27,365	1,852
Total, Fair Value	1,334,193	1,067,495
Total, Unrealized Losses	$ 33,285	$ 22,577

Loans - Summary of Composition

Loans - Summary of Composition of Loan Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 2,175,944	$ 1,923,933
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	732,491	696,173
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	287,731	241,357
Consumer Portfolio Segment Other Than Overdrafts [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	18,458	11,013
Municipal [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	106,599	135,418
Construction and Land Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	18,931	14,928
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	763,807	612,503
Home Equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	247,345	211,857
Overdrafts [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 582	$ 684

Loans - Additional Information

Loans - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Net of discount on loans	$ 272,000	$ 313,000
Net deferred fee	362,000	641,000
Mortgage loans sold to others	229,533,000	229,730,000
Loans held for sale	0	0
Mortgage servicing rights, net	1,525,000	1,629,000
Total recorded investment in impaired loans	7,114,000	3,830,000	$ 3,225,000
Impaired loans	6,581,000	3,105,000
Specific reserves	$ 164,000	$ 173,000

Loans - Composition of Nonaccru

Loans - Composition of Nonaccrual Loans and Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Loans on nonaccrual	$ 1,684	$ 1,084	$ 2,336
Loans 90 days past due and still accruing	0	0	0
Impaired loans on nonaccrual included above	254	304	332
Total recorded investment in impaired loans	7,114	3,830	3,225
Average recorded investment of impaired loans	5,608	3,661	4,490
Accruing troubled debt restructures	2,749	3,526	2,893
Interest income not recorded on nonaccrual loans according to their original terms	51	37	91
Interest income on nonaccrual loans actually recorded	0	0	0
Interest income recognized on impaired loans	$ 182	$ 140	$ 104

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, 2017USD ($)
Debt Disclosure [Abstract]
Beginning balance	$ 10,982
Additions	572
Repayments and Deletions	5,729
Ending balance	$ 5,825

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Allowance for loan losses, beginning of year				$ 24,406				$ 23,075	$ 24,406	$ 23,075	$ 22,318
Loans charged-off									(390)	(389)	(781)
Recoveries on loans previously charged-off									449	434	1,338
Net recoveries (charge-offs)									59	45	557
Provision charged to expense	$ 450	$ 450	$ 490	$ 400	$ 200	$ 375	$ 350	$ 450	1,790	1,375	200
Reclassification to other liabilities										(89)
Allowance for loan losses, end of year	$ 26,255				$ 24,406				$ 26,255	$ 24,406	$ 23,075

Allowance for Loan Losses - Sum

Allowance for Loan Losses - Summary of Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	$ 24,406	$ 23,075	$ 22,318
Charge-offs	(390)	(389)	(781)
Recoveries	449	434	1,338
Reclassification to other liabilities		(89)
Provision	1,790	(1,375)
Allowance for loan losses, end of year	26,255	24,406	23,075
Amount of allowance for loan losses for loans deemed to be impaired	164	173
Amount of allowance for loan losses for loans not deemed to be impaired	26,091	24,233
Loans:
Total loans, net	2,175,944	1,923,933
Loans deemed to be impaired	7,114	3,830	3,225
Loans not deemed to be impaired	2,168,830	1,920,103
Construction and Land Development [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	1,012	2,041
Reclassification to other liabilities		(5)
Provision	633	1,024
Allowance for loan losses, end of year	1,645	1,012	2,041
Amount of allowance for loan losses for loans deemed to be impaired		3
Amount of allowance for loan losses for loans not deemed to be impaired	1,645	1,009
Loans:
Total loans, net	18,931	14,928
Loans deemed to be impaired		94
Loans not deemed to be impaired	18,931	14,834
Commercial and Industrial [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	6,972	5,899
Charge-offs	(49)
Recoveries	110	132
Reclassification to other liabilities		(25)
Provision	2,618	(966)
Allowance for loan losses, end of year	9,651	6,972	5,899
Amount of allowance for loan losses for loans deemed to be impaired	7	23
Amount of allowance for loan losses for loans not deemed to be impaired	9,644	6,949
Loans:
Total loans, net	763,807	612,503
Loans deemed to be impaired	348	389
Loans not deemed to be impaired	763,459	612,114
Municipal [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	1,612	994
Provision	108	(618)
Allowance for loan losses, end of year	1,720	1,612	994
Amount of allowance for loan losses for loans not deemed to be impaired	1,720	1,612
Loans:
Total loans, net	106,599	135,418
Loans not deemed to be impaired	106,599	135,418
Commercial Real Estate [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	11,135	10,589
Reclassification to other liabilities		(9)
Provision	(1,407)	(555)
Allowance for loan losses, end of year	9,728	11,135	10,589
Amount of allowance for loan losses for loans deemed to be impaired	99	140
Amount of allowance for loan losses for loans not deemed to be impaired	9,629	10,995
Loans:
Total loans, net	732,491	696,173
Loans deemed to be impaired	2,554	3,149
Loans not deemed to be impaired	729,937	693,024
Residential Real Estate [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	1,698	1,320
Recoveries	2	6
Reclassification to other liabilities		(3)
Provision	173	(375)
Allowance for loan losses, end of year	1,873	1,698	1,320
Amount of allowance for loan losses for loans deemed to be impaired	58	7
Amount of allowance for loan losses for loans not deemed to be impaired	1,815	1,691
Loans:
Total loans, net	287,731	241,357
Loans deemed to be impaired	4,212	198
Loans not deemed to be impaired	283,519	241,159
Consumer [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	582	644
Charge-offs	(341)	(362)
Recoveries	255	296
Reclassification to other liabilities		(3)
Provision	(123)	(7)
Allowance for loan losses, end of year	373	582	644
Amount of allowance for loan losses for loans not deemed to be impaired	373	582
Loans:
Total loans, net	19,040	11,697
Loans not deemed to be impaired	19,040	11,697
Home Equity [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	1,102	1,077
Charge-offs		(27)
Recoveries	82
Reclassification to other liabilities		(44)
Provision	(195)	(96)
Allowance for loan losses, end of year	989	1,102	1,077
Amount of allowance for loan losses for loans not deemed to be impaired	989	1,102
Loans:
Total loans, net	247,345	211,857
Loans not deemed to be impaired	247,345	211,857
Unallocated [Member]
Allowance for Loan Losses:
Allowance for loan losses, beginning of year	293	511
Provision	(17)	218
Allowance for loan losses, end of year	276	293	$ 511
Amount of allowance for loan losses for loans not deemed to be impaired	$ 276	$ 293

Allowance for Loan Losses - Loa

Allowance for Loan Losses - Loans by Risk Rating (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Loans by risk rating
Financing Receivable, Net	$ 2,175,944	$ 1,923,933
Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	18,931	14,928
Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	763,807	612,503
Municipal [Member]
Loans by risk rating
Financing Receivable, Net	106,599	135,418
Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	732,491	696,173
1-3 (Pass) [Member] | Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	18,931	14,834
1-3 (Pass) [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	758,093	612,114
1-3 (Pass) [Member] | Municipal [Member]
Loans by risk rating
Financing Receivable, Net	106,599	135,418
1-3 (Pass) [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	705,235	661,271
4 (Monitor) [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	5,366
4 (Monitor) [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	24,702	31,753
Impaired [Member] | Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net		94
Impaired [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	348	389
Impaired [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	$ 2,554	$ 3,149

Allowance for Loan Losses - L70

Allowance for Loan Losses - Loans by Credit Rating (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	$ 1,074,923	$ 917,026
Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	586,000	407,515
A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	331,788	351,565
Baa1 - Baa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	148,970	154,336
Ba2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	8,165	3,610
Commercial and Industrial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	674,504	523,451
Commercial and Industrial [Member] | Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	478,905	334,674
Commercial and Industrial [Member] | A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	195,599	188,777
Municipal [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	104,799	130,190
Municipal [Member] | Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	62,029	66,245
Municipal [Member] | A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	7,635	33,365
Municipal [Member] | Baa1 - Baa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	26,970	26,970
Municipal [Member] | Ba2 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	8,165	3,610
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	295,620	263,385
Commercial Real Estate [Member] | Aaa - Aa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	45,066	6,596
Commercial Real Estate [Member] | A1 - A3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	128,554	129,423
Commercial Real Estate [Member] | Baa1 - Baa3 [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans and leases receivable net of deferred income credit quality	$ 122,000	$ 127,366

Allowance for Loan Losses - Agi

Allowance for Loan Losses - Aging of Past Due Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 5,642	$ 1,614
Non Accrual	1,684	1,084	$ 2,336
Accruing Greater Than 90 Days	0	0	$ 0
Total Past Due	7,326	2,698
Current Loans	2,168,618	1,921,235
Total loans, net	2,175,944	1,923,933
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Non Accrual		94
Accruing Greater Than 90 Days	0	0
Total Past Due		94
Current Loans	18,931	14,834
Total loans, net	18,931	14,928
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	65	37
Non Accrual	44	65
Accruing Greater Than 90 Days	0	0
Total Past Due	109	102
Current Loans	763,698	612,401
Total loans, net	763,807	612,503
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing Greater Than 90 Days	0	0
Current Loans	106,599	135,418
Total loans, net	106,599	135,418
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	672	597
Non Accrual	215	150
Accruing Greater Than 90 Days	0	0
Total Past Due	887	747
Current Loans	731,604	695,426
Total loans, net	732,491	696,173
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	4,282	245
Non Accrual	724	656
Accruing Greater Than 90 Days	0	0
Total Past Due	5,006	901
Current Loans	282,725	240,456
Total loans, net	287,731	241,357
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	5
Non Accrual	6	11
Accruing Greater Than 90 Days	0	0
Total Past Due	11	11
Current Loans	19,029	11,686
Total loans, net	19,040	11,697
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	618	735
Non Accrual	695	108
Accruing Greater Than 90 Days	0	0
Total Past Due	1,313	843
Current Loans	246,032	211,014
Total loans, net	$ 247,345	$ 211,857

Allowance for Loan Losses - Inf

Allowance for Loan Losses - Information Pertaining to Impaired Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	$ 533	$ 725
With no required reserve recorded, Unpaid Balance Principal	873	1,001
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value Recognized	413	530
With no required reserve recorded, Interest Income	25	46
With required reserve recorded, Carrying Value	6,581	3,105
With required reserve recorded, Unpaid Balance Principal	6,582	3,241
With required reserve recorded, Required Reserve	164	173
With required reserve recorded, Average Carrying Value Recognized	5,195	3,131
With required reserve recorded, Interest Income	157	94
Carrying Value	7,114	3,830	$ 3,225
Unpaid Balance Principal	7,455	4,242
With required reserve recorded, Required Reserve	164	173
Average Carrying Value Recognized	5,608	3,661	$ 4,490
Interest Income	182	140
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Required Reserve	0	0
With required reserve recorded, Carrying Value		94
With required reserve recorded, Unpaid Balance Principal		108
With required reserve recorded, Required Reserve		3
With required reserve recorded, Average Carrying Value Recognized	43	96
Carrying Value		94
Unpaid Balance Principal		108
With required reserve recorded, Required Reserve		3
Average Carrying Value Recognized	43	96
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	113	45
With no required reserve recorded, Unpaid Balance Principal	325	232
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value Recognized	54	53
With no required reserve recorded, Interest Income	4
With required reserve recorded, Carrying Value	235	344
With required reserve recorded, Unpaid Balance Principal	235	360
With required reserve recorded, Required Reserve	7	23
With required reserve recorded, Average Carrying Value Recognized	318	360
With required reserve recorded, Interest Income	12	18
Carrying Value	348	389
Unpaid Balance Principal	560	592
With required reserve recorded, Required Reserve	7	23
Average Carrying Value Recognized	372	413
Interest Income	16	18
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	420	590
With no required reserve recorded, Unpaid Balance Principal	548	590
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value Recognized	286	375
With no required reserve recorded, Interest Income	21	39
With required reserve recorded, Carrying Value	2,134	2,559
With required reserve recorded, Unpaid Balance Principal	2,135	2,665
With required reserve recorded, Required Reserve	99	140
With required reserve recorded, Average Carrying Value Recognized	2,501	2,324
With required reserve recorded, Interest Income	72	71
Carrying Value	2,554	3,149
Unpaid Balance Principal	2,683	3,255
With required reserve recorded, Required Reserve	99	140
Average Carrying Value Recognized	2,787	2,699
Interest Income	93	110
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value		90
With no required reserve recorded, Unpaid Balance Principal		179
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value Recognized	73	102
With no required reserve recorded, Interest Income		7
With required reserve recorded, Carrying Value	4,212	108
With required reserve recorded, Unpaid Balance Principal	4,212	108
With required reserve recorded, Required Reserve	58	7
With required reserve recorded, Average Carrying Value Recognized	2,333	323
With required reserve recorded, Interest Income	73	5
Carrying Value	4,212	198
Unpaid Balance Principal	4,212	287
With required reserve recorded, Required Reserve	58	7
Average Carrying Value Recognized	2,406	425
Interest Income	73	12
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
With required reserve recorded, Average Carrying Value Recognized		28
Average Carrying Value Recognized		$ 28

Allowance for Loan Losses - Add

Allowance for Loan Losses - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)Contracts
Financing Receivable, Allowance for Credit Losses [Line Items]
Commitment to lend additional funds to TDR borrowers	$ 0
Troubled debt restructurings, subsequently defaulted	$ 0	$ 0
Commercial Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Number of troubled debt restructurings | Contracts		1
Pre-modification outstanding recorded investment		$ 2,091,000
Post-modification outstanding recorded investment		2,091,000
Financing allowance reduction in principal payments		16,000
Financing allowance reduction in interest payments		$ 5,000

Bank Premises and Equipment - S

Bank Premises and Equipment - Schedule of Bank Premises and Equipment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	$ 64,696	$ 61,823
Accumulated depreciation and amortization	(41,169)	(38,406)
Total	23,527	23,417
Land [Member]
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	3,850	3,478
Bank Premises [Member]
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	21,055	19,272
Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	27,117	26,271
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Bank premises and equipment, gross	$ 12,674	$ 12,802
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Lease expense	$ 2,608,000	$ 2,834,000	$ 2,755,000
Lease rental income	$ 321,000	318,000	314,000
Operating lease premises and equipment lease expiration year, maximum	2,028
Depreciation of leased property	$ 3,208,000	3,099,000	2,728,000
Marshall M Sloane [Member]
Property, Plant and Equipment [Line Items]
Lease expense	$ 439,000	$ 424,000	$ 413,000

Bank Premises and Equipment -76

Bank Premises and Equipment - Summary of Future Minimum Rental Commitments for Non-Cancelable Operating Leases (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Property, Plant and Equipment [Abstract]
2,018	$ 2,309
2,019	2,149
2,020	1,856
2,021	1,382
2,022	1,022
Thereafter	1,942
Future minimum rental commitments for non-cancelable operating leases, total	$ 10,660

Goodwill and Identifiable Int77

Goodwill and Identifiable Intangible Assets - Carrying Amount of Goodwill and Intangibles (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Intangible Assets And Goodwill [Line Items]
Goodwill, Beginning Balance	$ 2,714	$ 2,714	$ 2,714
Total, Beginning Balance	4,343	4,019
Additions	276	708
Amortization Expense	(380)	(384)
Total, Ending Balance	4,239	4,343
Mortgage Servicing Rights [Member]
Intangible Assets And Goodwill [Line Items]
Beginning Balance	1,629	1,305
Additions	276	708
Amortization Expense	(380)	(384)
Ending Balance	$ 1,525	$ 1,629

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, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	$ 397,475	$ 499,297
Fair Value Measurements, Level 1 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	303	344
Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	314,572	334,077
Fair Value Measurements, Level 3 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	82,600	164,876
Financial Instruments Measured at Fair Value on a Non-recurring Basis
Impaired Loans	246	260
U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	1,984	2,000
U.S. Treasury [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	1,984	2,000
SBA Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	80,950	57,767
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	80,950	57,767
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	225,775	243,325
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	225,775	243,325
Privately Issued Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	892	1,109
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	892	1,109
Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	82,600	164,876
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	82,600	164,876
Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	4,971	4,924
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	4,971	4,924
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	303	344
Equity Securities [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	303	344
U.S. Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS		24,952
U.S. Government Sponsored Enterprises [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS		24,952
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	397,475	499,297
Financial Instruments Measured at Fair Value on a Non-recurring Basis
Impaired Loans	246	260
Carrying Value [Member] | U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	1,984	2,000
Carrying Value [Member] | SBA Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	80,950	57,767
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	225,775	243,325
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	892	1,109
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	82,600	164,876
Carrying Value [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	4,971	4,924
Carrying Value [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	$ 303	344
Carrying Value [Member] | U.S. Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS		$ 24,952

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Specific adjustments to impaired loans recognized	$ 3,000	$ (135,000)
Transfers between level 1, 2 and 3	0	0
Liabilities measured at fair value on a recurring or nonrecurring basis	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	82,849,000	165,281,000
Unrealized loss	$ 249,000	$ 405,000

Fair Value Measurements - Asset

Fair Value Measurements - Assets Measured at Fair Value (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Securities AFS [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Fair Value	$ 82,600	$ 164,876
Valuation Technique	Discounted cash flow
Unobservable Input	Discount rate
Impaired Loans [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Fair Value	$ 246	$ 260
Valuation Technique	Appraisal of collateral
Unobservable Input	Appraisal adjustments
Minimum [Member] | Securities AFS [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unobservable Input Value or Range	1.00%	0.00%
Minimum [Member] | Impaired Loans [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unobservable Input Value or Range	0.00%	0.00%
Maximum [Member] | Securities AFS [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unobservable Input Value or Range	3.50%	1.00%
Maximum [Member] | Impaired Loans [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Unobservable Input Value or Range	30.00%	30.00%

Fair Value Measurements - Chang

Fair Value Measurements - Changes in Level 3 Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 164,876	$ 156,997
Purchases	99,136	216,646
Maturities/redemptions	(181,394)	(209,027)
Amortization	(179)	(218)
Changes in fair value	161	478
Ending Balance	82,600	164,876
Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	4,298	3,820
Changes in fair value	161	478
Ending Balance	4,459	4,298
Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	160,578	153,140
Purchases	99,136	216,646
Maturities/redemptions	(181,394)	(208,990)
Amortization	(179)	(218)
Ending Balance	$ 78,141	160,578
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance		37
Maturities/redemptions		$ (37)

Fair Value Measurements - Ass82

Fair Value Measurements - Assets Measured at Fair Value (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017
Municipal [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities maturity period	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, 2017	Dec. 31, 2016
Maturities of Time Deposits [Abstract]
Within one year	$ 436,911	$ 262,406
Over one year to two years	121,802	87,952
Over two years to three years	30,098	83,067
Over three years to five years	36,550	44,934
Time Deposits, Total	$ 625,361	$ 478,359
Within one year, Percent	70.00%	55.00%
Over one year to two years, Percent	19.00%	18.00%
Over two years to three years, Percent	5.00%	17.00%
Over three years to five years, Percent	6.00%	10.00%
Total, Percent	100.00%	100.00%

Deposits - Additional Informati

Deposits - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Banking and Thrift [Abstract]
Time deposits 250000 or more	$ 345,183,000	$ 250,476,000
Deposits to related parties	$ 35,486,000	$ 26,191,000

Securities Sold under Agreeme85

Securities Sold under Agreements to Repurchase - Summary of Securities Sold Under Agreements to Repurchase (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Banking and Thrift [Abstract]
Amount outstanding at December 31	$ 158,990,000	$ 182,280,000	$ 197,850,000
Weighted average rate at December 31	0.32%	0.21%	0.21%
Maximum amount outstanding at any month end	$ 228,848,000	$ 241,110,000	$ 299,890,000
Daily average balance outstanding during the year	$ 189,684,000	$ 222,956,000	$ 245,276,000
Weighted average rate during the year	0.26%	0.21%	0.20%

Securities Sold under Agreeme86

Securities Sold under Agreements to Repurchase - Additional Information (Detail) - U.S. Government Sponsored Enterprises [Member] - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
U.S. Government Sponsored Enterprise securities with a total amortized cost	$ 162,927,000	$ 183,829,000	$ 199,152,000
Fair value of the collateral repurchase agreement	$ 159,051,000	$ 182,074,000	$ 197,318,000

Other Borrowed Funds and Subo87

Other Borrowed Funds and Subordinated Debentures - Summary of Other Borrowed Funds and Subordinated Debentures (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Amount outstanding at December 31	$ 383,861,000	$ 329,083,000	$ 404,083,000
Weighted average rate at December 31	2.26%	2.39%	2.29%
Maximum amount outstanding at any month end	$ 491,583,000	$ 467,083,000	$ 521,583,000
Daily average balance outstanding during the year	$ 309,102,000	$ 357,974,000	$ 374,109,000
Weighted average rate during the year	2.42%	2.48%	2.38%

Other Borrowed Funds and Subo88

Other Borrowed Funds and Subordinated Debentures - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Dec. 31, 2004	May 31, 1998	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
FHLBB advances			$ 20,000,000	$ 45,000,000	$ 55,000,000
Subordinated debt securities issued			36,083,000	$ 36,083,000
Century Bancorp Capital Trust II [Member]
Debt Instrument [Line Items]
Subordinated debt securities issued	$ 36,083,000
Subordinated debt securities due year	2,034
Federal Home Loan Bank Borrowings [Member]
Debt Instrument [Line Items]
Bank's remaining term borrowing capacity at the FHLBB			127,631,000
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
Period of dividend			10 years
LIBOR rate			Three-month 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			3.46%	2.83%
Subordinated Debt [Member] | Century Bancorp Capital Trust [Member]
Debt Instrument [Line Items]
Subordinated debt securities issued		$ 29,639,000
Subordinated debt securities due year		2,029
Other Borrowed Funds [Member]
Debt Instrument [Line Items]
Federal funds purchased			$ 0	$ 0
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%
Cumulative Preferred Stock Subject to Mandatory Redemption [Member] | Subordinated Debt [Member] | Century Bancorp Capital Trust [Member]
Debt Instrument [Line Items]
Shares of cumulative trust preferred securities			2,875,000
Liquidation value of shares of cumulative trust preferred securities			$ 10
Trust preferred securities annual dividend rate			8.30%

Other Borrowed Funds and Subo89

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal Home Loan Bank, Advances, Fiscal Year Maturity [Abstract]
Within one year	$ 164,500	$ 77,500	$ 100,000
Over one year to two years	63,000	54,500	57,500
Over two years to three years	28,000	58,000	54,500
Over three years to five years	28,500	58,000	91,000
Over five years	63,778	45,000	65,000
Federal Home Loan Bank, Advances, Total	$ 347,778	$ 293,000	$ 368,000
Within one year, Weighted Average Rate	1.82%	2.21%	1.89%
Over one year to two years, Weighted Average Rate	2.17%	2.25%	2.72%
Over two years to three years, Weighted Average Rate	2.29%	1.87%	2.25%
Over three years to five years, Weighted Average Rate	3.19%	2.68%	1.85%
Over five years, Weighted Average Rate	2.38%	2.85%	3.23%
Weighted Average Rate, Total	2.13%	2.34%	2.30%

Reclassifications Out of Accu90

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities									$ 47	$ 64	$ 594
Provision for income taxes	$ (9,645)	$ (617)	$ (552)	$ (144)	$ 394	$ 52	$ (20)	$ (64)	(10,958)	362	(533)
Net income	$ 25	$ 8,023	$ 8,157	$ 6,096	$ 6,500	$ 6,486	$ 6,241	$ 5,307	22,301	24,534	23,021
Salaries and employee benefits									(41,913)	(40,048)	$ (38,596)
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									47	52
Provision for income taxes									(19)	(20)
Net income									28	32
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									(2,292)	(4,317)
Provision for income taxes									1,258	1,505
Net income									(1,034)	(2,812)
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									(10)	(10)
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									(1,540)	(1,606)
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									(1,550)	(1,616)
Provision for income taxes									619	646
Net income									$ (931)	$ (970)

Earnings per Share ("EPS") - Ad

Earnings per Share ("EPS") - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 25	$ 8,023	$ 8,157	$ 6,096	$ 6,500	$ 6,486	$ 6,241	$ 5,307	$ 22,301	$ 24,534	$ 23,021
Class A Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income									$ 17,526	$ 19,270	$ 18,081
Weighted average shares outstanding, basic	3,605,829	3,605,829	3,603,729	3,600,729	3,600,729	3,600,729	3,600,729	3,600,729	3,604,029	3,600,729	3,600,729
Basic earnings per share	$ 0.01	$ 1.75	$ 1.78	$ 1.33	$ 1.42	$ 1.41	$ 1.36	$ 1.16	$ 4.86	$ 5.35	$ 5.02
Net income									$ 17,526	$ 19,270	$ 18,081
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		$ 1.44	$ 1.47	$ 1.09	$ 1.17	$ 1.16	$ 1.12	$ 0.95	$ 4.01	$ 4.41	$ 4.13
Class B Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income									$ 4,775	$ 5,264	$ 4,940
Weighted average shares outstanding, basic	1,962,080	1,962,080	1,964,180	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,963,880	1,967,180	1,967,180
Basic earnings per share		$ 0.87	$ 0.89	$ 0.66	$ 0.71	$ 0.71	$ 0.68	$ 0.58	$ 2.43	$ 2.68	$ 2.51
Net income									$ 4,775	$ 5,264	$ 4,940
Weighted average shares outstanding, diluted	1,962,080	1,962,080	1,964,180	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,963,880	1,967,180	1,967,180
Diluted earnings per share		$ 0.87	$ 0.89	$ 0.66	$ 0.71	$ 0.71	$ 0.68	$ 0.58	$ 2.43	$ 2.68	$ 2.51

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Stock Option Plan [Member]
Class of Stock [Line Items]
Number of outstanding options	0	0
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%
Minimum [Member] | Stock Option Plan [Member]
Class of Stock [Line Items]
Share based compensation of option price based on fair value	85.00%
Maximum [Member] | Stock Option Plan [Member]
Class of Stock [Line Items]
Share based compensation number of share authorized	150,000
Stock option exercisable period	10 years

Stockholders' Equity - Summary

Stockholders' Equity - Summary of the Bank's Actual Capital Amounts and Ratios (Detail) - Century Bancorp Capital Trust II [Member] - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 329,666	$ 293,143
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	303,411	268,737
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	303,411	268,737
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	$ 303,411	$ 268,737
Total Capital (to Risk-Weighted Assets), Ratio	12.70%	12.27%
Tier 1 Capital (to Risk-Weighted Assets), Ratio	11.69%	11.25%
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Ratio	11.69%	11.25%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	6.55%	6.02%
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	$ 207,707	$ 191,081
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	155,780	143,311
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	116,835	107,483
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	$ 185,199	$ 178,469
Total Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	8.00%	8.00%
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	6.00%	6.00%
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	4.00%	4.00%
Total Capital (to Risk-Weighted Assets), Well Capitalized Amount	$ 259,633	$ 238,851
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	207,707	191,081
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	168,762	155,253
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$ 231,499	$ 223,086
Total Capital (to Risk-Weighted Assets), Well Capitalized Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	8.00%	8.00%
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	5.00%	5.00%

Stockholders' Equity - Summar95

Stockholders' Equity - Summary of the Company's Actual Capital Amounts and Ratios (Detail) - Company [Member] - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements for Mortgage Companies [Line Items]
Total Capital (to Risk-Weighted Assets), Actual Amount	$ 341,033	$ 305,065
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	314,778	280,659
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	279,778	249,753
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	$ 314,778	$ 280,659
Total Capital (to Risk-Weighted Assets), Ratio	13.05%	12.72%
Tier 1 Capital (to Risk-Weighted Assets), Ratio	12.05%	11.70%
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Ratio	10.71%	10.41%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	6.78%	6.28%
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	$ 209,049	$ 191,904
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	156,787	143,928
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	117,590	107,946
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	$ 185,657	$ 178,903
Total Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	8.00%	8.00%
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	6.00%	6.00%
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	4.00%	4.00%
Total Capital (to Risk-Weighted Assets), Well Capitalized Amount	$ 261,312	$ 239,880
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	209,049	191,904
Common Equity Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	169,853	155,922
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$ 232,072	$ 223,628
Total Capital (to Risk-Weighted Assets), Well Capitalized Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	8.00%	8.00%
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	5.00%	5.00%

Income Taxes - Summary of Curre

Income Taxes - Summary of Current and Deferred Components of Income Tax (Benefit) Expense (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current expense:
Federal									$ 3,628	$ 3,875	$ 3,393
State									412	439	399
Total current expense									4,040	4,314	3,792
Deferred (benefit) expense:
Federal									6,496	(4,450)	(3,098)
State									422	(334)	(161)
Valuation Allowance										108
Total deferred expense (benefit)									6,918	(4,676)	(3,259)
Provision for income taxes	$ 9,645	$ 617	$ 552	$ 144	$ (394)	$ (52)	$ 20	$ 64	$ 10,958	$ (362)	$ 533

Income Taxes - Income Tax Accou

Income Taxes - Income Tax Accounts Included in Other Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Currently receivable	$ 15,940	$ 633
Deferred income tax asset, net	20,892	43,129
Total	$ 36,832	$ 43,762

Income Taxes - Summary of Diffe

Income Taxes - Summary of Differences between Income Tax (Benefit) Expense at the Statutory Federal Income Tax Rate and Total Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rates									$ 11,308	$ 8,218	$ 8,008
State income tax, net of federal income tax benefit									550	69	157
Insurance income									(371)	(406)	(375)
Effect of tax-exempt interest									(8,683)	(8,259)	(6,915)
Net tax credit									(341)	(395)	(460)
Valuation allowance										108
Deferred tax remeasurement									8,448
Other									47	303	118
Provision for income taxes	$ 9,645	$ 617	$ 552	$ 144	$ (394)	$ (52)	$ 20	$ 64	$ 10,958	$ (362)	$ 533
Effective tax rate									32.95%	(1.50%)	2.30%

Income Taxes - Gross Deferred I

Income Taxes - Gross Deferred Income Tax Assets and Gross Deferred Income Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets:
Allowance for loan losses	$ 7,855	$ 10,419
AMT credit	14,001	10,234
Deferred compensation	7,555	9,684
Pension and SERP liability	8,436	11,320
Unrealized losses on securities transferred to held-to-maturity	1,303	3,161
Depreciation	631	968
Accrued bonus		612
Unrealized (gains) losses on securities available-for-sale	14	357
Charitable contributions carryforward	442	266
Acquisition premium	17	128
Nonaccrual interest	97	125
Limited partnerships	21	30
Investments write down	17	26
Other	173	220
Gross deferred income tax asset	26,561	47,550
Valuation allowance	(108)	(108)
Gross deferred income tax asset, net of valuation allowance	26,453	47,442
Deferred income tax liabilities:
Pension asset (liability)	(4,403)	(3,662)
Deferred origination costs	(481)
Prepaid expenses	(248)
Mortgage servicing rights	(429)	(651)
Gross deferred income tax liability	(5,561)	(4,313)
Deferred income tax asset, net	$ 20,892	$ 43,129

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Valuation allowance on charitable contribution carryforward			$ 108,000
Federal tax rate			34.00%
Additional tax expense recorded	$ 3,800,000		$ 8,448,000
Alternative minimum tax credit			$ 14,001,000	$ 10,234,000
Scenario, Forecast [Member]
Income Taxes [Line Items]
Federal tax rate		21.00%
Minimum [Member]
Income Taxes [Line Items]
Charitable contribution carryforward period			3 years
Maximum [Member]
Income Taxes [Line Items]
Charitable contribution carryforward period			4 years

Employee Benefits - Additional

Employee Benefits - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Benefits expected to be paid in 2018	$ 1,530,000
Benefits expected to be paid in 2019	1,569,000
Benefits expected to be paid in 2020	1,732,000
Benefits expected to be paid in 2021	1,832,000
Benefits expected to be paid in 2022	1,988,000
Aggregate benefits expected to paid	11,531,000
Transfers in or out of level 3	$ 0	$ 0
Voluntary contribution of employees	33.30%
Contributions matched by compensation contribution	6.00%
Voluntary contribution of employees, amount	$ 445,000	418,000	$ 403,000
Discretionary bonus expense	1,859,000	1,418,000	$ 1,178,000
Change in Assumptions for Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in periodic plan cost		$ 859,000
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefits expected to be paid in 2018	2,096,000
Benefits expected to be paid in 2019	2,068,000
Benefits expected to be paid in 2020	2,002,000
Benefits expected to be paid in 2021	1,939,000
Benefits expected to be paid in 2022	1,966,000
Aggregate benefits expected to paid	$ 13,107,000
Equity Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	40.00%
Equity Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	64.00%
Fixed Income Funds [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	15.00%
Fixed Income Funds [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	25.00%
Hedge Funds [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	25.00%
Hedge Funds [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Allocation mix for the common and collective trust portfolio	41.00%

Employee Benefits - Fair Value

Employee Benefits - Fair Value of Plan Assets and Major Categories (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Investments measured at net asset value, Percentage	60.00%	59.80%
Fair value of plan assets, Percentage	100.00%	100.00%
Total, investments measured at net asset value	$ 29,035	$ 22,394
Total, fair value of plan assets	$ 48,422	$ 37,447
Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	40.00%	40.20%
Total, fair value of plan assets	$ 19,387	$ 15,053
Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	19,387	15,053
Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 19,387	$ 15,053
Collective Funds [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	3.60%	6.90%
Total, fair value of plan assets	$ 1,741	$ 2,600
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	$ 1,741	$ 2,600
Equity Securities [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	10.70%	19.70%
Total, fair value of plan assets	$ 5,195	$ 7,363
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	$ 5,195	$ 7,363
Diversified [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	17.80%	12.30%
Total, fair value of plan assets	$ 8,615	$ 4,615
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	$ 8,615	$ 4,615
Short-Term Investments [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	7.90%	1.30%
Total, fair value of plan assets	$ 3,836	$ 475
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	$ 3,836	$ 475

Employee Benefits - Schedule of

Employee Benefits - Schedule of Investments Measured Using Net Asset Value Per Share Practical Expedient (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	60.00%	59.80%
Investments, Fair Value	$ 29,035	$ 22,394
Collective Funds [Member] | Equity [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	31.60%	24.10%
Investments, Fair Value	$ 15,304	$ 9,013
Collective Funds [Member] | Diversified [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	0.70%	0.10%
Investments, Fair Value	$ 344	$ 47
Collective Funds [Member] | US Debt Securities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	9.40%	11.30%
Investments, Fair Value	$ 4,569	$ 4,241
Collective Funds [Member] | International Equities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	9.10%	9.80%
Investments, Fair Value	$ 4,419	$ 3,684
Limited Partnerships [Member] | Emerging Markets [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	2.80%	0.00%
Investments, Fair Value	$ 1,353
Limited Partnerships [Member] | Multi-strategy [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	1.50%	7.00%
Investments, Fair Value	$ 705	$ 2,623
Hedge Funds [Member] | Hedge Funds, Multi-strategy [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	3.50%	5.60%
Investments, Fair Value	$ 1,674	$ 2,082
Hedge Funds [Member] | Global Opportunities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	0.70%	1.10%
Investments, Fair Value	$ 345	$ 422
Hedge Funds [Member] | Private Investment Entities and/or Separately Managed Accounts [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Investments, Percent	0.70%	0.80%
Investments, Fair Value	$ 322	$ 282

Employee Benefits - Components

Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets
Balance at beginning of year	$ 37,447
Balance at end of year	48,422	$ 37,447
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Total recognized in other comprehensive income	18,117	16,733
Defined Benefit Pension Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	42,255	38,597
Service cost	1,241	1,273
Interest cost	1,450	1,358
Actuarial (gain)/loss	3,456	2,593
Benefits paid	(1,337)	(1,566)
Projected benefit obligation at end of year	47,065	42,255
Change in plan assets
Balance at beginning of year	37,447	33,717
Actual return on plan assets	5,312	3,221
Employer contributions	7,000	2,075
Benefits paid	(1,337)	(1,566)
Balance at end of year	48,422	37,447
(Unfunded) Funded status	1,357	(4,808)
Accumulated benefit obligation	$ 47,065	$ 42,255
Weighted-average assumptions as of December 31
Discount rate - Liability	3.49%	3.99%
Discount rate - Expense	3.99%	4.18%
Expected return on plan assets	8.00%	8.00%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	$ 1,241	$ 1,273
Interest cost	1,450	1,358
Expected return on plan assets	(2,985)	(2,776)
Recognized prior service cost	(104)	(104)
Recognized net losses	903	801
Net periodic cost (benefit)	505	552
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost	104	104
Net (gain) loss	409	1,347
Total recognized in other comprehensive income	513	1,451
Total recognized in net periodic benefit cost and other comprehensive income	1,018	2,003
Supplemental Insurance/ Retirement Plan [Member]
Change projected in benefit obligation
Benefit obligation at beginning of year	38,610	38,204
Service cost	1,582	1,820
Interest cost	1,382	1,334
Actuarial (gain)/loss	2,087	(1,653)
Benefits paid	(1,082)	(1,095)
Projected benefit obligation at end of year	42,579	38,610
Change in plan assets
Benefits paid	(1,082)	(1,095)
(Unfunded) Funded status	(42,579)	(38,610)
Accumulated benefit obligation	$ 40,375	$ 36,392
Weighted-average assumptions as of December 31
Discount rate - Liability	3.42%	3.85%
Discount rate - Expense	3.85%	4.01%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost
Service cost	$ 1,582	$ 1,820
Interest cost	1,382	1,334
Recognized prior service cost	114	114
Recognized net losses	636	805
Net periodic cost (benefit)	3,714	4,073
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost	(114)	(114)
Net (gain) loss	1,752	(2,458)
Total recognized in other comprehensive income	1,638	(2,572)
Total recognized in net periodic benefit cost and other comprehensive income	$ 5,352	$ 1,501

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	$ (435)	$ (445)
Net actuarial loss	(29,576)	(27,415)
Total	(30,011)	(27,860)
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(104)	(104)
Net actuarial loss	(3,456)	(2,593)
Total	(47,065)	(42,255)	$ (38,597)
Defined Benefit Pension Plan [Member] | Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	100	204
Net actuarial loss	(14,408)	(13,999)
Total	(14,308)	(13,795)
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	114	114
Net actuarial loss	(2,087)	1,653
Total	(42,579)	(38,610)	$ (38,204)
Supplemental Insurance/ Retirement Plan [Member] | Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(535)	(649)
Net actuarial loss	(15,168)	(13,416)
Total	$ (15,703)	$ (14,065)

Employee Benefits - Summary 106

Employee Benefits - Summary of Accumulated Other Comprehensive Loss Expected to be Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Amortization of loss to be recognized in 2018	$ 2,315	$ 297	$ 2,890
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost to be recognized in 2018	(100)
Amortization of loss to be recognized in 2018	904
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost to be recognized in 2018	114
Amortization of loss to be recognized in 2018	$ 707

Financial Instruments with O107

Financial Instruments with Off-Balance-Sheet Risk - Summary of Financial Instruments with Off-Balance-Sheet Risk (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	$ 5,520	$ 6,796
Commitments to Originate 1-4 Family Mortgages [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	5,748	13,877
Unused Lines of Credit [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	434,618	362,357
Unadvanced Portions of Construction Loans [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	15,152	22,049
Unadvanced Portions of Other Loans [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	$ 35,602	$ 52,224

Other Operating Expenses - Summ

Other Operating Expenses - Summary of Other Operating Expenses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Income and Expenses [Abstract]
Marketing	$ 2,315	$ 2,185	$ 1,849
Software maintenance/amortization	1,859	1,863	1,670
Legal and audit	1,543	1,255	1,269
Contributions	993	789	690
Processing services	1,160	1,040	1,002
Consulting	1,199	1,168	1,050
Postage and delivery	966	987	905
Supplies	945	948	965
Telephone	1,020	1,032	804
Directors' fees	440	413	377
Insurance	308	323	301
Other	1,845	1,812	1,826
Total	$ 14,593	$ 13,815	$ 12,708

Fair Values of Financial Ins109

Fair Values of Financial Instruments - Carrying Amount and Fair Value of Company's Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Securities held-to-maturity	$ 1,701,233	$ 1,653,986
Loans	2,149,689	1,899,527
Financial liabilities:
Time deposits	625,361	478,359
Other borrowed funds	347,778	293,000
Subordinated debentures	36,083	36,083
Carrying Value [Member]
Financial assets:
Securities held-to-maturity	1,701,233	1,653,986
Loans	2,149,689	1,899,527
Financial liabilities:
Time deposits	625,361	478,359
Other borrowed funds	347,778	293,000
Subordinated debentures	36,083	36,083
Estimated Fair Value [Member]
Financial assets:
Securities held-to-maturity	1,668,827	1,635,808
Loans	2,094,517	1,873,703
Financial liabilities:
Time deposits	627,517	480,133
Other borrowed funds	349,364	294,940
Subordinated debentures	36,083	36,083
Fair Value Measurements, Level 2 Inputs [Member]
Financial assets:
Securities held-to-maturity	1,668,827	1,635,808
Financial liabilities:
Time deposits	627,517	480,133
Other borrowed funds	349,364	294,940
Fair Value Measurements, Level 3 Inputs [Member]
Financial assets:
Loans	2,094,517	1,873,703
Financial liabilities:
Subordinated debentures	$ 36,083	$ 36,083

Quarterly Results of Operati110

Quarterly Results of Operations - Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information [Line Items]
Interest income	$ 29,470	$ 28,521	$ 28,806	$ 26,639	$ 24,689	$ 25,005	$ 23,742	$ 23,263	$ 113,436	$ 96,699	$ 90,093
Interest expense	7,768	7,168	6,701	6,183	5,927	5,791	5,486	5,413	27,820	22,617	20,134
Net interest income	21,702	21,353	22,105	20,456	18,762	19,214	18,256	17,850	85,616	74,082	69,959
Provision for loan losses	450	450	490	400	200	375	350	450	1,790	1,375	200
Net interest income after provision for loan losses	21,252	20,903	21,615	20,056	18,562	18,839	17,906	17,400	83,826	72,707	69,759
Other operating income	4,410	3,942	4,291	3,909	3,700	4,225	4,643	3,654	16,552	16,222	15,993
Operating expenses	15,992	16,205	17,197	17,725	16,156	16,630	16,288	15,683	67,119	64,757	62,198
Income before income taxes	9,670	8,640	8,709	6,240	6,106	6,434	6,261	5,371	33,259	24,172	23,554
Provision for income taxes	9,645	617	552	144	(394)	(52)	20	64	10,958	(362)	533
Net income	$ 25	$ 8,023	$ 8,157	$ 6,096	$ 6,500	$ 6,486	$ 6,241	$ 5,307	$ 22,301	$ 24,534	$ 23,021
Class A Common Stock [Member]
Share data:
Average shares outstanding, basic	3,605,829	3,605,829	3,603,729	3,600,729	3,600,729	3,600,729	3,600,729	3,600,729	3,604,029	3,600,729	3,600,729
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	$ 0.01	$ 1.75	$ 1.78	$ 1.33	$ 1.42	$ 1.41	$ 1.36	$ 1.16	$ 4.86	$ 5.35	$ 5.02
Earnings per share, diluted		$ 1.44	$ 1.47	$ 1.09	$ 1.17	$ 1.16	$ 1.12	$ 0.95	$ 4.01	$ 4.41	$ 4.13
Class B Common Stock [Member]
Share data:
Average shares outstanding, basic	1,962,080	1,962,080	1,964,180	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,963,880	1,967,180	1,967,180
Average shares outstanding, diluted	1,962,080	1,962,080	1,964,180	1,967,180	1,967,180	1,967,180	1,967,180	1,967,180	1,963,880	1,967,180	1,967,180
Earnings per share, basic		$ 0.87	$ 0.89	$ 0.66	$ 0.71	$ 0.71	$ 0.68	$ 0.58	$ 2.43	$ 2.68	$ 2.51
Earnings per share, diluted		$ 0.87	$ 0.89	$ 0.66	$ 0.71	$ 0.71	$ 0.68	$ 0.58	$ 2.43	$ 2.68	$ 2.51

Parent Company Financial Sta111

Parent Company Financial Statements - Balance Sheets of Parent Company (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Cash	$ 77,199	$ 62,400
Other assets	124,260	116,360
Total assets	4,785,572	4,462,608
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	4,525,275	4,222,567
Subordinated debentures	36,083	36,083
Stockholders' equity	260,297	240,041	$ 214,544	$ 192,500
Total liabilities and stockholders' equity	4,785,572	4,462,608
Century Bancorp, Inc. [Member]
ASSETS
Cash	1,981	2,768
Investment in subsidiary, at equity	283,881	263,070
Other assets	16,833	10,335
Total assets	302,695	276,173
LIABILITIES AND STOCKHOLDERS' EQUITY:
Liabilities	6,315	49
Subordinated debentures	36,083	36,083
Stockholders' equity	260,297	240,041
Total liabilities and stockholders' equity	$ 302,695	$ 276,173

Parent Company Financial Sta112

Parent Company Financial Statements - Statements of Income of Parent Company (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income:
Other income									$ 3,906	$ 3,441	$ 3,042
Total interest income	$ 29,470	$ 28,521	$ 28,806	$ 26,639	$ 24,689	$ 25,005	$ 23,742	$ 23,263	113,436	96,699	90,093
Interest expense	7,768	7,168	6,701	6,183	5,927	5,791	5,486	5,413	27,820	22,617	20,134
Operating expenses	15,992	16,205	17,197	17,725	16,156	16,630	16,288	15,683	67,119	64,757	62,198
Income before income taxes	9,670	8,640	8,709	6,240	6,106	6,434	6,261	5,371	33,259	24,172	23,554
Benefit from income taxes	9,645	617	552	144	(394)	(52)	20	64	10,958	(362)	533
Net income	$ 25	$ 8,023	$ 8,157	$ 6,096	$ 6,500	$ 6,486	$ 6,241	$ 5,307	22,301	24,534	23,021
Century Bancorp, Inc. [Member]
Income:
Dividends from subsidiary									2,500	2,000	1,500
Interest income from deposits in bank									1	3	13
Other income									34	28	24
Total interest income									2,535	2,031	1,537
Interest expense									1,121	937	792
Operating expenses									209	220	212
Income before income taxes									1,205	874	533
Benefit from income taxes									(440)	(383)	(328)
Income before equity in undistributed income of subsidiary									1,645	1,257	861
Equity in undistributed income of subsidiary									20,656	23,277	22,160
Net income									$ 22,301	$ 24,534	$ 23,021

Parent Company Financial Sta113

Parent Company Financial Statements - Statements of Cash Flows of Parent Company (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 25	$ 8,023	$ 8,157	$ 6,096	$ 6,500	$ 6,486	$ 6,241	$ 5,307	$ 22,301	$ 24,534	$ 23,021
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization									3,047	3,561	3,296
Increase in other assets									(16,195)	(2,953)	(10,862)
Net cash provided by (used in) operating activities									21,701	22,006	11,053
CASH FLOWS FROM FINANCING ACTIVITIES:
Cash dividends paid									(2,200)	(2,201)	(2,200)
Net cash provided by (used in) financing activities									293,037	485,387	293,259
Net increase (decrease) in cash and cash equivalents									120,279	15,427	(84,633)
Century Bancorp, Inc. [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income									22,301	24,534	23,021
Adjustments to reconcile net income to net cash provided by operating activities
Undistributed income of subsidiary									(20,656)	(23,277)	(22,160)
Depreciation and amortization											3
Increase in other assets									(6,498)	(1,527)	(1,112)
Decrease in liabilities									6,266	9	4
Net cash provided by (used in) operating activities									1,413	(261)	(244)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net proceeds from the exercise of stock options									0	0	0
Cash dividends paid									(2,200)	(2,201)	(2,200)
Net cash provided by (used in) financing activities									(2,200)	(2,201)	(2,200)
Net increase (decrease) in cash and cash equivalents									(787)	(2,462)	(2,444)
Cash and cash equivalents at beginning of year				$ 2,768				$ 5,230	2,768	5,230	7,674
Cash and cash equivalents at end of year	$ 1,981				$ 2,768				$ 1,981	$ 2,768	$ 5,230