Cambridge Bancorp (CATC) 10-Q 8 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	CATC
Entity Registrant Name	CAMBRIDGE BANCORP
Entity Central Index Key	711,772
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		4,105,500

CONSOLIDATED BALANCE SHEETS (UN

CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Cash and cash equivalents	$ 16,431	$ 103,591
Investment securities
Available for sale, at fair value (amortized cost $186,715 and $208,911, respectively)	180,363	205,017
Held to maturity, at amortized cost (fair value $266,685 and $233,554, respectively)	271,005	232,188
Total investment securities	451,368	437,205
Loans
Residential mortgage	573,525	538,920
Commercial mortgage	688,536	633,649
Home equity	70,173	74,444
Commercial & Industrial	82,243	65,295
Consumer	37,304	38,591
Total loans	1,451,781	1,350,899
Less: allowance for loan losses	(16,106)	(15,320)
Net loans	1,435,675	1,335,579
Federal Home Loan Bank of Boston Stock, at cost	6,666	4,242
Bank owned life insurance	30,800	31,083
Banking premises and equipment, net	8,836	9,310
Deferred income taxes, net	8,990	8,273
Accrued interest receivable	5,536	5,128
Other assets	24,642	15,523
Total assets	1,988,944	1,949,934
Deposits
Demand	505,951	493,613
Interest bearing checking	385,617	462,957
Money market	119,241	69,259
Savings	606,843	589,741
Certificates of deposit	113,627	159,830
Total deposits	1,731,279	1,775,400
Short-term borrowings	66,700
Long-term borrowings	3,452	3,579
Other liabilities	26,737	22,998
Total liabilities	1,828,168	1,801,977
Shareholders’ Equity
Common stock, par value $1.00; Authorized 10,000,000 shares; Outstanding: 4,105,742 shares and 4,082,188 shares, respectively	4,106	4,082
Additional paid-in capital	37,789	35,663
Retained earnings	127,883	114,093
Accumulated other comprehensive loss	(9,002)	(5,881)
Total shareholders’ equity	160,776	147,957
Total liabilities and shareholders’ equity	$ 1,988,944	$ 1,949,934

CONSOLIDATED BALANCE SHEETS (_2

CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Available for sale securities, amortized cost	$ 186,715	$ 208,911
Held-to-maturity securities, fair value	$ 266,685	$ 233,554
Common stock, par value	$ 1	$ 1
Common stock, shares authorized	10,000,000	10,000,000
Common stock, shares, outstanding	4,105,742	4,082,188

CONSOLIDATED STATEMENTS OF INCO

CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest and dividend income
Interest on taxable loans	$ 14,808	$ 13,038	$ 42,318	$ 37,966
Interest on tax-exempt loans	91	121	279	391
Interest on taxable investment securities	1,936	1,712	5,570	4,762
Interest on tax-exempt investment securities	591	641	1,817	1,966
Dividends on FHLB of Boston stock	93	107	202	192
Interest on overnight investments	83	54	484	170
Total interest and dividend income	17,602	15,673	50,670	45,447
Interest expense
Interest on deposits	1,275	809	3,290	2,183
Interest on borrowed funds	156	225	202	434
Total interest expense	1,431	1,034	3,492	2,617
Net interest and dividend income	16,171	14,639	47,178	42,830
Provision for Loan Losses	457	310	787	360
Net interest and dividend income after provision for loan losses	15,714	14,329	46,391	42,470
Noninterest income
Bank owned life insurance income	129	139	393	448
Gain (loss) on disposition of investment securities			2	(3)
Gain on loans held for sale	37	39	82	324
Loan related derivative income	636	284	1,220	647
Other income	272	282	1,029	890
Total noninterest income	8,929	7,977	24,951	22,649
Noninterest expense
Salaries and employee benefits	10,326	9,000	30,842	27,026
Occupancy and equipment	2,290	2,358	6,736	6,936
Data processing	1,329	1,111	3,848	3,830
Professional services	761	954	2,477	2,650
Marketing	595	355	1,369	1,098
FDIC Insurance	151	154	437	466
Other expenses	427	670	1,436	2,274
Total noninterest expense	15,879	14,602	47,145	44,280
Income before income taxes	8,764	7,704	24,197	20,839
Income tax expense	2,105	2,694	5,622	6,987
Net income	$ 6,659	$ 5,010	$ 18,575	$ 13,852
Share data:
Weighted average number of shares outstanding, basic	4,064,620	4,037,026	4,059,608	4,027,378
Weighted average number of shares outstanding, diluted	4,101,378	4,070,332	4,095,447	4,062,743
Basic earnings per share	$ 1.62	$ 1.23	$ 4.53	$ 3.40
Diluted earnings per share	$ 1.61	$ 1.22	$ 4.49	$ 3.37
Wealth Management Revenue
Noninterest income
Noninterest income	$ 6,779	$ 6,131	$ 19,044	$ 17,077
Deposit Account Fees
Noninterest income
Noninterest income	782	768	2,306	2,387
ATM/Debit Card Income
Noninterest income
Noninterest income	$ 294	$ 334	$ 875	$ 879

UNAUDITED CONSOLIDATED STATEMEN

UNAUDITED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 6,659	$ 5,010	$ 18,575	$ 13,852
Unrealized (losses)/gains on available for sale securities
Unrealized holding (losses)/gains arising during period	(172)	286	(1,947)	968
Less: reclassification adjustment for (gains)/losses included in net income			(2)	1
Total unrealized (losses)/gains on securities	(172)	286	(1,949)	969
Defined benefit retirement plans
Change in retirement liabilities	10	133	30	398
Other comprehensive (loss)/income	(162)	419	(1,919)	1,367
Comprehensive income	$ 6,497	$ 5,429	$ 16,656	$ 15,219

UNAUDITED CONSOLIDATED STATEM_2

UNAUDITED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive (Loss) Income
Beginning balance at Dec. 31, 2016	$ 134,671	$ 4,037	$ 33,253	$ 107,262	$ (9,881)
Net income	13,852			13,852
Other comprehensive income	1,367				1,367
Share based compensation	1,824	45	2,182	(403)
Dividends declared	(5,663)			(5,663)
Ending balance at Sep. 30, 2017	146,051	4,082	35,435	115,048	(8,514)
Beginning balance at Dec. 31, 2017	147,957	4,082	35,663	114,093	(5,881)
Cumulative effect of accounting changes				1,202	(1,202)
Net income	18,575			18,575
Other comprehensive income	(1,919)				(1,919)
Share based compensation	2,150	24	2,126
Dividends declared	(5,987)			(5,987)
Ending balance at Sep. 30, 2018	$ 160,776	$ 4,106	$ 37,789	$ 127,883	$ (9,002)

UNAUDITED CONSOLIDATED STATEM_3

UNAUDITED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS EQUITY (Parenthetical) - $ / shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Retained Earnings
Dividends declared, per share	$ 1.46	$ 1.39

UNAUDITED CONSOLIDATED STATEM_4

UNAUDITED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 18,575	$ 13,852
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for loan losses	787	360
Amortization of deferred charges and fees, net	676	720
Depreciation and amortization	1,454	1,470
Bank owned life insurance income	(393)	(448)
(Gain)/loss on disposition of investment securities	(2)	3
Share based compensation	2,150	1,824
Change in accrued interest receivable	(408)	(272)
Deferred income tax benefit	(156)	(531)
Change in other assets, net	(9,702)	(848)
Change in other liabilities, net	4,365	1,438
Change in loans held for sale		5,946
Net cash provided by operating activities	17,346	23,514
CASH FLOWS FROM INVESTING ACTIVITIES
Origination of loans	(393,864)	(257,931)
Proceeds from principal payments of loans	293,091	222,187
Proceeds from calls/maturities of securities available for sale	21,084	41,982
Proceeds from sales of securities available for sale	702	77,369
Purchase of securities available for sale		(5,091)
Proceeds from calls/maturities of securities held to maturity	25,750	27,100
Purchase of securities held to maturity	(64,960)	(164,684)
Proceeds from settlement of bank owned life insurance policies	676
(Purchase) sale of FHLB of Boston stock	(2,424)	(840)
Purchase of banking premises and equipment	(980)	(521)
Net cash used by investing activities	(120,925)	(60,429)
CASH FLOWS FROM FINANCING ACTIVITIES
Change in demand, interest bearing, money market and savings accounts	2,082	3,144
Change in certificates of deposit	(46,249)	(12,323)
Change in short-term borrowings	66,700	11,500
Repayment of long-term borrowings	(127)	(125)
Cash dividends paid on common stock	(5,987)	(5,663)
Net cash provided by financing activities	16,419	(3,467)
Net decrease in cash and cash equivalents	(87,160)	(40,382)
Cash and cash equivalents at beginning of period	103,591	54,050
Cash and cash equivalents at end of period	16,431	13,668
Cash paid during the period for:
Interest	3,516	2,626
Income taxes	$ 6,275	$ 7,305

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Basis of Presentation	1. BASIS OF PRESENTATION The unaudited consolidated financial statements include the accounts of Cambridge Bancorp (the “Company”) and its wholly owned subsidiary, Cambridge Trust Company (the “Bank”), and the Bank’s wholly owned subsidiaries, Cambridge Trust Company of New Hampshire Inc., CTC Security Corporation, and CTC Security Corporation III. References to the Company herein relate to the consolidated group of companies. All significant intercompany accounts and transactions have been eliminated in preparation of the consolidated financial statements. The Company is a state-chartered, federally registered bank holding company headquartered in Cambridge, Massachusetts and was incorporated in 1983. The Company is the sole shareholder of the Bank, a Massachusetts trust company chartered in 1890, which is a commercial bank. We offer a full range of private banking and wealth management services to our clients. The private banking business, the Company’s only reportable operating segment, is managed as a single strategic unit. The unaudited consolidated financial statements reflect all adjustments (consisting of normal recurring adjustments) and disclosures necessary to present fairly the Company’s financial position, as of September 30, 2018 and December 31, 2017, respectively, and the results of operations and cash flows for the interim periods presented in accordance with U.S. generally accepted accounting principles (“GAAP”). Interim results are not necessarily reflective of the results of the entire year. For further information, refer to the consolidated financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, filed with the Securities and Exchange Commission.

Use of Estimates

Use of Estimates	9 Months Ended
	Sep. 30, 2018
Use Of Estimates [Abstract]
Use of Estimates	2. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements. Actual results could differ from those estimates. The allowance for loan losses and the valuation of deferred tax assets are particularly subject to change.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	3. Subsequent Events Management has reviewed events occurring through November 8, 2018, the date the unaudited consolidated financial statements were available to be issued, and determined that no other subsequent events occurred requiring adjustment to or disclosure in these financial statements.

Recently Issued and Adopted Acc

Recently Issued and Adopted Accounting Guidance	9 Months Ended
	Sep. 30, 2018
New Accounting Pronouncements And Changes In Accounting Principles [Abstract]
Recently Issued and Adopted Accounting Guidance	4. Recently Issued and Adopted Accounting Guidance Accounting Standards Update 2018-16 - Inclusion of the Secured Overnight Financing Rate (SOFR) Overnight Index Swap (OIS) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes The amendments in the ASU will be effective concurrently with ASU 2017-12. For public companies that have already adopted ASU 2017-12, the new amendments are effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. Early adoption is permitted in any interim period upon issuance of this ASU if a company or organization has already adopted Update 2017-12. We are currently assessing the impact the adoption of this guidance will have on our consolidated balance sheets, statements of income, and cash flows. Accounting Standards Update 2018-15 - Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract . Accounting Standards Update 2018-14 - Changes to the Disclosure Requirements for Defined Benefit Plans . Accounting Standards Update 2018-13 - Changes to the Disclosure Requirements for Fair Value Measurement . Accounting Standards Update 2018-07 - Improvements to Nonemployee Share-Based Payment Accounting Revenue from Contracts with Customers Accounting Standards Update No. 2018-02 - Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income • a description of the accounting policy for releasing income tax effects from AOCI, • discussion regarding whether we elect to reclassify the stranded income tax effects from the Tax Cuts and Jobs Act of 2017, and • information about the other income tax effects that are reclassified. The amendments in this ASU affect any organization that is required to apply the provisions of Topic 220, Income Statement —Reporting Comprehensive Income Accounting Standard Update No. Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities h is s e e s ta da r a a l l o c p a i e b t a l i e i e g a c c o n t i n a r i s a n a e ac t i v i t i es d c c o s a c m l e x i t a p l y i h d g a c c o n t i n T s ta d a r r q i r e c p a n i e c h a n h r c o n i t i a r s n t a t i o o t f c t h d g a c c o n ti b y • e li m i n a ti t q u i r e t s e a r a t l m a s r a p e g i e ff e c ti v n s s a n • re q i r i c m t p e s n a h l m n e a c o n i n a a e c e a a i n c m s t a e l i e d e i The standard also permits hedge accounting for strategies for which hedge accounting was not historically permitted and includes new alternatives for measuring the hedged item for fair value hedges of interest rate risk. Furthermore, the standard eases the requirements for effectiveness testing, hedge documentation, applying the critical terms match method, and introduces new alternatives that will permit companies to reduce the risk of material error corrections if they misapply the shortcut method. The new accounting standard is effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. Early adoption is permitted. The new standard requires a modified retrospective transition method in which the Company will recognize the cumulative effect of the change on the opening balance of each affected component of equity in the statement of financial position as of the date of adoption. We are currently assessing the impact the adoption of this guidance will have on our consolidated balance sheets, statements of income, and cash flows. Accounting Standards Update No. 2017-08 - Premium Amortization on Purchased Callable Debt Securities Accounting Standards Update No. 2017-07 - Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost Accounting Standards Update No. 2016-18 - Restricted Cash Accounting Standards Update No. 2016-15 - Classification of Certain Cash Receipts and Cash Payments . Accounting Standards Update No. 2016-13 - Financial Instruments - Measurement of Credit Losses on Financial Instruments Accounting Standards Update No. 2016-02 - Leases Accounting Standards Update No. 2016-01 - Recognition and Measurement of Financial Assets and Financial Liabilities 1. Requires equity investments (with certain exceptions) to be measured at fair value with changes in fair value recognized in net income. 2. Requires an entity to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. 3. Requires separate presentation of financial assets and financial liabilities by measurement category and form of financial asset (that is, securities or loans and receivables) on the statement of condition or the accompanying notes to the financial statements. 4. Clarifies that an entity must assess valuation allowances on a deferred tax asset related to available for sale debt securities in combination with its other deferred tax assets. 5. Require public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes. 6. Eliminates the requirement for public entities to disclose the method(s) and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost on the statement of condition. The amendments, in general, are required to be applied by means of a cumulative-effect adjustment on the statement of condition as of the beginning of the period of adoption. The Company adopted the standard on January 1, 2018, and it did not have a material impact on our consolidated balance sheets, statements of income, or cash flows. Accounting Standards Update No. 2014-09 – Revenue from Contracts with Customers Topic 606 On January 1, 2018, the Company adopted ASU No. 2014-09 and all subsequent ASUs that modified Topic 606. Topic 606 is applicable to noninterest revenue streams such as trust and asset management income, deposit related fees, interchange fees, and merchant income. The Company completed its overall assessment of revenue streams and review of related contracts potentially affected by the ASU, including trust and asset management fees, deposit related fees, and other income within noninterest income. Based on this assessment, the Company concluded that ASU 2014-09 did not materially change the method in which the Company currently recognizes revenue for these revenue streams. The Company adopted ASU 2014-09 and its related amendments utilizing the modified retrospective approach. Since there was no net income impact upon adoption of the new guidance, a cumulative effect adjustment to opening retained earnings was not deemed necessary. Noninterest income considered in-scope of Topic 606 is discussed below. Wealth management and trust fees Wealth management fees are earned for providing investment management, retirement plan advisory, family office, and financial planning services to clients. The Company’s performance obligation under these contracts is satisfied over time as the wealth management services are provided. Fees are recognized monthly based on the average monthly value of the assets under management and the applicable fee rate, depending on the terms of the contract. No performance based incentives are earned on wealth management contracts. Receivables are recorded on the consolidated balance sheet in the other assets line item. The Company earns trust fees for serving as trustee for certain clients. As trustee, the Company administers the client’s trust and manages the assets of the trust including investments and property. The Company’s performance obligation under these agreements is satisfied over time as the administration and management services are provided. Fees are recognized monthly based on a percentage of the market value of the account as outlined in the agreement. Receivables are recorded on the consolidated balance sheet in the other assets line item. The Company also earned fees for trust related activities. The Company’s performance obligation under these agreements is satisfied at a point in time and recognized when these services have been performed. All of the wealth management and trust fee income on the consolidated statement of income is considered in-scope of Topic 606. Other banking fee income The Company charges a variety of fees to its clients for services provided on the deposit and deposit management related accounts. Each fee is either transaction-based or assessed monthly. The types of fees include service charges on accounts, overdraft fees, wire transfer fees, maintenance fees, ATM fee charges, and other miscellaneous charges related to the accounts. These fees are not governed by individual contracts with clients. They are charges to clients based on disclosures presented to clients upon opening these accounts along with updated disclosures when changes are made to the fee structures. The transaction-based fees are recognized in revenue when charged to the client based on specific activity on the client’s account. Monthly service and maintenance charges are recognized in the month they are earned and are charged directly to the client’s account.

Cash and Cash Equivalents

Cash and Cash Equivalents	9 Months Ended
	Sep. 30, 2018
Cash And Cash Equivalents [Abstract]
Cash and Cash Equivalents	5. Cash and CASH EQUIVALENTS At September 30, 2018 and December 31, 2017, cash and cash equivalents totaled $16.4 million and $103.6 million, respectively. Of this amount, $15.1 million and $12.8 million, respectively, were maintained to satisfy the reserve requirements of the Federal Reserve Bank of Boston. Additionally, at September 30, 2018 and December 31, 2017, the Company pledged $ to the New Hampshire Banking Department relating to Cambridge Trust Company of New Hampshire, Inc.’s operations in that state.

Investment Securities

Investment Securities	9 Months Ended
	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Investment Securities	6. Investment Securities Investment securities have been classified in the unaudited consolidated balance sheets according to management’s intent. The carrying amounts of securities and their estimated fair values were as follows: September 30, 2018 December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (dollars in thousands) Available for sale securities U.S. GSE obligations $ 85,008 $ — $ (1,651 ) $ 83,357 $ 90,021 $ — $ (1,230 ) $ 88,791 Mortgage-backed securities 96,688 131 (4,764 ) 92,055 113,184 248 (2,806 ) 110,626 Corporate debt securities 5,019 8 (76 ) 4,951 5,034 12 (45 ) 5,001 Mutual funds — — — — 672 — (73 ) 599 Total available for sale securities $ 186,715 $ 139 $ (6,491 ) $ 180,363 $ 208,911 $ 260 $ (4,154 ) $ 205,017 Held to maturity securities U.S. GSE obligations $ 32,572 $ — $ (503 ) $ 32,069 $ 32,572 $ — $ (166 ) $ 32,406 Mortgage-backed securities 155,723 3 (3,839 ) 151,887 117,155 7 (906 ) 116,256 Corporate debt securities 6,970 — (156 ) 6,814 1,998 4 — 2,002 Municipal securities 75,740 949 (774 ) 75,915 80,463 2,544 (117 ) 82,890 Total held to maturity securities $ 271,005 $ 952 $ (5,272 ) $ 266,685 $ 232,188 $ 2,555 $ (1,189 ) $ 233,554 Total $ 457,720 $ 1,091 $ (11,763 ) $ 447,048 $ 441,099 $ 2,815 $ (5,343 ) $ 438,571 All of the Company’s mortgage-backed securities have been issued by, or are collateralized by securities issued by, either Government National Mortgage Association (Ginnie Mae), Federal National Mortgage Association (Fannie Mae), or Federal Home Loan Mortgage Corporation (Freddie Mac). The following tables show the Company’s securities with gross unrealized losses aggregated by investment category and length of time that individual securities have been in a continuous loss position: September 30, 2018 Less than 12 months 12 months or longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses (dollars in thousands) Temporarily Impaired Securities Available for sale securities U.S. GSE obligations $ — $ — $ 83,357 $ (1,651 ) $ 83,357 $ (1,651 ) Mortgage-backed securities — — 89,466 (4,764 ) 89,466 (4,764 ) Corporate debt securities — — 3,944 (76 ) 3,944 (76 ) Total available for sale securities $ — $ — $ 176,767 $ (6,491 ) $ 176,767 $ (6,491 ) Held to maturity securities U.S. GSE obligations $ 12,374 $ (200 ) $ 19,695 $ (303 ) $ 32,069 $ (503 ) Mortgage-backed securities 94,453 (1,993 ) 57,280 (1,846 ) 151,733 (3,839 ) Corporate debt securities 6,814 (156 ) — — 6,814 (156 ) Municipal securities 14,831 (339 ) 7,135 (435 ) 21,966 (774 ) Total held to maturity securities $ 128,472 $ (2,688 ) $ 84,110 $ (2,584 ) $ 212,582 $ (5,272 ) Total temporarily impaired securities $ 128,472 $ (2,688 ) $ 260,877 $ (9,075 ) $ 389,349 $ (11,763 ) December 31, 2017 Less than 12 months 12 months or longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses (dollars in thousands) Temporarily Impaired Securities Available for sale securities U.S. GSE obligations $ 4,979 $ (21 ) $ 83,812 $ (1,209 ) $ 88,791 $ (1,230 ) Mortgage-backed securities 12,526 (157 ) 94,663 (2,649 ) 107,189 (2,806 ) Corporate debt securities — — 3,990 (45 ) 3,990 (45 ) Mutual funds — — 599 (73 ) 599 (73 ) Total available for sale securities $ 17,505 $ (178 ) $ 183,064 $ (3,976 ) $ 200,569 $ (4,154 ) Held to maturity securities U.S. GSE obligations $ 27,407 $ (166 ) $ — $ — $ 27,407 $ (166 ) Mortgage-backed securities 115,926 (906 ) 3 — 115,929 (906 ) Corporate debt securities — — — — — — Municipal securities 2,041 (19 ) 6,459 (98 ) 8,500 (117 ) Total held to maturity securities $ 145,374 $ (1,091 ) $ 6,462 $ (98 ) $ 151,836 $ (1,189 ) Total temporarily impaired securities $ 162,879 $ (1,269 ) $ 189,526 $ (4,074 ) $ 352,405 $ (5,343 ) Management evaluates securities for other-than-temporary impairment on at least a quarterly basis, and more frequently, when economic or market conditions warrant such evaluation. Consideration is given to: (1) the length of time and the extent to which the fair value has been less than cost; (2) the financial condition and near-term prospects of the issuer; and (3) the intent and ability of the Company to retain its investment in the issuer for a period of time sufficient to allow for any anticipated recovery in fair value. As of September 30, 2018, 159 debt securities had gross unrealized losses, with an aggregate depreciation of 2.93% from the Company’s amortized cost basis. The largest unrealized loss percentage of any single security was 10.34%, or $52,000 of its amortized cost. The largest unrealized dollar loss of any single security was $260,000, or 5.20% of its amortized cost. As of December 31, 2017 The Company believes that the nature and duration of impairment on its debt security positions are primarily a function of interest rate movements and changes in investment spreads and does not consider full repayment of principal on the reported debt obligations to be at risk. Since all of these securities are rated “investment grade” and (a) the Company does not intend to sell these securities before recovery and (b) that it is more likely than not that the Company will not be required to sell these securities before recovery, the Company does not consider these securities to be other-than-temporarily impaired as of September 30, 2018 and December 31, 2017. The amortized cost and fair value of debt securities, aggregated by the earlier of guaranteed call date or contractual maturity, are shown below. Maturities of mortgage-backed securities do not take into consideration scheduled amortization or prepayments. Actual maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. Within One Year After One, But Within Five Years After Five, But Within Ten Years After Ten Years Total Amortized Cost Fair Value Amortized Cost Fair Value Amortized Cost Fair Value Amortized Cost Fair Value Amortized Cost Fair Value At September 30, 2018 (dollars in thousands) Available for sale securities U.S. GSE obligations $ 20,008 $ 19,910 $ 65,000 $ 63,447 $ — $ — $ — $ — $ 85,008 $ 83,357 Mortgage-backed securities — — 92 94 35,457 33,969 61,139 57,992 96,688 92,055 Corporate debt securities 2,012 1,994 3,007 2,957 — — — — 5,019 4,951 Total available for sale securities $ 22,020 $ 21,904 $ 68,099 $ 66,498 $ 35,457 $ 33,969 $ 61,139 $ 57,992 $ 186,715 $ 180,363 Held to maturity securities U.S. GSE obligations $ 5,002 $ 4,982 $ 27,570 $ 27,087 $ — $ — $ — $ — $ 32,572 $ 32,069 Mortgage-backed securities 63 64 35 35 36,295 35,397 119,330 116,391 155,723 151,887 Corporate debt securities — — 6,970 6,814 — — — — 6,970 6,814 Municipal securities 4,386 4,416 12,050 12,191 42,149 42,591 17,155 16,717 75,740 75,915 Total held to maturity securities $ 9,451 $ 9,462 $ 46,625 $ 46,127 $ 78,444 $ 77,988 $ 136,485 $ 133,108 $ 271,005 $ 266,685 Total $ 31,471 $ 31,366 $ 114,724 $ 112,625 $ 113,901 $ 111,957 $ 197,624 $ 191,100 $ 457,720 $ 447,048 The following table sets forth information regarding sales of investment securities and the resulting gains (losses) from such sales: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (dollars in thousands) Amortized cost of securities sold $ — $ — $ 700 $ 77,372 Gain/(loss) realized on securities sold — — 2 (3 ) Net proceeds from securities sold $ — $ — $ 702 $ 77,369

Loans and the Allowance for Loa

Loans and the Allowance for Loan Losses	9 Months Ended
	Sep. 30, 2018
Receivables [Abstract]
Loans and the Allowance for Loan Losses	7. LOANS AND THE ALLOWANCE FOR LOAN LOSSES The Company’s lending activities are conducted principally in Eastern Massachusetts. The Company grants single- and multi-family residential loans, commercial & industrial (“C&I”), commercial real estate (“CRE”), construction loans, and a variety of consumer loans. Most of the loans granted by the Company are secured by real estate collateral. Repayment of the Company’s residential loans is generally dependent on the health of the employment market in the borrowers’ geographic areas and that of the general economy with liquidation of the underlying real estate collateral being typically viewed as the primary source of repayment in the event of borrower default. The repayment of C&I loans depends primarily on the cash flow and credit worthiness of the borrower and secondarily on the underlying collateral provided by the borrower. As borrower cash flow may be difficult to predict, liquidation of the underlying collateral securing these loans is typically viewed as the primary source of repayment in the event of borrower default. However, collateral in C&I loans typically consists of equipment, inventory, accounts receivable, or other business assets that may fluctuate in value, so the liquidation of collateral in the event of default is often an insufficient source of repayment. The Company’s CRE loans are primarily made based on the cash flow from the collateral property and secondarily on the underlying collateral provided by the borrower, with liquidation of the underlying real estate collateral typically being viewed as the primary source of repayment in the event of borrower default. The Company’s construction loans are primarily made based on the borrower’s expected ability to execute and the future completed value of the collateral property, with sale of the underlying real estate collateral typically being viewed as the primary source of repayment. Loans outstanding are detailed by category as follows: September 30, 2018 December 31, 2017 (dollars in thousands) Residential mortgage Mortgages - fixed rate $ 295,452 $ 298,851 Mortgages - adjustable rate 276,930 239,027 Deferred costs net of unearned fees 1,143 1,042 Total residential mortgages 573,525 538,920 Commercial mortgage Mortgages - nonowner occupied 610,966 562,203 Mortgages - owner occupied 37,347 35,343 Construction 40,076 35,904 Deferred costs net of unearned fees 147 199 Total commercial mortgages 688,536 633,649 Home equity Home equity - lines of credit 65,138 70,326 Home equity - term loans 4,788 3,863 Deferred costs net of unearned fees 247 255 Total home equity 70,173 74,444 Commercial & industrial Commercial & industrial 82,272 65,305 Deferred costs (fees) net of unearned fees (29 ) (10 ) Total commercial & industrial 82,243 65,295 Consumer Secured 35,890 37,272 Unsecured 1,399 1,303 Deferred costs net of unearned fees 15 16 Total consumer 37,304 38,591 Total loans $ 1,451,781 $ 1,350,899 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. At September 30, 2018 and December 31, 2017, total loans outstanding to such directors and officers were $391,000 and $516,000, respectively. During the nine months ended September 30, 2018, $14,000 of additions and $139,000 of repayments were made to these loans. There were $124,000 of additions and $298,000 of repayments during the year ended December 31, 2017. At September 30, 2018 and December 31, 2017, all of the loans to directors and officers were performing according to their original terms. The following tables set forth information regarding non-performing loans disaggregated by loan category: September 30, 2018 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Total (dollars in thousands) Non-performing loans: Non-accrual loans $ 520 $ — $ 14 $ 148 $ — $ 682 Loans past due >90 days, but still accruing — — — — — — Troubled debt restructurings 114 — — 12 — 126 Total $ 634 $ — $ 14 $ 160 $ — $ 808 December 31, 2017 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Total (dollars in thousands) Non-performing loans: Non-accrual loans $ 918 $ 213 $ 17 $ — $ — $ 1,148 Loans past due >90 days, but still accruing — — — — — — Troubled debt restructurings 121 — — 29 — 150 Total $ 1,039 $ 213 $ 17 $ 29 $ — $ 1,298 There were no Troubled Debt Restructurings (“TDRs”) Loans are considered restructured in a troubled debt restructuring when the Company has granted concessions to a borrower due to the borrower’s financial condition that it otherwise would not have considered. These concessions may include modifications of the terms of the debt such as deferral of payments, extension of maturity, reduction of principal balance, reduction of the stated interest rate other than normal market rate adjustments, or a combination of these concessions. Debt may be bifurcated with separate terms for each tranche of the restructured debt. Restructuring a loan in lieu of aggressively enforcing the collection of the loan may benefit the Company by increasing the ultimate probability of collection. Restructured loans are classified as accruing or non-accruing based on management’s assessment of the collectability of the loan. Loans which are already on nonaccrual status at the time of the restructuring generally remain on nonaccrual status for approximately six months or longer before management considers such loans for return to accruing status. Accruing restructured loans are placed into nonaccrual status if and when the borrower fails to comply with the restructured terms and management deems it unlikely that the borrower will return to a status of compliance in the near term. Troubled debt restructurings are classified as impaired loans. The Company identifies loss allocations for impaired loans on an individual loan basis. There were no new TDRs during the three months and nine months ended September 30, 2018. At September 30, 2018, three no During the year ended December 31, 2017, the Company modified one loan with a pre-modification carrying value (which consists of the unpaid principal balance, net of charge-offs and unamortized deferred loan origination fees and costs, at the time of the restructuring) of $65,000 and a post-modification carrying value of $48,000. At December 31, 2017, this loan had a carrying value of $29,000. At December 31, 2017, three loans were determined to be TDRs with a total carrying value of $150,000. Two loans designated as TDRs were charged-off during the fourth quarter of 2017. There were no TDR defaults during the year ended December 31, 2017. There were no specific allowances for TDRs at September 30, 2018 and December 31, 2017. As of September 30, 2018, there were no significant commitments to lend additional funds to borrowers whose loans were restructured. Loans by Credit Quality Indicator. The following tables contain period-end balances of loans receivable disaggregated by credit quality indicator: September 30, 2018 Residential Mortgages Home Equity Consumer (dollars in thousands) Credit risk profile based on payment activity: Performing $ 572,891 $ 70,159 $ 37,304 Non-performing 634 14 — Total $ 573,525 $ 70,173 $ 37,304 Commercial Mortgages Commercial & Industrial Credit risk profile by internally assigned grade: 1-6 (Pass) $ 683,883 $ 72,858 7 (Special Mention) 4,653 5,456 8 (Substandard) — 3,929 9 (Doubtful) — — 10 (Loss) — — Total $ 688,536 $ 82,243 December 31, 2017 Residential Mortgages Home Equity Consumer (dollars in thousands) Credit risk profile based on payment activity: Performing $ 537,881 $ 74,427 $ 38,591 Non-performing 1,039 17 — Total $ 538,920 $ 74,444 $ 38,591 Commercial Mortgages Commercial & Industrial Credit risk profile by internally assigned grade: 1-6 (Pass) $ 629,852 $ 56,755 7 (Special Mention) 3,584 8,126 8 (Substandard) 213 414 9 (Doubtful) — — 10 (Loss) — — Total $ 633,649 $ 65,295 With respect to residential mortgages, home equity, and consumer loans, the Bank utilizes the following categories as indicators of credit quality: • Performing – These loans are accruing and are considered having low to moderate risk. • Non-performing – These loans either have been placed on non-accrual, or are past due more than 90 days but are still accruing, and may contain greater than average risk. With respect to CRE and C&I loans, the Bank utilizes a 10 grade internal loan rating system as an indicator of credit quality. The grades are as follows: • Loans rated 1-6 (Pass) – These loans are considered “pass” rated with low to moderate risk. • Loans rated 7 (Special Mention) – These loans have potential weaknesses warranting close attention, which, if left uncorrected, may result in deterioration of the credit at some future date. • Loans rated 8 (Substandard) – These loans have well-defined weaknesses that jeopardize the orderly liquidation of the debt under the original loan terms. Loss potential exists but is not identifiable in any one customer. • Loans rated 9 (Doubtful) – These loans have pronounced weaknesses that make full collection highly questionable and improbable. • Loans rated 10 (Loss) – These loans are considered uncollectible and continuance as a bankable asset is not warranted. Delinquencies The past due status of a loan is determined in accordance with its contractual repayment terms. All loan types are reported past due when one scheduled payment is due and unpaid for 30 days or more. Loan delinquencies can be attributed to many factors, such as but not limited to, a continuing weakness in, or deteriorating economic conditions in, the region in which the collateral is located, the loss of a tenant or lower lease rates for commercial borrowers, or the loss of income for consumers and the resulting liquidity impacts on the borrowers. The following tables contain period-end balances of loans receivable disaggregated by past due status: September 30, 2018 30-59 Days Past Due 60-89 Days Past Due 90 Days or Greater Total Past Due Current Loans Total (dollars in thousands) Residential Mortgages $ 1,503 $ 264 $ 351 $ 2,118 $ 571,407 $ 573,525 Commercial Mortgages 301 — — 301 688,235 688,536 Home Equity — — 14 14 70,159 70,173 Commercial & Industrial 29 — 148 177 82,066 82,243 Consumer loans 6 2 — 8 37,296 37,304 Total $ 1,839 $ 266 $ 513 $ 2,618 $ 1,449,163 $ 1,451,781 December 31, 2017 30-59 Days Past Due 60-89 Days Past Due 90 Days or Greater Total Past Due Current Loans Total (dollars in thousands) Residential Mortgages $ 1,353 $ 706 $ 64 $ 2,123 $ 536,797 $ 538,920 Commercial Mortgages — 32 — 32 633,617 633,649 Home Equity 1 — 17 18 74,426 74,444 Commercial & Industrial — — — — 65,295 65,295 Consumer loans 176 — — 176 38,415 38,591 Total $ 1,530 $ 738 $ 81 $ 2,349 $ 1,348,550 $ 1,350,899 Foreclosure Proceedings As of September 30, 2018 and December 31, 2017, loans secured by one- to four-family residential property amounting to $ and $64,000, respectively, were in process of foreclosure Impaired Loans Impaired loans are loans for which it is probable that the Company will not be able to collect all amounts due according to the contractual terms of the loan agreements and loans restructured in a troubled debt restructuring. The recorded investment in impaired loans consists of unpaid principal balance net of charge-offs, interest payments received applied to principal, and unamortized deferred loan origination fees and costs. The following tables present information pertaining to impaired loans: For the Three Months Ended September 30, 2018 Carrying Value Average Carrying Value Unpaid Principal Balance Related Allowance Interest Income Recognized (dollars in thousands) With no required reserve recorded: Commercial and industrial $ 12 $ 14 $ 12 $ — $ — Commercial mortgage — — — — — Residential mortgage 565 569 712 — — Home equity 77 79 112 — — Total 654 662 836 — — With required reserve recorded: Commercial and industrial 148 148 148 77 1 Commercial mortgage — — — — — Residential mortgage — — — — — Home equity — — — — — Total 148 148 148 77 1 Total: Commercial and industrial 160 162 160 77 1 Commercial mortgage — — — — — Residential mortgage 565 569 712 — — Home equity 77 79 112 — — Total $ 802 $ 810 $ 984 $ 77 $ 1 For the Nine Months Ended September 30, 2018 Carrying Value Average Carrying Value Unpaid Principal Balance Related Allowance Interest Income Recognized (dollars in thousands) With no required reserve recorded: Commercial and industrial $ 12 $ 20 $ 12 $ — $ 1 Commercial mortgage — — — — — Residential mortgage 565 574 712 — — Home equity 77 80 112 — — Total 654 674 836 — 1 With required reserve recorded: Commercial and industrial 148 148 148 77 6 Commercial mortgage — — — — — Residential mortgage — — — — — Home equity — — — — — Total 148 148 148 77 6 Total: Commercial and industrial 160 168 160 77 7 Commercial mortgage — — — — — Residential mortgage 565 574 712 — — Home equity 77 80 112 — — Total $ 802 $ 822 $ 984 $ 77 $ 7 For the Three Months Ended September 30, 2017 Carrying Value Average Carrying Value Unpaid Principal Balance Related Allowance Interest Income Recognized (dollars in thousands) With no required reserve recorded: Commercial and industrial $ 38 $ 40 $ 38 $ — $ 1 Commercial mortgage 217 221 229 — — Residential mortgage 991 997 1,187 — — Home equity 98 99 128 — — Total 1,344 1,357 1,582 — 1 With required reserve recorded: Commercial and industrial 269 273 281 259 — Commercial mortgage — — — — — Residential mortgage 65 65 65 98 — Home equity — — — — — Total 334 338 346 357 — Total: Commercial and industrial 307 313 319 259 1 Commercial mortgage 217 221 229 — — Residential mortgage 1,056 1,062 1,252 98 — Home equity 98 99 128 — — Total $ 1,678 $ 1,695 $ 1,928 $ 357 $ 1 For the Nine Months Ended September 30, 2017 Carrying Value Average Carrying Value Unpaid Principal Balance Related Allowance Interest Income Recognized (dollars in thousands) With no required reserve recorded: Commercial and industrial $ 38 $ 38 $ 38 $ — $ 2 Commercial mortgage 217 227 229 — 3 Residential mortgage 991 1,012 1,187 — — Home equity 98 102 128 — — Total 1,344 1,379 1,582 — 5 With required reserve recorded: Commercial and industrial 269 279 281 259 — Commercial mortgage — — — — — Residential mortgage 65 66 65 98 1 Home equity — — — — — Total 334 345 346 357 1 Total: Commercial and industrial 307 317 319 259 2 Commercial mortgage 217 227 229 — 3 Residential mortgage 1,056 1,078 1,252 98 1 Home equity 98 102 128 — — Total $ 1,678 $ 1,724 $ 1,928 $ 357 $ 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. We provide for loan losses based upon the consistent application of our documented allowance for loan loss methodology. All loan losses are charged to the allowance for loan losses and all recoveries are credited to it. Additions to the allowance for loan losses are provided by charges to income based on various factors which, in our judgment, deserve current recognition in estimating probable losses. We regularly review the loan portfolio, including a review of our classified assets, and make provisions for loan losses in order to maintain the allowance for loan losses in accordance with GAAP. The allowance for loan losses consists primarily of two components: 1. Specific allowances established for impaired loans, as defined by GAAP. The amount of impairment provided for as a specific allowance is measured based on the deficiency, if any, between the present value of expected future cash flows discounted at the loan’s effective interest rate at the time of impairment or, as a practical expedient, at the loan’s observable market price, or the fair value of the collateral if the loan is collateral-dependent, and the carrying value of the loan; and 2. General allowances established for loan losses on a portfolio basis for loans that do not meet the definition of impaired loans. The portfolio is grouped into homogenous pools by similar risk characteristics, primarily by loan type and regulatory classification. We apply an estimated incurred loss rate to each loan group. The loss rates applied are based upon our historical loss experience over a designated look back period adjusted, as appropriate, for the quantitative, qualitative, and environmental factors discussed below. This evaluation is inherently subjective, as it requires material estimates that may be susceptible to significant revisions based upon changes in economic and real estate market conditions. Actual loan losses may be significantly more than the allowance for loan losses we have established, which could have a material negative effect on our financial results. The adjustments to historical loss experience are based on our evaluation of several quantitative, qualitative, and environmental factors, including: • the loss emergence period, which represents the average amount of time between when loss events occur for specific loan types and when such problem loans are identified and the related loss amounts are confirmed through charge-offs; • changes in any concentration of credit (including, but not limited to, concentrations by geography, industry, or collateral type); • changes in the number and amount of non-accrual loans and past due loans; • changes in national, state, and local economic trends; • changes in the types of loans in the loan portfolio; • changes in the experience and ability of personnel; • changes in lending strategies; and • changes in lending policies and procedures. In addition, we may establish an unallocated allowance to provide for probable losses that have been incurred as of the reporting date but are not reflected in the allocated allowance. We evaluate the allowance for loan losses based upon the combined total of the specific and general components. Generally, when the loan portfolio increases, absent other factors, the allowance for loan loss methodology results in a higher dollar amount of estimated probable losses than would be the case without the increase. Generally, when the loan portfolio decreases, absent other factors, the allowance for loan losses methodology results in a lower dollar amount of estimated probable losses than would be the case without the decrease. Periodically, management conducts an analysis to estimate the loss emergence period for various loan categories based on samples of historical charge-offs. Model output by loan category is reviewed to evaluate the reasonableness of the reserve levels in comparison to the estimated loss emergence period applied to historical loss experience. We evaluate the loan portfolio on a quarterly basis, and the allowance is adjusted accordingly. While we use the best information available to make evaluations, future adjustments to the allowance may be necessary if conditions differ substantially from the information used in making the evaluations. In addition, various regulatory agencies, as an integral part of their examination process, will periodically review the allowance for loan losses. Such agencies may require us to recognize additions to the allowance based on their analysis of information available to them at the time of their examination. The following tables contain changes in the allowance for loan losses disaggregated by loan category for the periods noted. For the Three Months Ended September 30, 2018 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Impaired Total (dollars in thousands) Allowance for loan losses: Balance at June 30, 2018 $ 4,879 $ 8,674 $ 601 $ 1,195 $ 296 $ — $ 15,645 Charge-offs — — — — (10 ) — (10 ) Recoveries — — — 13 1 — 14 Provision for (Release of) 267 149 (56 ) 9 11 77 457 Balance at September 30, 2018 $ 5,146 $ 8,823 $ 545 $ 1,217 $ 298 $ 77 $ 16,106 For the Nine Months Ended September 30, 2018 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Impaired Total (dollars in thousands) Allowance for loan losses: Balance at December 31, 2017 $ 5,047 $ 8,289 $ 630 $ 946 $ 315 $ 93 $ 15,320 Charge-offs — — — (11 ) (31 ) — (42 ) Recoveries — — — 35 6 — $ 41 Provision for (Release of) 99 534 (85 ) 247 8 (16 ) 787 Balance at September 30, 2018 $ 5,146 $ 8,823 $ 545 $ 1,217 $ 298 $ 77 $ 16,106 For the Three Months Ended September 30, 2017 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Impaired Total (dollars in thousands) Allowance for loan losses: Balance at June 30, 2017 $ 4,783 $ 8,449 $ 632 $ 758 $ 324 $ 357 $ 15,303 Charge-offs — — — — (1 ) — (1 ) Recoveries — — — 6 2 — 8 Provision for (Release of) 303 (126 ) (4 ) 94 43 — 310 Balance at September 30, 2017 $ 5,086 $ 8,323 $ 628 $ 858 $ 368 $ 357 $ 15,620 For the Nine Months Ended September 30, 2017 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Impaired Total (dollars in thousands) Allowance for loan losses: Balance at December 31, 2016 $ 4,898 $ 8,451 $ 651 $ 807 $ 264 $ 190 $ 15,261 Charge-offs — — — (1 ) (14 ) — (15 ) Recoveries — — — 8 6 — 14 Provision for (Release of) 188 (128 ) (23 ) 44 112 167 360 Balance at September 30, 2017 $ 5,086 $ 8,323 $ 628 $ 858 $ 368 $ 357 $ 15,620 The following tables contain period-end balances of the allowance for loan losses and related loans receivable disaggregated by impairment method: September 30, 2018 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Total (dollars in thousands) Allowance for loan losses Individually evaluated for impairment $ — $ — $ — $ 77 $ — $ 77 Collectively evaluated for impairment 5,146 8,823 545 1,217 298 16,029 Total $ 5,146 $ 8,823 $ 545 $ 1,294 $ 298 $ 16,106 Loans receivable Individually evaluated for impairment $ 578 $ — $ 64 $ 160 $ — $ 802 Collectively evaluated for impairment 572,947 688,536 70,109 82,083 37,304 1,450,979 Total $ 573,525 $ 688,536 $ 70,173 $ 82,243 $ 37,304 $ 1,451,781 December 31, 2017 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Total (dollars in thousands) Allowance for loan losses Individually evaluated for impairment $ 93 $ — $ — $ — $ — $ 93 Collectively evaluated for impairment 5,047 8,289 630 946 315 15,227 Total $ 5,140 $ 8,289 $ 630 $ 946 $ 315 $ 15,320 Loans receivable Individually evaluated for impairment $ 968 $ 213 $ 86 $ 29 $ — $ 1,296 Collectively evaluated for impairment 537,952 633,436 74,358 65,266 38,591 1,349,603 Total $ 538,920 $ 633,649 $ 74,444 $ 65,295 $ 38,591 $ 1,350,899

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	8. Income Taxes In accordance with the Tax Cuts and Jobs Act of 2017, the Company’s statutory federal tax rate reduced from 35% to 21% effective January 1, 2018. The effective tax rate was 24.0% and 23.2% for the three and nine months ended September 30, 2018, respectively, as compared to 35.0% and 33.5% for the three and nine months ended September 30, 2017, respectively. Net deferred tax assets totaled $9.0 million at September 30, 2018 and $ 8.3 The components of income tax expense were as follows: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (dollars in thousands) Current Federal $ 1,812 $ 2,651 $ 4,047 $ 5,966 State 799 690 1,731 1,552 Total current expense 2,611 3,341 5,778 7,518 Deferred Federal (344 ) (504 ) (106 ) (414 ) State (162 ) (143 ) (50 ) (117 ) Total deferred (506 ) (647 ) (156 ) (531 ) Total income tax expense $ 2,105 $ 2,694 $ 5,622 $ 6,987

Pension and Retirement Plans

Pension and Retirement Plans	9 Months Ended
	Sep. 30, 2018
Compensation And Retirement Disclosure [Abstract]
Pension and Retirement Plans	9. Pension and Retirement Plans The components of net periodic benefit cost (credit) were as follows: Three Months Ended September 30, Pension Plan Supplemental Retirement Plan Retirement Healthcare Plan 2018 2017 2018 2017 2018 2017 (dollars in thousands) Net periodic benefit cost Service cost $ — $ 339 $ 88 $ 66 $ 6 $ 4 Interest cost 389 412 78 91 6 5 Expected return on assets (723 ) (619 ) — — — — Amortization of prior service cost (credit) (1 ) (1 ) — — — — Amortization of net actuarial loss 27 179 1 — — (2 ) Net periodic benefit cost (credit) $ (308 ) $ 310 $ 167 $ 157 $ 12 $ 7 Nine Months Ended September 30, Pension Plan Supplemental Retirement Plan Retirement Healthcare Plan 2018 2017 2018 2017 2018 2017 (dollars in thousands) Net periodic benefit cost Service cost $ — $ 1,162 $ 265 $ 200 $ 17 $ 12 Interest cost 1,167 1,413 232 273 17 15 Expected return on assets (2,168 ) (2,122 ) — — — — Amortization of prior service (credit) (3 ) (3 ) — — — — Amortization of net actuarial loss 80 614 3 — — (6 ) Net periodic benefit cost (credit) $ (924 ) $ 1,064 $ 500 $ 473 $ 34 $ 21 The Company made no contributions to the qualified defined benefit pension plan during the three and nine months ended September 30, 2018. We do not expect to make any contributions to the qualified defined benefit plan during the remainder of 2018. The Company froze the accrual of benefits for the Pension Plan effective December 31, 2017. The Company adopted Accounting Standards Update No. 2017-07 - Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost Employee Profit Sharing Plan The Company maintains a Profit Sharing Plan (“PSP”) that provides for deferral of federal and state income taxes on employee contributions allowed under Section 401(k) of federal law. The PSP plan was amended effective January 1, 2018. The Company matches employee contributions up to 100 service. Additionally, employees will be eligible to participate in the discretionary contribution portion of the PSP upon attaining 1,000 hours of service and Employee Stock Ownership Plan The Company has an Employee Stock Ownership Plan (“ESOP”) for its eligible employees. An employee becomes eligible to participate in the ESOP on January 1 or July 1 following the completion of 12 months of service consisting of at least 1,000 hours and must be at least 21 years old. The Company may make a discretionary contribution to the ESOP on an annual basis. Shares of the Company’s common stock can then be purchased and allocated to eligible participant accounts at the then quoted market price. Participants can elect to re-invested cash dividends paid during the year to purchase additional shares. Total expenses related to the PSP and ESOP for the three months ended September 30, 2018 and September 30, 2017 amounted to approximately $595,000 and $238,000, respectively. Total expenses related to the PSP and ESOP for the nine months ended September 30, 2018 and September 30, 2017 amounted to approximately $2.0 million and $713,000, respectively.

Stock Based Compensation

Stock Based Compensation	9 Months Ended
	Sep. 30, 2018
Share Based Compensation [Abstract]
Stock Based Compensation	10. STOCK BASED COMPENSATION The following table presents the pre-tax expense associated with all outstanding non-vested restricted stock awards, restricted stock units, performance based restricted stock units, and the related tax benefits recognized: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (dollars in thousands) Stock based compensation expense $ 705 $ 253 $ 2,052 $ 805 Related tax benefits $ 198 $ 103 $ 577 $ 329

Financial Instruments with Off-

Financial Instruments with Off-Balance-Sheet Risk	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Financial Instruments with Off-Balance-Sheet Risk	11. Financial Instruments with Off-Balance-Sheet Risk To meet the financing needs of its customers, the Company is a party to financial instruments with off-balance-sheet risk in the normal course of business. These financial instruments are primarily comprised of commitments to extend credit, commitments to sell residential mortgage loans, derivatives contracts, risk participation agreements, and standby letters of credit. Those instruments involve, to varying degrees, elements of credit and interest rate risk in excess of the amounts recognized in the consolidated balance sheets. The Company’s exposure to credit loss in the event of nonperformance by the other party to the financial instrument for loan commitments and standby letters of credit is represented by the contractual amount of those instruments assuming that the amounts are fully advanced and that collateral, or other security, is of no value. The Company uses the same credit policies in making commitments and conditional obligations as it does for on-balance-sheet instruments. Off-balance-sheet financial instruments with contractual amounts that present credit risk include the following: September 30, 2018 December 31, 2017 (dollars in thousands) Financial instruments whose contractual amount represents credit risk: Commitments to extend credit: Unused portion of existing lines of credit $ 359,798 $ 304,298 Origination of new loans 30,283 45,061 Standby letters of credit 8,639 8,322 Financial instruments whose notional amount exceeds the amount of credit risk: Commitments to sell residential mortgage loans 1,871 1,490 Customer related derivative contracts: Interest rate swaps with customers 96,450 74,758 Mirror swaps with counterparties 96,450 74,758 Risk participation agreements with counterparties 59,812 38,494

Shareholders' Equity

Shareholders' Equity	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Shareholders' Equity	12. Shareholders’ Equity As of September 30, 2018 and December 31, 2017, the Company and the Bank met all applicable minimum capital requirements and were considered “well-capitalized” by both the FRB and the FDIC. Actual Minimum Capital Required For Capital Adequacy Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount Ratio Amount Ratio Amount Ratio Amount Ratio Amount Ratio (dollars in thousands) At September 30, 2018 Cambridge Bancorp: Total capital (to risk-weighted assets) $ 185,522 14.0 % $ 106,268 8.0 % $ 131,174 9.875 % $ 139,477 10.5 % N/A N/A Tier I capital (to risk-weighted assets) 169,366 12.8 % 79,701 6.0 % 104,607 7.875 % 112,910 8.5 % N/A N/A Common equity tier I capital (to risk-weighted assets) 169,366 12.8 % 59,776 4.5 % 84,682 6.375 % 92,984 7.0 % N/A N/A Tier I capital (to average assets) 169,366 8.5 % 79,576 4.0 % 79,576 4.000 % 79,576 4.0 % N/A N/A Cambridge Trust Company: Total capital (to risk-weighted assets) $ 181,025 13.6 % $ 106,268 8.0 % $ 131,174 9.875 % $ 139,477 10.5 % $ 132,835 10.0 % Tier I capital (to risk-weighted assets) 164,869 12.4 % 79,701 6.0 % 104,607 7.875 % 112,910 8.5 % 106,268 8.0 % Common equity tier I capital (to risk-weighted assets) 164,869 12.4 % 59,776 4.5 % 84,682 6.375 % 92,984 7.0 % 86,343 6.5 % Tier I capital (to average assets) 164,869 8.3 % 79,576 4.0 % 79,576 4.000 % 79,576 4.0 % 99,470 5.0 % At December 31, 2017 Cambridge Bancorp: Total capital (to risk-weighted assets) $ 168,615 13.7 % $ 98,136 8.0 % $ 113,470 9.250 % $ 128,804 10.5 % N/A N/A Tier I capital (to risk-weighted assets) 153,281 12.5 % 73,602 6.0 % 88,936 7.250 % 104,270 8.5 % N/A N/A Common equity tier I capital (to risk-weighted assets) 153,281 12.5 % 55,202 4.5 % 70,535 5.750 % 85,869 7.0 % N/A N/A Tier I capital (to average assets) 153,281 8.1 % 76,026 4.0 % 76,026 4.000 % 76,026 4.0 % N/A N/A Cambridge Trust Company: Total capital (to risk-weighted assets) $ 164,880 13.4 % $ 98,136 8.0 % $ 113,470 9.250 % $ 128,804 10.5 % $ 122,670 10.0 % Tier I capital (to risk-weighted assets) 149,546 12.2 % 73,602 6.0 % 88,936 7.250 % 104,270 8.5 % 98,136 8.0 % Common equity tier I capital (to risk-weighted assets) 149,546 12.2 % 55,202 4.5 % 70,535 5.750 % 85,869 7.0 % 79,736 6.5 % Tier I capital (to average assets) 149,546 7.9 % 76,026 4.0 % 76,026 4.000 % 76,026 4.0 % 95,033 5.0 %

Other Comprehensive Income

Other Comprehensive Income	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Other Comprehensive Income	13. Other Comprehensive Income The following tables present the changes in accumulated other comprehensive income (loss) during the period, by component, net of tax: Three Months Ended September 30, 2018 Three Months Ended September 30, 2017 Before Tax Amount Tax (Expense) or Benefit Net-of-tax Amount Before Tax Amount Tax (Expense) or Benefit Net-of-tax Amount (dollars in thousands) Unrealized (losses)/gains on available for sale securities Unrealized holding (losses)/gains arising during period $ (228 ) $ 56 $ (172 ) $ 452 $ (166 ) $ 286 Reclassification adjustment for (gains)/losses recognized in net income — — — — — — Defined benefit retirement plans Change in retirement liability 14 (4 ) 10 225 (92 ) 133 Total Other Comprehensive (Loss)/Income $ (214 ) $ 52 $ (162 ) $ 677 $ (258 ) $ 419 Nine Months Ended September 30, 2018 Nine Months Ended September 30, 2017 Before Tax Amount Tax (Expense) or Benefit Net-of-tax Amount Before Tax Amount Tax (Expense) or Benefit Net-of-tax Amount (dollars in thousands) Unrealized (losses)/gains on available for sale securities Unrealized holding (losses)/gains arising during period $ (2,456 ) $ 509 $ (1,947 ) $ 1,524 $ (556 ) $ 968 Reclassification adjustment for (gains)/losses recognized in net income (2 ) — (2 ) 3 (2 ) 1 Defined benefit retirement plans Change in retirement liability 43 (13 ) 30 673 (275 ) 398 Total Other Comprehensive (Loss)/Income $ (2,415 ) $ 496 $ (1,919 ) $ 2,200 $ (833 ) $ 1,367 Reclassifications out of Accumulated Other Comprehensive Income (“AOCI”) that have an impact on net income are presented below: Three Months Ended September 30, Details about Accumulated Other Comprehensive Income Components 2018 2017 Affected Line Item in the Statement where Net Income is Presented (dollars in thousands) Unrealized gains and losses on available for sale securities $ — $ — Gain/(loss) on disposition of investment securities Tax benefit or (expense) — — Provision for income taxes Net of tax $ — $ — Net income Nine Months Ended September 30, Details about Accumulated Other Comprehensive Income Components 2018 2017 Affected Line Item in the Statement where Net Income is Presented (dollars in thousands) Unrealized gains and losses on available for sale securities $ 2 $ (3 ) Gain/(loss) on disposition of investment securities Tax benefit or (expense) — 2 Provision for income taxes Net of tax $ 2 $ (1 ) Net income

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Earnings Per Share	14. Earnings per Share The following represents a reconciliation between basic and diluted earnings per share: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (dollars in thousands, except per share data) Earnings per common share - basic: Numerator: Net income $ 6,659 $ 5,010 $ 18,575 $ 13,852 Less dividends and undistributed earnings allocated to participating securities (66 ) (53 ) (189 ) (148 ) Net income applicable to common shareholders $ 6,593 $ 4,957 $ 18,386 $ 13,704 Denominator: Weighted average common shares outstanding 4,065 4,037 4,060 4,027 Earnings per common share - basic $ 1.62 $ 1.23 $ 4.53 $ 3.40 Earnings per common share - diluted: Numerator: Net income $ 6,659 $ 5,010 $ 18,575 $ 13,852 Less dividends and undistributed earnings allocated to participating securities (66 ) (53 ) (189 ) (148 ) Net income applicable to common shareholders $ 6,593 $ 4,957 $ 18,386 $ 13,704 Denominator: Weighted average common shares outstanding 4,065 4,037 4,060 4,027 Dilutive effect of common stock equivalents 36 33 35 36 Weighted average diluted common shares outstanding 4,101 4,070 4,095 4,063 Earnings per common share - diluted $ 1.61 $ 1.22 $ 4.49 $ 3.37

Derivative Financial Instrument

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2018
Derivative Instruments Not Designated As Hedging Instruments [Abstract]
Derivative Financial Instruments	15. Derivative Financial Instruments The Company enters into interest rate derivatives to accommodate the business requirements of its customers. Derivatives are recognized as either assets or liabilities on the balance sheet and are measured at fair value. The accounting for changes in the fair value of derivatives depends on the intended use of the derivative and resulting designation. Interest Rate Swaps The Company has entered into interest rate swap contracts to help commercial loan borrowers manage their interest rate risk. The interest rate swap contracts with commercial loan borrowers allow them to convert floating-rate loan payments to fixed-rate loan payments. When the Bank enters into an interest rate swap contract with a commercial loan borrower, it simultaneously enters into a “mirror” swap contract with a third party. The third party exchanges the client’s fixed‑rate loan payments for floating-rate loan payments. As of September 30, 2018 and December 31, 2017, the Bank had interest rate swap contracts with commercial loan borrowers with notional amounts of $96.4 million and $74.8 million, respectively, and equal amounts of “mirror” swap contracts with third-party financial institutions. These derivatives are not designated as hedges and therefore, changes in fair value are recognized in earnings. Because these derivatives have mirror-image contractual terms, the changes in fair value substantially offset each other through earnings. Fees earned in connection with the execution of derivatives related to this program are recognized in earnings through loan related derivative income. The credit risk associated with swap transactions is the risk of default by the counterparty. To minimize this risk, the Company enters into interest rate agreements only with highly rated counterparties that management believes to be creditworthy. The notional amounts of these agreements do not represent amounts exchanged by the parties under a collateral pledging agreement, and, thus, are not a measure of the potential loss exposure. Risk Participation Agreements The Company has entered into risk participation agreements (“RPAs”) with other banks participating in commercial loan arrangements. Participating banks guarantee the performance on borrower-related interest rate swap contracts. RPAs are derivative financial instruments and are recorded at fair value and reported in other assets or other liabilities. These derivatives are not designated as hedges, and therefore, changes in fair value are recognized in earnings with a corresponding offset within other liabilities. Under a risk participation-out agreement (a derivative asset), the Company participates out a portion of the credit risk associated with the interest rate swap position executed with the commercial borrower for a fee paid to the participating bank. Under a risk participation-in agreement (a derivative liability), the Company assumes, or participates in, a portion of the credit risk associated with the interest rate swap position with the commercial borrower for a fee received from the other bank. As of September 30, 2018, the notional amounts of the risk participation-in agreements and risk participation-out agreements were $59.8 million and $0, respectively. As of December 31, 2017, the notional amounts of the risk participation-in agreements and risk participation-out agreements were $38.5 million and $0, respectively. The following table presents the fair values of derivative instruments in the Company’s unaudited consolidated balance sheets: Derivative Assets Derivative Liabilities Balance Sheet Location September 30, 2018 December 31, 2017 Balance Sheet Location September 30, 2018 December 31, 2017 (dollars in thousands) (dollars in thousands) Derivatives not Designated as Hedging Instruments Loan related derivative contracts Interest rate swaps with customers Other Assets $ 3,866 $ 1,859 Other Liabilities $ — $ — Mirror swaps with counterparties Other Assets — — Other Liabilities 3,866 1,859 Risk participation agreements Other Assets — — Other Liabilities 94 81 Total $ 3,866 $ 1,859 $ 3,960 $ 1,940 For the three months ended September 30, 2018 and September 30, 2017, the Company recorded $636,000 and $284,000, respectively, of loan related derivative income. For the nine months ended September 30, 2018 and September 30, 2017, the Company recorded $1.2 million and $647,000, respectively, of loan related derivative income. See Note 11 – Financial Instruments with Off-Balance Sheet Risk – for further information regarding the Company’s derivatives.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	16. Fair Value Measurements The Company follows ASC 820, “Fair Value Measurements and Disclosures,” • Level 1 – Quoted prices for identical assets or liabilities in active markets. • Level 2 – Quoted prices for similar assets or liabilities in active markets; quoted prices for identical or similar assets or liabilities in inactive markets; and model-derived valuations in which all significant inputs and significant value drivers are observable in active markets. • Level 3 – Valuations derived from techniques in which one or more significant inputs or significant value drivers are unobservable in the markets and which reflect the Company’s market assumptions. The Company uses fair value measurements to record fair value adjustments to certain assets and liabilities and to determine fair value disclosures. Securities available for sale, derivative instruments, and hedges are recorded at fair value on a recurring basis. Additionally, from time to time, the Company may be required to record at fair value other assets on a nonrecurring basis such as collateral dependent impaired loans. In accordance with the requirements of ASU 2016-01, the Company uses an exit price approach for its fair value disclosures as of September 30, 2018. The following is a summary of the carrying values and estimated fair values of the Company’s significant financial instruments as of the dates indicated: September 30, 2018 December 31, 2017 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value (dollars in thousands) Financial assets Cash and cash equivalents $ 16,431 $ 16,431 $ 103,591 $ 103,591 Securities available for sale 180,363 180,363 205,017 205,017 Securities held to maturity 271,005 266,685 232,188 233,554 Loans, net 1,435,675 1,377,456 1,335,579 1,304,719 Loans held for sale — — — — FHLB Boston stock 6,666 6,666 4,242 4,242 Accrued interest receivable 5,536 5,536 5,128 5,128 Mortgage servicing rights 701 1,002 793 1,049 Loan level interest rate swaps 3,866 3,866 1,859 1,859 Financial liabilities Deposits 1,731,279 1,728,606 1,775,400 1,772,838 Short-term borrowings 66,700 66,700 — — Long-term borrowings 3,452 3,387 3,579 3,559 Loan level interest rate swaps 3,866 3,866 1,859 1,859 Risk participation agreements 94 94 81 81 The following tables summarize certain assets reported at fair value on a recurring basis: Fair Value as of September 30, 2018 Level 1 Level 2 Level 3 Total (dollars in thousands) Measured on a recurring basis Securities available for sale U.S. GSE obligations $ — $ 83,357 $ — $ 83,357 Mortgage-backed securities — 92,055 — 92,055 Corporate debt securities — 4,951 — 4,951 Mutual funds — — — — Other assets Interest rate swaps with customers — 3,866 — 3,866 Other liabilities Mirror swaps with counterparties — 3,866 — 3,866 Risk participation agreements — 94 — 94 Fair Value as of December 31, 2017 Level 1 Level 2 Level 3 Total (dollars in thousands) Measured on a recurring basis Securities available for sale U.S. GSE obligations $ — $ 88,791 $ — $ 88,791 Mortgage-backed securities — 110,626 — 110,626 Corporate debt securities — 5,001 — 5,001 Mutual funds 599 — — 599 Other assets Interest rate swaps with customers — 1,859 — 1,859 Other liabilities Mirror swaps with counterparties — 1,859 — 1,859 Risk participation agreements — 81 — 81 The following table presents the carrying value of assets held at September 30, 2018, which were measured at fair value on a non-recurring basis during the three and nine months ended September 30, 2018: September 30, 2018 Level 1 Level 2 Level 3 Total (dollars in thousands) Items recorded at fair value on a nonrecurring basis Assets Collateral dependent impaired loans $ — $ — $ 148 $ 148 Loans held for sale — — — — Total $ — $ — $ 148 $ 148 There were no transfers between levels for the three and nine months ended September 30, 2018 and the three and nine months ended September 30, 2017. The following is a description of the principal valuation methodologies used by the Company to estimate the fair values of its financial instruments. Investment Securities For investment securities, fair values are primarily based upon valuations obtained from a national pricing service which uses matrix pricing with inputs that are observable in the market or can be derived from, or corroborated by, observable market data. When available, quoted prices in active markets for identical securities are utilized. Loans Held for Sale For loans held for sale, fair values are estimated using projected future cash flows, discounted at rates based upon either trades of similar loans or mortgage-backed securities, or at current rates at which similar loans would be made to borrowers with similar credit ratings and for similar remaining maturities. Loans For most categories of loans, fair values are estimated using projected future cash flows, discounted at rates based upon current rates at which similar loans would be made to borrowers with similar credit ratings, and for similar remaining maturities. Projected estimated cash flows are adjusted for prepayment assumptions, liquidity premium assumptions, and credit loss assumptions. Loans that are deemed to be impaired in accordance with ASC 310, “Receivables,” are valued based upon the lower of cost or fair value of the underlying collateral. FHLB of Boston Stock The fair value of FHLB of Boston stock equals its carrying value since such stock is only redeemable at its par value. Deposits The fair value of non-maturity deposit accounts is the amount payable on demand at the reporting date. This amount does not take into account the value of the Bank’s long-term relationships with core depositors. The fair value of fixed-maturity certificates of deposit is estimated using a replacement cost of funds approach and is based upon rates currently offered for deposits of similar remaining maturities. Long-Term Borrowings For long-term borrowings, fair values are estimated using future cash flows, discounted at rates based upon current costs for debt securities with similar terms and remaining maturities. Other Financial Assets and Liabilities Cash and cash equivalents, accrued interest receivable, and short-term borrowings have fair values which approximate their respective carrying values because these instruments are payable on demand or have short-term maturities and present relatively low credit risk and interest rate risk. Derivative Instruments and Hedges The valuation of these instruments is determined using widely accepted valuation techniques including discounted cash flow analysis on the expected cash flows of each derivative. This analysis reflects the contractual terms of the derivatives, including the period to maturity, and uses observable market-based inputs, including interest rate curves and implied volatilities. The Bank incorporates credit valuation adjustments to appropriately reflect nonperformance risk in the fair value measurements. In adjusting the fair value of its derivative contracts for the effect of nonperformance risk, the Bank has considered the impact of netting and any applicable credit enhancements, such as collateral postings. Off-Balance-Sheet Financial Instruments In the course of originating loans and extending credit, the Bank will charge fees in exchange for its commitment. While these commitment fees have value, the Bank has not estimated their value due to the short-term nature of the underlying commitments and their immateriality. Values Not Determined In accordance with ASC 820, the Company has not estimated fair values for non-financial assets such as banking premises and equipment, goodwill, the intangible value of the Bank’s portfolio of loans serviced for itself, and the intangible value inherent in the Bank’s deposit relationships (i.e., core deposits), among others. Accordingly, the aggregate fair value amounts presented do not represent the underlying value of the Company.

Basis of Presentation (Policies

Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Use of Estimates	2. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements. Actual results could differ from those estimates. The allowance for loan losses and the valuation of deferred tax assets are particularly subject to change.
Recently Issued and Adopted Accounting Guidance	Recently Issued and Adopted Accounting Guidance Accounting Standards Update 2018-16 - Inclusion of the Secured Overnight Financing Rate (SOFR) Overnight Index Swap (OIS) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes The amendments in the ASU will be effective concurrently with ASU 2017-12. For public companies that have already adopted ASU 2017-12, the new amendments are effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. Early adoption is permitted in any interim period upon issuance of this ASU if a company or organization has already adopted Update 2017-12. We are currently assessing the impact the adoption of this guidance will have on our consolidated balance sheets, statements of income, and cash flows. Accounting Standards Update 2018-15 - Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract . Accounting Standards Update 2018-14 - Changes to the Disclosure Requirements for Defined Benefit Plans . Accounting Standards Update 2018-13 - Changes to the Disclosure Requirements for Fair Value Measurement . Accounting Standards Update 2018-07 - Improvements to Nonemployee Share-Based Payment Accounting Revenue from Contracts with Customers Accounting Standards Update No. 2018-02 - Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income • a description of the accounting policy for releasing income tax effects from AOCI, • discussion regarding whether we elect to reclassify the stranded income tax effects from the Tax Cuts and Jobs Act of 2017, and • information about the other income tax effects that are reclassified. The amendments in this ASU affect any organization that is required to apply the provisions of Topic 220, Income Statement —Reporting Comprehensive Income Accounting Standard Update No. Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities h is s e e s ta da r a a l l o c p a i e b t a l i e i e g a c c o n t i n a r i s a n a e ac t i v i t i es d c c o s a c m l e x i t a p l y i h d g a c c o n t i n T s ta d a r r q i r e c p a n i e c h a n h r c o n i t i a r s n t a t i o o t f c t h d g a c c o n ti b y • e li m i n a ti t q u i r e t s e a r a t l m a s r a p e g i e ff e c ti v n s s a n • re q i r i c m t p e s n a h l m n e a c o n i n a a e c e a a i n c m s t a e l i e d e i The standard also permits hedge accounting for strategies for which hedge accounting was not historically permitted and includes new alternatives for measuring the hedged item for fair value hedges of interest rate risk. Furthermore, the standard eases the requirements for effectiveness testing, hedge documentation, applying the critical terms match method, and introduces new alternatives that will permit companies to reduce the risk of material error corrections if they misapply the shortcut method. The new accounting standard is effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. Early adoption is permitted. The new standard requires a modified retrospective transition method in which the Company will recognize the cumulative effect of the change on the opening balance of each affected component of equity in the statement of financial position as of the date of adoption. We are currently assessing the impact the adoption of this guidance will have on our consolidated balance sheets, statements of income, and cash flows. Accounting Standards Update No. 2017-08 - Premium Amortization on Purchased Callable Debt Securities Accounting Standards Update No. 2017-07 - Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost Accounting Standards Update No. 2016-18 - Restricted Cash Accounting Standards Update No. 2016-15 - Classification of Certain Cash Receipts and Cash Payments . Accounting Standards Update No. 2016-13 - Financial Instruments - Measurement of Credit Losses on Financial Instruments Accounting Standards Update No. 2016-02 - Leases Accounting Standards Update No. 2016-01 - Recognition and Measurement of Financial Assets and Financial Liabilities 1. Requires equity investments (with certain exceptions) to be measured at fair value with changes in fair value recognized in net income. 2. Requires an entity to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. 3. Requires separate presentation of financial assets and financial liabilities by measurement category and form of financial asset (that is, securities or loans and receivables) on the statement of condition or the accompanying notes to the financial statements. 4. Clarifies that an entity must assess valuation allowances on a deferred tax asset related to available for sale debt securities in combination with its other deferred tax assets. 5. Require public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes. 6. Eliminates the requirement for public entities to disclose the method(s) and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost on the statement of condition. The amendments, in general, are required to be applied by means of a cumulative-effect adjustment on the statement of condition as of the beginning of the period of adoption. The Company adopted the standard on January 1, 2018, and it did not have a material impact on our consolidated balance sheets, statements of income, or cash flows. Accounting Standards Update No. 2014-09 – Revenue from Contracts with Customers Topic 606 On January 1, 2018, the Company adopted ASU No. 2014-09 and all subsequent ASUs that modified Topic 606. Topic 606 is applicable to noninterest revenue streams such as trust and asset management income, deposit related fees, interchange fees, and merchant income. The Company completed its overall assessment of revenue streams and review of related contracts potentially affected by the ASU, including trust and asset management fees, deposit related fees, and other income within noninterest income. Based on this assessment, the Company concluded that ASU 2014-09 did not materially change the method in which the Company currently recognizes revenue for these revenue streams. The Company adopted ASU 2014-09 and its related amendments utilizing the modified retrospective approach. Since there was no net income impact upon adoption of the new guidance, a cumulative effect adjustment to opening retained earnings was not deemed necessary. Noninterest income considered in-scope of Topic 606 is discussed below. Wealth management and trust fees Wealth management fees are earned for providing investment management, retirement plan advisory, family office, and financial planning services to clients. The Company’s performance obligation under these contracts is satisfied over time as the wealth management services are provided. Fees are recognized monthly based on the average monthly value of the assets under management and the applicable fee rate, depending on the terms of the contract. No performance based incentives are earned on wealth management contracts. Receivables are recorded on the consolidated balance sheet in the other assets line item. The Company earns trust fees for serving as trustee for certain clients. As trustee, the Company administers the client’s trust and manages the assets of the trust including investments and property. The Company’s performance obligation under these agreements is satisfied over time as the administration and management services are provided. Fees are recognized monthly based on a percentage of the market value of the account as outlined in the agreement. Receivables are recorded on the consolidated balance sheet in the other assets line item. The Company also earned fees for trust related activities. The Company’s performance obligation under these agreements is satisfied at a point in time and recognized when these services have been performed. All of the wealth management and trust fee income on the consolidated statement of income is considered in-scope of Topic 606. Other banking fee income The Company charges a variety of fees to its clients for services provided on the deposit and deposit management related accounts. Each fee is either transaction-based or assessed monthly. The types of fees include service charges on accounts, overdraft fees, wire transfer fees, maintenance fees, ATM fee charges, and other miscellaneous charges related to the accounts. These fees are not governed by individual contracts with clients. They are charges to clients based on disclosures presented to clients upon opening these accounts along with updated disclosures when changes are made to the fee structures. The transaction-based fees are recognized in revenue when charged to the client based on specific activity on the client’s account. Monthly service and maintenance charges are recognized in the month they are earned and are charged directly to the client’s account.

Investment Securities (Tables)

Investment Securities (Tables)	9 Months Ended
	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Summary of Carrying Amounts of Securities and Their Estimated Fair Values	Investment securities have been classified in the unaudited consolidated balance sheets according to management’s intent. The carrying amounts of securities and their estimated fair values were as follows: September 30, 2018 December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (dollars in thousands) Available for sale securities U.S. GSE obligations $ 85,008 $ — $ (1,651 ) $ 83,357 $ 90,021 $ — $ (1,230 ) $ 88,791 Mortgage-backed securities 96,688 131 (4,764 ) 92,055 113,184 248 (2,806 ) 110,626 Corporate debt securities 5,019 8 (76 ) 4,951 5,034 12 (45 ) 5,001 Mutual funds — — — — 672 — (73 ) 599 Total available for sale securities $ 186,715 $ 139 $ (6,491 ) $ 180,363 $ 208,911 $ 260 $ (4,154 ) $ 205,017 Held to maturity securities U.S. GSE obligations $ 32,572 $ — $ (503 ) $ 32,069 $ 32,572 $ — $ (166 ) $ 32,406 Mortgage-backed securities 155,723 3 (3,839 ) 151,887 117,155 7 (906 ) 116,256 Corporate debt securities 6,970 — (156 ) 6,814 1,998 4 — 2,002 Municipal securities 75,740 949 (774 ) 75,915 80,463 2,544 (117 ) 82,890 Total held to maturity securities $ 271,005 $ 952 $ (5,272 ) $ 266,685 $ 232,188 $ 2,555 $ (1,189 ) $ 233,554 Total $ 457,720 $ 1,091 $ (11,763 ) $ 447,048 $ 441,099 $ 2,815 $ (5,343 ) $ 438,571
Gross Unrealized Losses of Aggregated by Investment Category and Length of Time that Individual Securities have been in Continuous Loss Position	The following tables show the Company’s securities with gross unrealized losses aggregated by investment category and length of time that individual securities have been in a continuous loss position: September 30, 2018 Less than 12 months 12 months or longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses (dollars in thousands) Temporarily Impaired Securities Available for sale securities U.S. GSE obligations $ — $ — $ 83,357 $ (1,651 ) $ 83,357 $ (1,651 ) Mortgage-backed securities — — 89,466 (4,764 ) 89,466 (4,764 ) Corporate debt securities — — 3,944 (76 ) 3,944 (76 ) Total available for sale securities $ — $ — $ 176,767 $ (6,491 ) $ 176,767 $ (6,491 ) Held to maturity securities U.S. GSE obligations $ 12,374 $ (200 ) $ 19,695 $ (303 ) $ 32,069 $ (503 ) Mortgage-backed securities 94,453 (1,993 ) 57,280 (1,846 ) 151,733 (3,839 ) Corporate debt securities 6,814 (156 ) — — 6,814 (156 ) Municipal securities 14,831 (339 ) 7,135 (435 ) 21,966 (774 ) Total held to maturity securities $ 128,472 $ (2,688 ) $ 84,110 $ (2,584 ) $ 212,582 $ (5,272 ) Total temporarily impaired securities $ 128,472 $ (2,688 ) $ 260,877 $ (9,075 ) $ 389,349 $ (11,763 ) December 31, 2017 Less than 12 months 12 months or longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses (dollars in thousands) Temporarily Impaired Securities Available for sale securities U.S. GSE obligations $ 4,979 $ (21 ) $ 83,812 $ (1,209 ) $ 88,791 $ (1,230 ) Mortgage-backed securities 12,526 (157 ) 94,663 (2,649 ) 107,189 (2,806 ) Corporate debt securities — — 3,990 (45 ) 3,990 (45 ) Mutual funds — — 599 (73 ) 599 (73 ) Total available for sale securities $ 17,505 $ (178 ) $ 183,064 $ (3,976 ) $ 200,569 $ (4,154 ) Held to maturity securities U.S. GSE obligations $ 27,407 $ (166 ) $ — $ — $ 27,407 $ (166 ) Mortgage-backed securities 115,926 (906 ) 3 — 115,929 (906 ) Corporate debt securities — — — — — — Municipal securities 2,041 (19 ) 6,459 (98 ) 8,500 (117 ) Total held to maturity securities $ 145,374 $ (1,091 ) $ 6,462 $ (98 ) $ 151,836 $ (1,189 ) Total temporarily impaired securities $ 162,879 $ (1,269 ) $ 189,526 $ (4,074 ) $ 352,405 $ (5,343 )
Schedule of Amortized Cost and Fair Value of Debt Securities, Aggregated By Earlier of Guaranteed Call Date or Contractual Maturity	The amortized cost and fair value of debt securities, aggregated by the earlier of guaranteed call date or contractual maturity, are shown below. Maturities of mortgage-backed securities do not take into consideration scheduled amortization or prepayments. Actual maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. Within One Year After One, But Within Five Years After Five, But Within Ten Years After Ten Years Total Amortized Cost Fair Value Amortized Cost Fair Value Amortized Cost Fair Value Amortized Cost Fair Value Amortized Cost Fair Value At September 30, 2018 (dollars in thousands) Available for sale securities U.S. GSE obligations $ 20,008 $ 19,910 $ 65,000 $ 63,447 $ — $ — $ — $ — $ 85,008 $ 83,357 Mortgage-backed securities — — 92 94 35,457 33,969 61,139 57,992 96,688 92,055 Corporate debt securities 2,012 1,994 3,007 2,957 — — — — 5,019 4,951 Total available for sale securities $ 22,020 $ 21,904 $ 68,099 $ 66,498 $ 35,457 $ 33,969 $ 61,139 $ 57,992 $ 186,715 $ 180,363 Held to maturity securities U.S. GSE obligations $ 5,002 $ 4,982 $ 27,570 $ 27,087 $ — $ — $ — $ — $ 32,572 $ 32,069 Mortgage-backed securities 63 64 35 35 36,295 35,397 119,330 116,391 155,723 151,887 Corporate debt securities — — 6,970 6,814 — — — — 6,970 6,814 Municipal securities 4,386 4,416 12,050 12,191 42,149 42,591 17,155 16,717 75,740 75,915 Total held to maturity securities $ 9,451 $ 9,462 $ 46,625 $ 46,127 $ 78,444 $ 77,988 $ 136,485 $ 133,108 $ 271,005 $ 266,685 Total $ 31,471 $ 31,366 $ 114,724 $ 112,625 $ 113,901 $ 111,957 $ 197,624 $ 191,100 $ 457,720 $ 447,048
Summary of Gains (Losses) from Sale of Investment Securities	The following table sets forth information regarding sales of investment securities and the resulting gains (losses) from such sales: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (dollars in thousands) Amortized cost of securities sold $ — $ — $ 700 $ 77,372 Gain/(loss) realized on securities sold — — 2 (3 ) Net proceeds from securities sold $ — $ — $ 702 $ 77,369

Loans and the Allowance for L_2

Loans and the Allowance for Loan Losses (Tables)	9 Months Ended
	Sep. 30, 2018
Receivables [Abstract]
Loans Outstanding by Category	Loans outstanding are detailed by category as follows: September 30, 2018 December 31, 2017 (dollars in thousands) Residential mortgage Mortgages - fixed rate $ 295,452 $ 298,851 Mortgages - adjustable rate 276,930 239,027 Deferred costs net of unearned fees 1,143 1,042 Total residential mortgages 573,525 538,920 Commercial mortgage Mortgages - nonowner occupied 610,966 562,203 Mortgages - owner occupied 37,347 35,343 Construction 40,076 35,904 Deferred costs net of unearned fees 147 199 Total commercial mortgages 688,536 633,649 Home equity Home equity - lines of credit 65,138 70,326 Home equity - term loans 4,788 3,863 Deferred costs net of unearned fees 247 255 Total home equity 70,173 74,444 Commercial & industrial Commercial & industrial 82,272 65,305 Deferred costs (fees) net of unearned fees (29 ) (10 ) Total commercial & industrial 82,243 65,295 Consumer Secured 35,890 37,272 Unsecured 1,399 1,303 Deferred costs net of unearned fees 15 16 Total consumer 37,304 38,591 Total loans $ 1,451,781 $ 1,350,899
Non-performing Loans Disaggregated by Loan Category	The following tables set forth information regarding non-performing loans disaggregated by loan category: September 30, 2018 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Total (dollars in thousands) Non-performing loans: Non-accrual loans $ 520 $ — $ 14 $ 148 $ — $ 682 Loans past due >90 days, but still accruing — — — — — — Troubled debt restructurings 114 — — 12 — 126 Total $ 634 $ — $ 14 $ 160 $ — $ 808 December 31, 2017 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Total (dollars in thousands) Non-performing loans: Non-accrual loans $ 918 $ 213 $ 17 $ — $ — $ 1,148 Loans past due >90 days, but still accruing — — — — — — Troubled debt restructurings 121 — — 29 — 150 Total $ 1,039 $ 213 $ 17 $ 29 $ — $ 1,298
Loans Receivable Disaggregated by Credit Quality Indicator	. The following tables contain period-end balances of loans receivable disaggregated by credit quality indicator: September 30, 2018 Residential Mortgages Home Equity Consumer (dollars in thousands) Credit risk profile based on payment activity: Performing $ 572,891 $ 70,159 $ 37,304 Non-performing 634 14 — Total $ 573,525 $ 70,173 $ 37,304 Commercial Mortgages Commercial & Industrial Credit risk profile by internally assigned grade: 1-6 (Pass) $ 683,883 $ 72,858 7 (Special Mention) 4,653 5,456 8 (Substandard) — 3,929 9 (Doubtful) — — 10 (Loss) — — Total $ 688,536 $ 82,243 December 31, 2017 Residential Mortgages Home Equity Consumer (dollars in thousands) Credit risk profile based on payment activity: Performing $ 537,881 $ 74,427 $ 38,591 Non-performing 1,039 17 — Total $ 538,920 $ 74,444 $ 38,591 Commercial Mortgages Commercial & Industrial Credit risk profile by internally assigned grade: 1-6 (Pass) $ 629,852 $ 56,755 7 (Special Mention) 3,584 8,126 8 (Substandard) 213 414 9 (Doubtful) — — 10 (Loss) — — Total $ 633,649 $ 65,295
Schedule of Loans Receivable Disaggregated by Past Due Status	The following tables contain period-end balances of loans receivable disaggregated by past due status: September 30, 2018 30-59 Days Past Due 60-89 Days Past Due 90 Days or Greater Total Past Due Current Loans Total (dollars in thousands) Residential Mortgages $ 1,503 $ 264 $ 351 $ 2,118 $ 571,407 $ 573,525 Commercial Mortgages 301 — — 301 688,235 688,536 Home Equity — — 14 14 70,159 70,173 Commercial & Industrial 29 — 148 177 82,066 82,243 Consumer loans 6 2 — 8 37,296 37,304 Total $ 1,839 $ 266 $ 513 $ 2,618 $ 1,449,163 $ 1,451,781 December 31, 2017 30-59 Days Past Due 60-89 Days Past Due 90 Days or Greater Total Past Due Current Loans Total (dollars in thousands) Residential Mortgages $ 1,353 $ 706 $ 64 $ 2,123 $ 536,797 $ 538,920 Commercial Mortgages — 32 — 32 633,617 633,649 Home Equity 1 — 17 18 74,426 74,444 Commercial & Industrial — — — — 65,295 65,295 Consumer loans 176 — — 176 38,415 38,591 Total $ 1,530 $ 738 $ 81 $ 2,349 $ 1,348,550 $ 1,350,899
Information Pertaining to Impaired Loans	The following tables present information pertaining to impaired loans: For the Three Months Ended September 30, 2018 Carrying Value Average Carrying Value Unpaid Principal Balance Related Allowance Interest Income Recognized (dollars in thousands) With no required reserve recorded: Commercial and industrial $ 12 $ 14 $ 12 $ — $ — Commercial mortgage — — — — — Residential mortgage 565 569 712 — — Home equity 77 79 112 — — Total 654 662 836 — — With required reserve recorded: Commercial and industrial 148 148 148 77 1 Commercial mortgage — — — — — Residential mortgage — — — — — Home equity — — — — — Total 148 148 148 77 1 Total: Commercial and industrial 160 162 160 77 1 Commercial mortgage — — — — — Residential mortgage 565 569 712 — — Home equity 77 79 112 — — Total $ 802 $ 810 $ 984 $ 77 $ 1 For the Nine Months Ended September 30, 2018 Carrying Value Average Carrying Value Unpaid Principal Balance Related Allowance Interest Income Recognized (dollars in thousands) With no required reserve recorded: Commercial and industrial $ 12 $ 20 $ 12 $ — $ 1 Commercial mortgage — — — — — Residential mortgage 565 574 712 — — Home equity 77 80 112 — — Total 654 674 836 — 1 With required reserve recorded: Commercial and industrial 148 148 148 77 6 Commercial mortgage — — — — — Residential mortgage — — — — — Home equity — — — — — Total 148 148 148 77 6 Total: Commercial and industrial 160 168 160 77 7 Commercial mortgage — — — — — Residential mortgage 565 574 712 — — Home equity 77 80 112 — — Total $ 802 $ 822 $ 984 $ 77 $ 7 For the Three Months Ended September 30, 2017 Carrying Value Average Carrying Value Unpaid Principal Balance Related Allowance Interest Income Recognized (dollars in thousands) With no required reserve recorded: Commercial and industrial $ 38 $ 40 $ 38 $ — $ 1 Commercial mortgage 217 221 229 — — Residential mortgage 991 997 1,187 — — Home equity 98 99 128 — — Total 1,344 1,357 1,582 — 1 With required reserve recorded: Commercial and industrial 269 273 281 259 — Commercial mortgage — — — — — Residential mortgage 65 65 65 98 — Home equity — — — — — Total 334 338 346 357 — Total: Commercial and industrial 307 313 319 259 1 Commercial mortgage 217 221 229 — — Residential mortgage 1,056 1,062 1,252 98 — Home equity 98 99 128 — — Total $ 1,678 $ 1,695 $ 1,928 $ 357 $ 1 For the Nine Months Ended September 30, 2017 Carrying Value Average Carrying Value Unpaid Principal Balance Related Allowance Interest Income Recognized (dollars in thousands) With no required reserve recorded: Commercial and industrial $ 38 $ 38 $ 38 $ — $ 2 Commercial mortgage 217 227 229 — 3 Residential mortgage 991 1,012 1,187 — — Home equity 98 102 128 — — Total 1,344 1,379 1,582 — 5 With required reserve recorded: Commercial and industrial 269 279 281 259 — Commercial mortgage — — — — — Residential mortgage 65 66 65 98 1 Home equity — — — — — Total 334 345 346 357 1 Total: Commercial and industrial 307 317 319 259 2 Commercial mortgage 217 227 229 — 3 Residential mortgage 1,056 1,078 1,252 98 1 Home equity 98 102 128 — — Total $ 1,678 $ 1,724 $ 1,928 $ 357 $ 6
Summary of Changes in Allowance for Loan Losses Disaggregated by Loan Type	The following tables contain changes in the allowance for loan losses disaggregated by loan category for the periods noted. For the Three Months Ended September 30, 2018 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Impaired Total (dollars in thousands) Allowance for loan losses: Balance at June 30, 2018 $ 4,879 $ 8,674 $ 601 $ 1,195 $ 296 $ — $ 15,645 Charge-offs — — — — (10 ) — (10 ) Recoveries — — — 13 1 — 14 Provision for (Release of) 267 149 (56 ) 9 11 77 457 Balance at September 30, 2018 $ 5,146 $ 8,823 $ 545 $ 1,217 $ 298 $ 77 $ 16,106 For the Nine Months Ended September 30, 2018 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Impaired Total (dollars in thousands) Allowance for loan losses: Balance at December 31, 2017 $ 5,047 $ 8,289 $ 630 $ 946 $ 315 $ 93 $ 15,320 Charge-offs — — — (11 ) (31 ) — (42 ) Recoveries — — — 35 6 — $ 41 Provision for (Release of) 99 534 (85 ) 247 8 (16 ) 787 Balance at September 30, 2018 $ 5,146 $ 8,823 $ 545 $ 1,217 $ 298 $ 77 $ 16,106 For the Three Months Ended September 30, 2017 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Impaired Total (dollars in thousands) Allowance for loan losses: Balance at June 30, 2017 $ 4,783 $ 8,449 $ 632 $ 758 $ 324 $ 357 $ 15,303 Charge-offs — — — — (1 ) — (1 ) Recoveries — — — 6 2 — 8 Provision for (Release of) 303 (126 ) (4 ) 94 43 — 310 Balance at September 30, 2017 $ 5,086 $ 8,323 $ 628 $ 858 $ 368 $ 357 $ 15,620 For the Nine Months Ended September 30, 2017 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Impaired Total (dollars in thousands) Allowance for loan losses: Balance at December 31, 2016 $ 4,898 $ 8,451 $ 651 $ 807 $ 264 $ 190 $ 15,261 Charge-offs — — — (1 ) (14 ) — (15 ) Recoveries — — — 8 6 — 14 Provision for (Release of) 188 (128 ) (23 ) 44 112 167 360 Balance at September 30, 2017 $ 5,086 $ 8,323 $ 628 $ 858 $ 368 $ 357 $ 15,620
Summary of Allowance for Loan Losses and Related Loans Receivable Disaggregated by Impairment Method	The following tables contain period-end balances of the allowance for loan losses and related loans receivable disaggregated by impairment method: September 30, 2018 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Total (dollars in thousands) Allowance for loan losses Individually evaluated for impairment $ — $ — $ — $ 77 $ — $ 77 Collectively evaluated for impairment 5,146 8,823 545 1,217 298 16,029 Total $ 5,146 $ 8,823 $ 545 $ 1,294 $ 298 $ 16,106 Loans receivable Individually evaluated for impairment $ 578 $ — $ 64 $ 160 $ — $ 802 Collectively evaluated for impairment 572,947 688,536 70,109 82,083 37,304 1,450,979 Total $ 573,525 $ 688,536 $ 70,173 $ 82,243 $ 37,304 $ 1,451,781 December 31, 2017 Residential Mortgages Commercial Mortgages Home Equity Commercial & Industrial Consumer Total (dollars in thousands) Allowance for loan losses Individually evaluated for impairment $ 93 $ — $ — $ — $ — $ 93 Collectively evaluated for impairment 5,047 8,289 630 946 315 15,227 Total $ 5,140 $ 8,289 $ 630 $ 946 $ 315 $ 15,320 Loans receivable Individually evaluated for impairment $ 968 $ 213 $ 86 $ 29 $ — $ 1,296 Collectively evaluated for impairment 537,952 633,436 74,358 65,266 38,591 1,349,603 Total $ 538,920 $ 633,649 $ 74,444 $ 65,295 $ 38,591 $ 1,350,899

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Components of Income Tax Expense	The components of income tax expense were as follows: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (dollars in thousands) Current Federal $ 1,812 $ 2,651 $ 4,047 $ 5,966 State 799 690 1,731 1,552 Total current expense 2,611 3,341 5,778 7,518 Deferred Federal (344 ) (504 ) (106 ) (414 ) State (162 ) (143 ) (50 ) (117 ) Total deferred (506 ) (647 ) (156 ) (531 ) Total income tax expense $ 2,105 $ 2,694 $ 5,622 $ 6,987

Pension and Retirement Plans (T

Pension and Retirement Plans (Tables)	9 Months Ended
	Sep. 30, 2018
Compensation And Retirement Disclosure [Abstract]
Components of Net Periodic Benefit Cost (Credit)	The components of net periodic benefit cost (credit) were as follows: Three Months Ended September 30, Pension Plan Supplemental Retirement Plan Retirement Healthcare Plan 2018 2017 2018 2017 2018 2017 (dollars in thousands) Net periodic benefit cost Service cost $ — $ 339 $ 88 $ 66 $ 6 $ 4 Interest cost 389 412 78 91 6 5 Expected return on assets (723 ) (619 ) — — — — Amortization of prior service cost (credit) (1 ) (1 ) — — — — Amortization of net actuarial loss 27 179 1 — — (2 ) Net periodic benefit cost (credit) $ (308 ) $ 310 $ 167 $ 157 $ 12 $ 7 Nine Months Ended September 30, Pension Plan Supplemental Retirement Plan Retirement Healthcare Plan 2018 2017 2018 2017 2018 2017 (dollars in thousands) Net periodic benefit cost Service cost $ — $ 1,162 $ 265 $ 200 $ 17 $ 12 Interest cost 1,167 1,413 232 273 17 15 Expected return on assets (2,168 ) (2,122 ) — — — — Amortization of prior service (credit) (3 ) (3 ) — — — — Amortization of net actuarial loss 80 614 3 — — (6 ) Net periodic benefit cost (credit) $ (924 ) $ 1,064 $ 500 $ 473 $ 34 $ 21

Stock Based Compensation (Table

Stock Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2018
Share Based Compensation [Abstract]
Schedule of Pre-tax Expense Associated with All Outstanding Non-vested Restricted Stock Awards, Restricted Stock Units, Performance Based Restricted Stock Units and Related Tax Benefits	The following table presents the pre-tax expense associated with all outstanding non-vested restricted stock awards, restricted stock units, performance based restricted stock units, and the related tax benefits recognized: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (dollars in thousands) Stock based compensation expense $ 705 $ 253 $ 2,052 $ 805 Related tax benefits $ 198 $ 103 $ 577 $ 329

Financial Instruments with Of_2

Financial Instruments with Off-Balance-Sheet Risk (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Summary of Off-Balance-Sheet Financial Instruments with Contractual Amounts Include Present Credit Risk	Off-balance-sheet financial instruments with contractual amounts that present credit risk include the following: September 30, 2018 December 31, 2017 (dollars in thousands) Financial instruments whose contractual amount represents credit risk: Commitments to extend credit: Unused portion of existing lines of credit $ 359,798 $ 304,298 Origination of new loans 30,283 45,061 Standby letters of credit 8,639 8,322 Financial instruments whose notional amount exceeds the amount of credit risk: Commitments to sell residential mortgage loans 1,871 1,490 Customer related derivative contracts: Interest rate swaps with customers 96,450 74,758 Mirror swaps with counterparties 96,450 74,758 Risk participation agreements with counterparties 59,812 38,494

Shareholders' Equity (Tables)

Shareholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Schedule of Minimum Capital Requirements Considered Well Capitalized by FRB and FDIC	As of September 30, 2018 and December 31, 2017, the Company and the Bank met all applicable minimum capital requirements and were considered “well-capitalized” by both the FRB and the FDIC. Actual Minimum Capital Required For Capital Adequacy Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount Ratio Amount Ratio Amount Ratio Amount Ratio Amount Ratio (dollars in thousands) At September 30, 2018 Cambridge Bancorp: Total capital (to risk-weighted assets) $ 185,522 14.0 % $ 106,268 8.0 % $ 131,174 9.875 % $ 139,477 10.5 % N/A N/A Tier I capital (to risk-weighted assets) 169,366 12.8 % 79,701 6.0 % 104,607 7.875 % 112,910 8.5 % N/A N/A Common equity tier I capital (to risk-weighted assets) 169,366 12.8 % 59,776 4.5 % 84,682 6.375 % 92,984 7.0 % N/A N/A Tier I capital (to average assets) 169,366 8.5 % 79,576 4.0 % 79,576 4.000 % 79,576 4.0 % N/A N/A Cambridge Trust Company: Total capital (to risk-weighted assets) $ 181,025 13.6 % $ 106,268 8.0 % $ 131,174 9.875 % $ 139,477 10.5 % $ 132,835 10.0 % Tier I capital (to risk-weighted assets) 164,869 12.4 % 79,701 6.0 % 104,607 7.875 % 112,910 8.5 % 106,268 8.0 % Common equity tier I capital (to risk-weighted assets) 164,869 12.4 % 59,776 4.5 % 84,682 6.375 % 92,984 7.0 % 86,343 6.5 % Tier I capital (to average assets) 164,869 8.3 % 79,576 4.0 % 79,576 4.000 % 79,576 4.0 % 99,470 5.0 % At December 31, 2017 Cambridge Bancorp: Total capital (to risk-weighted assets) $ 168,615 13.7 % $ 98,136 8.0 % $ 113,470 9.250 % $ 128,804 10.5 % N/A N/A Tier I capital (to risk-weighted assets) 153,281 12.5 % 73,602 6.0 % 88,936 7.250 % 104,270 8.5 % N/A N/A Common equity tier I capital (to risk-weighted assets) 153,281 12.5 % 55,202 4.5 % 70,535 5.750 % 85,869 7.0 % N/A N/A Tier I capital (to average assets) 153,281 8.1 % 76,026 4.0 % 76,026 4.000 % 76,026 4.0 % N/A N/A Cambridge Trust Company: Total capital (to risk-weighted assets) $ 164,880 13.4 % $ 98,136 8.0 % $ 113,470 9.250 % $ 128,804 10.5 % $ 122,670 10.0 % Tier I capital (to risk-weighted assets) 149,546 12.2 % 73,602 6.0 % 88,936 7.250 % 104,270 8.5 % 98,136 8.0 % Common equity tier I capital (to risk-weighted assets) 149,546 12.2 % 55,202 4.5 % 70,535 5.750 % 85,869 7.0 % 79,736 6.5 % Tier I capital (to average assets) 149,546 7.9 % 76,026 4.0 % 76,026 4.000 % 76,026 4.0 % 95,033 5.0 %

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Summary of Other Comprehensive Income	The following tables present the changes in accumulated other comprehensive income (loss) during the period, by component, net of tax: Three Months Ended September 30, 2018 Three Months Ended September 30, 2017 Before Tax Amount Tax (Expense) or Benefit Net-of-tax Amount Before Tax Amount Tax (Expense) or Benefit Net-of-tax Amount (dollars in thousands) Unrealized (losses)/gains on available for sale securities Unrealized holding (losses)/gains arising during period $ (228 ) $ 56 $ (172 ) $ 452 $ (166 ) $ 286 Reclassification adjustment for (gains)/losses recognized in net income — — — — — — Defined benefit retirement plans Change in retirement liability 14 (4 ) 10 225 (92 ) 133 Total Other Comprehensive (Loss)/Income $ (214 ) $ 52 $ (162 ) $ 677 $ (258 ) $ 419 Nine Months Ended September 30, 2018 Nine Months Ended September 30, 2017 Before Tax Amount Tax (Expense) or Benefit Net-of-tax Amount Before Tax Amount Tax (Expense) or Benefit Net-of-tax Amount (dollars in thousands) Unrealized (losses)/gains on available for sale securities Unrealized holding (losses)/gains arising during period $ (2,456 ) $ 509 $ (1,947 ) $ 1,524 $ (556 ) $ 968 Reclassification adjustment for (gains)/losses recognized in net income (2 ) — (2 ) 3 (2 ) 1 Defined benefit retirement plans Change in retirement liability 43 (13 ) 30 673 (275 ) 398 Total Other Comprehensive (Loss)/Income $ (2,415 ) $ 496 $ (1,919 ) $ 2,200 $ (833 ) $ 1,367
Summary of Reclassifications out of Accumulated Other Comprehensive Income ("AOCI")	Reclassifications out of Accumulated Other Comprehensive Income (“AOCI”) that have an impact on net income are presented below: Three Months Ended September 30, Details about Accumulated Other Comprehensive Income Components 2018 2017 Affected Line Item in the Statement where Net Income is Presented (dollars in thousands) Unrealized gains and losses on available for sale securities $ — $ — Gain/(loss) on disposition of investment securities Tax benefit or (expense) — — Provision for income taxes Net of tax $ — $ — Net income Nine Months Ended September 30, Details about Accumulated Other Comprehensive Income Components 2018 2017 Affected Line Item in the Statement where Net Income is Presented (dollars in thousands) Unrealized gains and losses on available for sale securities $ 2 $ (3 ) Gain/(loss) on disposition of investment securities Tax benefit or (expense) — 2 Provision for income taxes Net of tax $ 2 $ (1 ) Net income

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Summary of Reconciliation Between Basic and Diluted Earnings Per Share	The following represents a reconciliation between basic and diluted earnings per share: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 (dollars in thousands, except per share data) Earnings per common share - basic: Numerator: Net income $ 6,659 $ 5,010 $ 18,575 $ 13,852 Less dividends and undistributed earnings allocated to participating securities (66 ) (53 ) (189 ) (148 ) Net income applicable to common shareholders $ 6,593 $ 4,957 $ 18,386 $ 13,704 Denominator: Weighted average common shares outstanding 4,065 4,037 4,060 4,027 Earnings per common share - basic $ 1.62 $ 1.23 $ 4.53 $ 3.40 Earnings per common share - diluted: Numerator: Net income $ 6,659 $ 5,010 $ 18,575 $ 13,852 Less dividends and undistributed earnings allocated to participating securities (66 ) (53 ) (189 ) (148 ) Net income applicable to common shareholders $ 6,593 $ 4,957 $ 18,386 $ 13,704 Denominator: Weighted average common shares outstanding 4,065 4,037 4,060 4,027 Dilutive effect of common stock equivalents 36 33 35 36 Weighted average diluted common shares outstanding 4,101 4,070 4,095 4,063 Earnings per common share - diluted $ 1.61 $ 1.22 $ 4.49 $ 3.37

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Derivative Instruments Not Designated As Hedging Instruments [Abstract]
Summary of Fair Values of Derivative Instruments in the Company's Unaudited Consolidated Balance Sheets	The following table presents the fair values of derivative instruments in the Company’s unaudited consolidated balance sheets: Derivative Assets Derivative Liabilities Balance Sheet Location September 30, 2018 December 31, 2017 Balance Sheet Location September 30, 2018 December 31, 2017 (dollars in thousands) (dollars in thousands) Derivatives not Designated as Hedging Instruments Loan related derivative contracts Interest rate swaps with customers Other Assets $ 3,866 $ 1,859 Other Liabilities $ — $ — Mirror swaps with counterparties Other Assets — — Other Liabilities 3,866 1,859 Risk participation agreements Other Assets — — Other Liabilities 94 81 Total $ 3,866 $ 1,859 $ 3,960 $ 1,940

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Summary of Carrying Values and Estimated Fair Values of Financial Instruments	The following is a summary of the carrying values and estimated fair values of the Company’s significant financial instruments as of the dates indicated: September 30, 2018 December 31, 2017 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value (dollars in thousands) Financial assets Cash and cash equivalents $ 16,431 $ 16,431 $ 103,591 $ 103,591 Securities available for sale 180,363 180,363 205,017 205,017 Securities held to maturity 271,005 266,685 232,188 233,554 Loans, net 1,435,675 1,377,456 1,335,579 1,304,719 Loans held for sale — — — — FHLB Boston stock 6,666 6,666 4,242 4,242 Accrued interest receivable 5,536 5,536 5,128 5,128 Mortgage servicing rights 701 1,002 793 1,049 Loan level interest rate swaps 3,866 3,866 1,859 1,859 Financial liabilities Deposits 1,731,279 1,728,606 1,775,400 1,772,838 Short-term borrowings 66,700 66,700 — — Long-term borrowings 3,452 3,387 3,579 3,559 Loan level interest rate swaps 3,866 3,866 1,859 1,859 Risk participation agreements 94 94 81 81
Summary of Certain Assets Reported at Fair Value on a Recurring Basis	The following tables summarize certain assets reported at fair value on a recurring basis: Fair Value as of September 30, 2018 Level 1 Level 2 Level 3 Total (dollars in thousands) Measured on a recurring basis Securities available for sale U.S. GSE obligations $ — $ 83,357 $ — $ 83,357 Mortgage-backed securities — 92,055 — 92,055 Corporate debt securities — 4,951 — 4,951 Mutual funds — — — — Other assets Interest rate swaps with customers — 3,866 — 3,866 Other liabilities Mirror swaps with counterparties — 3,866 — 3,866 Risk participation agreements — 94 — 94 Fair Value as of December 31, 2017 Level 1 Level 2 Level 3 Total (dollars in thousands) Measured on a recurring basis Securities available for sale U.S. GSE obligations $ — $ 88,791 $ — $ 88,791 Mortgage-backed securities — 110,626 — 110,626 Corporate debt securities — 5,001 — 5,001 Mutual funds 599 — — 599 Other assets Interest rate swaps with customers — 1,859 — 1,859 Other liabilities Mirror swaps with counterparties — 1,859 — 1,859 Risk participation agreements — 81 — 81
Schedule of Carrying Value of Assets Measured at Fair Value on a Non-Recurring Basis	The following table presents the carrying value of assets held at September 30, 2018, which were measured at fair value on a non-recurring basis during the three and nine months ended September 30, 2018: September 30, 2018 Level 1 Level 2 Level 3 Total (dollars in thousands) Items recorded at fair value on a nonrecurring basis Assets Collateral dependent impaired loans $ — $ — $ 148 $ 148 Loans held for sale — — — — Total $ — $ — $ 148 $ 148

Recently Issued and Adopted A_2

Recently Issued and Adopted Accounting Guidance - Additional Information (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Federal tax rate		21.00%	35.00%
Accounting Standards Update 2018-02
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Reclassification from AOCI to retained earnings	$ 1,300,000
Reclassification effect on shareholder equity	$ 0

Cash and Cash Equivalents - Add

Cash and Cash Equivalents - Additional Information (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash And Cash Equivalents [Line Items]
Cash and cash equivalents	$ 16,431,000	$ 103,591,000	$ 13,668,000	$ 54,050,000
Federal Reserve Bank of Boston
Cash And Cash Equivalents [Line Items]
Reserve balance of cash and cash equivalents	15,100,000	12,800,000
New Hampshire
Cash And Cash Equivalents [Line Items]
Pledged amount to federal banking department	$ 500,000	$ 500,000

Investment Securities - Summary

Investment Securities - Summary of Carrying Amounts of Securities and Their Estimated Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Available for sale securities, Amortized Cost	$ 186,715	$ 208,911
Available for sale securities, Gross Unrealized Gains	139	260
Available for sale securities, Gross Unrealized Losses	(6,491)	(4,154)
Available for sale securities, Fair Value	180,363	205,017
Held to maturity securities, Amortized Cost	271,005	232,188
Held to maturity securities, Gross Unrealized Gains	952	2,555
Held to maturity securities, Gross Unrealized Losses	(5,272)	(1,189)
Held to maturity securities, Fair Value	266,685	233,554
Total, Amortized Cost	457,720	441,099
Total, Gross Unrealized Gains	1,091	2,815
Total, Gross Unrealized Losses	(11,763)	(5,343)
Total, Fair Value	447,048	438,571
U.S. GSE Obligations
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Available for sale securities, Amortized Cost	85,008	90,021
Available for sale securities, Gross Unrealized Losses	(1,651)	(1,230)
Available for sale securities, Fair Value	83,357	88,791
Held to maturity securities, Amortized Cost	32,572	32,572
Held to maturity securities, Gross Unrealized Losses	(503)	(166)
Held to maturity securities, Fair Value	32,069	32,406
Corporate Debt Securities
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Available for sale securities, Amortized Cost	5,019	5,034
Available for sale securities, Gross Unrealized Gains	8	12
Available for sale securities, Gross Unrealized Losses	(76)	(45)
Available for sale securities, Fair Value	4,951	5,001
Held to maturity securities, Amortized Cost	6,970	1,998
Held to maturity securities, Gross Unrealized Gains		4
Held to maturity securities, Gross Unrealized Losses	(156)
Held to maturity securities, Fair Value	6,814	2,002
Mortgage Backed Securities
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Available for sale securities, Amortized Cost	96,688	113,184
Available for sale securities, Gross Unrealized Gains	131	248
Available for sale securities, Gross Unrealized Losses	(4,764)	(2,806)
Available for sale securities, Fair Value	92,055	110,626
Held to maturity securities, Amortized Cost	155,723	117,155
Held to maturity securities, Gross Unrealized Gains	3	7
Held to maturity securities, Gross Unrealized Losses	(3,839)	(906)
Held to maturity securities, Fair Value	151,887	116,256
Mutual Funds
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Available for sale securities, Amortized Cost		672
Available for sale securities, Gross Unrealized Losses		(73)
Available for sale securities, Fair Value		599
Municipal Securities
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Held to maturity securities, Amortized Cost	75,740	80,463
Held to maturity securities, Gross Unrealized Gains	949	2,544
Held to maturity securities, Gross Unrealized Losses	(774)	(117)
Held to maturity securities, Fair Value	$ 75,915	$ 82,890

Investment Securities - Gross U

Investment Securities - Gross Unrealized Losses of Aggregated by Investment Category and Length of Time that Individual Securities have been in Continuous Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Temporarily Impaired Securities, Available for sale securities, Fair Value, Less than 12 months		$ 17,505
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses, Less than 12 months		(178)
Temporarily Impaired Securities, Available for sale securities, Fair Value, 12 months or longer	$ 176,767	183,064
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses, 12 months or longer	(6,491)	(3,976)
Temporarily Impaired Securities, Available for sale securities, Fair Value	176,767	200,569
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses	(6,491)	(4,154)
Temporarily Impaired Securities, Held to maturity securities, Fair Value, Less than 12 months	128,472	145,374
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses, Less than 12 months	(2,688)	(1,091)
Temporarily Impaired Securities, Held to maturity securities, Fair Value, 12 months or longer	84,110	6,462
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses, 12 months or longer	(2,584)	(98)
Temporarily Impaired Securities, Held to maturity securities, Fair Value	212,582	151,836
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses	(5,272)	(1,189)
Temporarily Impaired Securities, Fair Value, Less than 12 months	128,472	162,879
Temporarily Impaired Securities, Unrealized Losses, Less than 12 months	(2,688)	(1,269)
Temporarily Impaired Securities, Fair Value, 12 months or longer	260,877	189,526
Temporarily Impaired Securities, Unrealized Losses, 12 months or longer	(9,075)	(4,074)
Temporarily Impaired Securities, Fair Value	389,349	352,405
Temporarily Impaired Securities, Unrealized Losses	(11,763)	(5,343)
U.S. GSE Obligations
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Temporarily Impaired Securities, Available for sale securities, Fair Value, Less than 12 months		4,979
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses, Less than 12 months		(21)
Temporarily Impaired Securities, Available for sale securities, Fair Value, 12 months or longer	83,357	83,812
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses, 12 months or longer	(1,651)	(1,209)
Temporarily Impaired Securities, Available for sale securities, Fair Value	83,357	88,791
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses	(1,651)	(1,230)
Temporarily Impaired Securities, Held to maturity securities, Fair Value, Less than 12 months	12,374	27,407
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses, Less than 12 months	(200)	(166)
Temporarily Impaired Securities, Held to maturity securities, Fair Value, 12 months or longer	19,695
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses, 12 months or longer	(303)
Temporarily Impaired Securities, Held to maturity securities, Fair Value	32,069	27,407
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses	(503)	(166)
Mortgage Backed Securities
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Temporarily Impaired Securities, Available for sale securities, Fair Value, Less than 12 months		12,526
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses, Less than 12 months		(157)
Temporarily Impaired Securities, Available for sale securities, Fair Value, 12 months or longer	89,466	94,663
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses, 12 months or longer	(4,764)	(2,649)
Temporarily Impaired Securities, Available for sale securities, Fair Value	89,466	107,189
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses	(4,764)	(2,806)
Temporarily Impaired Securities, Held to maturity securities, Fair Value, Less than 12 months	94,453	115,926
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses, Less than 12 months	(1,993)	(906)
Temporarily Impaired Securities, Held to maturity securities, Fair Value, 12 months or longer	57,280	3
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses, 12 months or longer	(1,846)
Temporarily Impaired Securities, Held to maturity securities, Fair Value	151,733	115,929
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses	(3,839)	(906)
Corporate Debt Securities
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Temporarily Impaired Securities, Available for sale securities, Fair Value, 12 months or longer	3,944	3,990
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses, 12 months or longer	(76)	(45)
Temporarily Impaired Securities, Available for sale securities, Fair Value	3,944	3,990
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses	(76)	(45)
Temporarily Impaired Securities, Held to maturity securities, Fair Value, Less than 12 months	6,814
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses, Less than 12 months	(156)
Temporarily Impaired Securities, Held to maturity securities, Fair Value	6,814
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses	(156)
Mutual Funds
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Temporarily Impaired Securities, Available for sale securities, Fair Value, 12 months or longer		599
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses, 12 months or longer		(73)
Temporarily Impaired Securities, Available for sale securities, Fair Value		599
Temporarily Impaired Securities, Available for sale securities, Unrealized Losses		(73)
Municipal Securities
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Temporarily Impaired Securities, Held to maturity securities, Fair Value, Less than 12 months	14,831	2,041
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses, Less than 12 months	(339)	(19)
Temporarily Impaired Securities, Held to maturity securities, Fair Value, 12 months or longer	7,135	6,459
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses, 12 months or longer	(435)	(98)
Temporarily Impaired Securities, Held to maturity securities, Fair Value	21,966	8,500
Temporarily Impaired Securities, Held to maturity securities, Unrealized Losses	$ (774)	$ (117)

Investment Securities - Additio

Investment Securities - Additional Information (Details)	Sep. 30, 2018USD ($)Security	Dec. 31, 2017USD ($)Security
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Number of debt securities with unrealized losses | Security	159	118
Aggregate depreciation percentage of gross unrealized losses from amortized cost	2.93%	1.49%
Number of equity securities with unrealized losses | Security		1
Maximum
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Percentage of unrealized loss on amortized cost	10.34%	10.90%
Unrealized loss of amortized cost basis | $	$ 52,000	$ 73,000
Unrealized dollar loss of amortized cost basis | $	$ 260,000	$ 185,000
Percentage of unrealized dollar loss on amortized cost	5.20%	3.71%

Investment Securities - Schedul

Investment Securities - Schedule of Amortized Cost and Fair Value of Debt Securities, Aggregated By Earlier of Guaranteed Call Date or Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Available for sale securities, Within One Year, Amortized Cost	$ 22,020
Available for sale securities, Within One Year, Fair Value	21,904
Available for sale securities, After One, But Within Five Years, Amortized Cost	68,099
Available for sale securities, After One, But Within Five Years, Fair Value	66,498
Available for sale securities, After Five, But Within Ten Years, Amortized Cost	35,457
Available for sale securities, After Five, But Within Ten Years, Fair Value	33,969
Available for sale securities, After Ten Years, Amortized Cost	61,139
Available for sale securities, After Ten Years, Fair Value	57,992
Available for sale securities, Total, Amortized Cost	186,715
Available for sale securities, Total, Fair Value	180,363
Held to maturity securities, Within One Year, Amortized Cost	9,451
Held to maturity securities, Within One Year, Fair Value	9,462
Held to maturity securities, After One, But Within Five Years, Amortized Cost	46,625
Held to maturity securities, After One, But Within Five Years, Fair Value	46,127
Held to maturity securities, After Five, But Within Ten Years, Amortized Cost	78,444
Held to maturity securities, After Five, But Within Ten Years, Fair Value	77,988
Held to maturity securities, After Ten Years, Amortized Cost	136,485
Held to maturity securities, After Ten Years, Fair Value	133,108
Held to maturity securities, Amortized Cost	271,005	$ 232,188
Held to maturity securities, Fair Value	266,685	233,554
Available for sale securities and Held to maturity securities, Within One Year, Amortized Cost	31,471
Available for sale securities and Held to maturity securities, Within One Year, Fair Value	31,366
Available for sale securities and Held to maturity securities, After One, But Within Five Years, Amortized Cost	114,724
Available for sale securities and Held to maturity securities, After One, But Within Five Years, Fair Value	112,625
Available for sale securities and Held to maturity securities, After Five, But Within Ten Years, Amortized Cost	113,901
Available for sale securities and Held to maturity securities, After Five, But Within Ten Years, Fair Value	111,957
Available for sale securities and Held to maturity securities, After Ten Years, Amortized Cost	197,624
Available for sale securities and Held to maturity securities, After Ten Years, Fair Value	191,100
Available for sale securities and Held to maturity securities, Total, Amortized Cost	457,720
Available for sale securities and Held to maturity securities, Total, Fair Value	447,048
U.S. GSE Obligations
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Available for sale securities, Within One Year, Amortized Cost	20,008
Available for sale securities, Within One Year, Fair Value	19,910
Available for sale securities, After One, But Within Five Years, Amortized Cost	65,000
Available for sale securities, After One, But Within Five Years, Fair Value	63,447
Available for sale securities, Total, Amortized Cost	85,008
Available for sale securities, Total, Fair Value	83,357
Held to maturity securities, Within One Year, Amortized Cost	5,002
Held to maturity securities, Within One Year, Fair Value	4,982
Held to maturity securities, After One, But Within Five Years, Amortized Cost	27,570
Held to maturity securities, After One, But Within Five Years, Fair Value	27,087
Held to maturity securities, Amortized Cost	32,572
Held to maturity securities, Fair Value	32,069
Mortgage Backed Securities
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Available for sale securities, After One, But Within Five Years, Amortized Cost	92
Available for sale securities, After One, But Within Five Years, Fair Value	94
Available for sale securities, After Five, But Within Ten Years, Amortized Cost	35,457
Available for sale securities, After Five, But Within Ten Years, Fair Value	33,969
Available for sale securities, After Ten Years, Amortized Cost	61,139
Available for sale securities, After Ten Years, Fair Value	57,992
Available for sale securities, Total, Amortized Cost	96,688
Available for sale securities, Total, Fair Value	92,055
Held to maturity securities, Within One Year, Amortized Cost	63
Held to maturity securities, Within One Year, Fair Value	64
Held to maturity securities, After One, But Within Five Years, Amortized Cost	35
Held to maturity securities, After One, But Within Five Years, Fair Value	35
Held to maturity securities, After Five, But Within Ten Years, Amortized Cost	36,295
Held to maturity securities, After Five, But Within Ten Years, Fair Value	35,397
Held to maturity securities, After Ten Years, Amortized Cost	119,330
Held to maturity securities, After Ten Years, Fair Value	116,391
Held to maturity securities, Amortized Cost	155,723
Held to maturity securities, Fair Value	151,887
Corporate Debt Securities
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Available for sale securities, Within One Year, Amortized Cost	2,012
Available for sale securities, Within One Year, Fair Value	1,994
Available for sale securities, After One, But Within Five Years, Amortized Cost	3,007
Available for sale securities, After One, But Within Five Years, Fair Value	2,957
Available for sale securities, Total, Amortized Cost	5,019
Available for sale securities, Total, Fair Value	4,951
Held to maturity securities, After One, But Within Five Years, Amortized Cost	6,970
Held to maturity securities, After One, But Within Five Years, Fair Value	6,814
Held to maturity securities, Amortized Cost	6,970
Held to maturity securities, Fair Value	6,814
Municipal Securities
Schedule of Available for Sale Securities and Held to Maturity Securities [Line Items]
Held to maturity securities, Within One Year, Amortized Cost	4,386
Held to maturity securities, Within One Year, Fair Value	4,416
Held to maturity securities, After One, But Within Five Years, Amortized Cost	12,050
Held to maturity securities, After One, But Within Five Years, Fair Value	12,191
Held to maturity securities, After Five, But Within Ten Years, Amortized Cost	42,149
Held to maturity securities, After Five, But Within Ten Years, Fair Value	42,591
Held to maturity securities, After Ten Years, Amortized Cost	17,155
Held to maturity securities, After Ten Years, Fair Value	16,717
Held to maturity securities, Amortized Cost	75,740	80,463
Held to maturity securities, Fair Value	$ 75,915	$ 82,890

Investment Securities - Summa_2

Investment Securities - Summary of Gains (Losses) from Sale of Investment Securities (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Amortized cost of securities sold	$ 700	$ 77,372
Gain/(loss) realized on securities sold	2	(3)
Net proceeds from securities sold	$ 702	$ 77,369

Loans and the Allowance for L_3

Loans and the Allowance for Loan Losses - Loans Outstanding by Category (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Loans And Leases Receivable Disclosure [Line Items]
Total loans	$ 1,451,781	$ 1,350,899
Residential Mortgage
Loans And Leases Receivable Disclosure [Line Items]
Deferred costs (fees) net of unearned fees	1,143	1,042
Total loans	573,525	538,920
Residential Mortgage | Mortgages - Fixed Rate
Loans And Leases Receivable Disclosure [Line Items]
Loans outstanding by category gross	295,452	298,851
Residential Mortgage | Mortgages - Adjustable Rate
Loans And Leases Receivable Disclosure [Line Items]
Loans outstanding by category gross	276,930	239,027
Commercial Mortgage
Loans And Leases Receivable Disclosure [Line Items]
Deferred costs (fees) net of unearned fees	147	199
Total loans	688,536	633,649
Commercial Mortgage | Construction
Loans And Leases Receivable Disclosure [Line Items]
Loans outstanding by category gross	40,076	35,904
Commercial Mortgage | Mortgages - Nonowner Occupied
Loans And Leases Receivable Disclosure [Line Items]
Loans outstanding by category gross	610,966	562,203
Commercial Mortgage | Mortgages - Owner Occupied
Loans And Leases Receivable Disclosure [Line Items]
Loans outstanding by category gross	37,347	35,343
Home Equity
Loans And Leases Receivable Disclosure [Line Items]
Deferred costs (fees) net of unearned fees	247	255
Total loans	70,173	74,444
Home Equity | Home Equity - Lines of Credit
Loans And Leases Receivable Disclosure [Line Items]
Loans outstanding by category gross	65,138	70,326
Home Equity | Home Equity - Term Loans
Loans And Leases Receivable Disclosure [Line Items]
Loans outstanding by category gross	4,788	3,863
Commercial & Industrial
Loans And Leases Receivable Disclosure [Line Items]
Loans outstanding by category gross	82,272	65,305
Deferred costs (fees) net of unearned fees	(29)	(10)
Total loans	82,243	65,295
Consumer
Loans And Leases Receivable Disclosure [Line Items]
Deferred costs (fees) net of unearned fees	15	16
Total loans	37,304	38,591
Consumer | Secured
Loans And Leases Receivable Disclosure [Line Items]
Loans outstanding by category gross	35,890	37,272
Consumer | Unsecured
Loans And Leases Receivable Disclosure [Line Items]
Loans outstanding by category gross	$ 1,399	$ 1,303

Loans and the Allowance for L_4

Loans and the Allowance for Loan Losses - Additional Information (Details)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018USD ($)Loan	Sep. 30, 2018USD ($)Loan	Dec. 31, 2017USD ($)Loan
Financing Receivable Modifications [Line Items]
Loans outstanding to directors and officers	$ 391,000	$ 391,000	$ 516,000
Loans to directors and officers, additions		14,000	124,000
Loans to directors and officers, repayments		139,000	298,000
Commitments to lend additional funds to borrowers whose loans were on non-accrual status	$ 0	$ 0	$ 0
Number of loans modified as troubled debt restructuring | Loan	0	0	1
Number of loans determined to be troubled debt restructurings | Loan		3	3
Troubled debt restructuring carrying value	$ 125,000	$ 125,000	$ 150,000
Number of TDRs defaulted during the period | Loan		0	0
Pre-modification carrying value			$ 65,000
Post-modification carrying value			$ 48,000
Number of loans designated to be troubled debt restructurings | Loan			2
Specific allowances for troubled debt restructurings	0	$ 0	$ 0
Loans secured by one- to-four family residential properties in process of foreclosure	$ 351,000,000	$ 351,000,000	64,000,000
Troubled Debt Restructurings
Financing Receivable Modifications [Line Items]
Troubled debt restructuring carrying value			$ 29,000

Loans and the Allowance for L_5

Loans and the Allowance for Loan Losses - Non-performing Loans Disaggregated by Loan Category (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable Recorded Investment Past Due [Line Items]
Troubled debt restructurings	$ 125,000	$ 150,000
Non-Performing Loans
Financing Receivable Recorded Investment Past Due [Line Items]
Non-accrual loans	682,000	1,148,000
Troubled debt restructurings	126,000	150,000
Total	808,000	1,298,000
Non-Performing Loans | Residential Mortgage
Financing Receivable Recorded Investment Past Due [Line Items]
Non-accrual loans	520,000	918,000
Troubled debt restructurings	114,000	121,000
Total	634,000	1,039,000
Non-Performing Loans | Commercial Mortgage
Financing Receivable Recorded Investment Past Due [Line Items]
Non-accrual loans		213,000
Total		213,000
Non-Performing Loans | Home Equity
Financing Receivable Recorded Investment Past Due [Line Items]
Non-accrual loans	14,000	17,000
Total	14,000	17,000
Non-Performing Loans | Commercial & Industrial
Financing Receivable Recorded Investment Past Due [Line Items]
Non-accrual loans	148,000
Troubled debt restructurings	12,000	29,000
Total	$ 160,000	$ 29,000

Loans and the Allowance for L_6

Loans and the Allowance for Loan Losses - Loans Receivable Disaggregated by Credit Quality Indicator (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable Recorded Investment [Line Items]
Loans receivable	$ 1,451,781	$ 1,350,899
Residential Mortgage
Financing Receivable Recorded Investment [Line Items]
Loans receivable	573,525	538,920
Residential Mortgage | Performing
Financing Receivable Recorded Investment [Line Items]
Loans receivable	572,891	537,881
Residential Mortgage | Non-Performing Loans
Financing Receivable Recorded Investment [Line Items]
Loans receivable	634	1,039
Home Equity
Financing Receivable Recorded Investment [Line Items]
Loans receivable	70,173	74,444
Home Equity | Performing
Financing Receivable Recorded Investment [Line Items]
Loans receivable	70,159	74,427
Home Equity | Non-Performing Loans
Financing Receivable Recorded Investment [Line Items]
Loans receivable	14	17
Consumer
Financing Receivable Recorded Investment [Line Items]
Loans receivable	37,304	38,591
Consumer | Performing
Financing Receivable Recorded Investment [Line Items]
Loans receivable	37,304	38,591
Commercial Mortgage
Financing Receivable Recorded Investment [Line Items]
Loans receivable	688,536	633,649
Commercial Mortgage | 1-6 (Pass)
Financing Receivable Recorded Investment [Line Items]
Loans receivable	683,883	629,852
Commercial Mortgage | 7 (Special Mention)
Financing Receivable Recorded Investment [Line Items]
Loans receivable	4,653	3,584
Commercial Mortgage | 8 (Substandard)
Financing Receivable Recorded Investment [Line Items]
Loans receivable		213
Commercial & Industrial
Financing Receivable Recorded Investment [Line Items]
Loans receivable	82,243	65,295
Commercial & Industrial | 1-6 (Pass)
Financing Receivable Recorded Investment [Line Items]
Loans receivable	72,858	56,755
Commercial & Industrial | 7 (Special Mention)
Financing Receivable Recorded Investment [Line Items]
Loans receivable	5,456	8,126
Commercial & Industrial | 8 (Substandard)
Financing Receivable Recorded Investment [Line Items]
Loans receivable	$ 3,929	$ 414

Loans and the Allowance for L_7

Loans and the Allowance for Loan Losses - Schedule of Loans Receivable Disaggregated by Past Due Status (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	$ 2,618	$ 2,349
Current Loans	1,449,163	1,348,550
Total loans	1,451,781	1,350,899
30-59 Days Past Due
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	1,839	1,530
60-89 Days Past Due
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	266	738
90 Days or Greater
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	513	81
Residential Mortgage
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	2,118	2,123
Current Loans	571,407	536,797
Total loans	573,525	538,920
Residential Mortgage | 30-59 Days Past Due
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	1,503	1,353
Residential Mortgage | 60-89 Days Past Due
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	264	706
Residential Mortgage | 90 Days or Greater
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	351	64
Commercial Mortgage
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	301	32
Current Loans	688,235	633,617
Total loans	688,536	633,649
Commercial Mortgage | 30-59 Days Past Due
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	301
Commercial Mortgage | 60-89 Days Past Due
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due		32
Commercial & Industrial
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	177
Current Loans	82,066	65,295
Total loans	82,243	65,295
Commercial & Industrial | 30-59 Days Past Due
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	29
Commercial & Industrial | 90 Days or Greater
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	148
Consumer loans
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	8	176
Current Loans	37,296	38,415
Total loans	37,304	38,591
Consumer loans | 30-59 Days Past Due
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	6	176
Consumer loans | 60-89 Days Past Due
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	2
Home Equity
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	14	18
Current Loans	70,159	74,426
Total loans	70,173	74,444
Home Equity | 30-59 Days Past Due
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due		1
Home Equity | 90 Days or Greater
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due	$ 14	$ 17

Loans and the Allowance for L_8

Loans and the Allowance for Loan Losses - Information Pertaining to Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable Allowance For Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	$ 654	$ 1,344	$ 654	$ 1,344
With no required reserve recorded, Average Carrying Value	662	1,357	674	1,379
With no required reserve recorded, Unpaid Principal Balance	836	1,582	836	1,582
With no required reserve recorded, Interest Income Recognized		1	1	5
With required reserve recorded, Carrying Value	148	334	148	334
With required reserve recorded, Average Carrying Value	148	338	148	345
With required reserve recorded, Unpaid Principal Balance	148	346	148	346
With required reserve recorded, Related Allowance	77	357	77	357
With required reserve recorded, Interest Income Recognized	1		6	1
Carrying Value	802	1,678	802	1,678
Average Carrying Value	810	1,695	822	1,724
Unpaid Principal Balance	984	1,928	984	1,928
Interest Income Recognized	1	1	7	6
Commercial and Industrial
Financing Receivable Allowance For Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	12	38	12	38
With no required reserve recorded, Average Carrying Value	14	40	20	38
With no required reserve recorded, Unpaid Principal Balance	12	38	12	38
With no required reserve recorded, Interest Income Recognized		1	1	2
With required reserve recorded, Carrying Value	148	269	148	269
With required reserve recorded, Average Carrying Value	148	273	148	279
With required reserve recorded, Unpaid Principal Balance	148	281	148	281
With required reserve recorded, Related Allowance	77	259	77	259
With required reserve recorded, Interest Income Recognized	1		6
Carrying Value	160	307	160	307
Average Carrying Value	162	313	168	317
Unpaid Principal Balance	160	319	160	319
Interest Income Recognized	1	1	7	2
Commercial Mortgage
Financing Receivable Allowance For Credit Losses [Line Items]
With no required reserve recorded, Carrying Value		217		217
With no required reserve recorded, Average Carrying Value		221		227
With no required reserve recorded, Unpaid Principal Balance		229		229
With no required reserve recorded, Interest Income Recognized				3
Carrying Value		217		217
Average Carrying Value		221		227
Unpaid Principal Balance		229		229
Interest Income Recognized				3
Residential Mortgage
Financing Receivable Allowance For Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	565	991	565	991
With no required reserve recorded, Average Carrying Value	569	997	574	1,012
With no required reserve recorded, Unpaid Principal Balance	712	1,187	712	1,187
With required reserve recorded, Carrying Value		65		65
With required reserve recorded, Average Carrying Value		65		66
With required reserve recorded, Unpaid Principal Balance		65		65
With required reserve recorded, Related Allowance		98		98
With required reserve recorded, Interest Income Recognized				1
Carrying Value	565	1,056	565	1,056
Average Carrying Value	569	1,062	574	1,078
Unpaid Principal Balance	712	1,252	712	1,252
Interest Income Recognized				1
Home Equity
Financing Receivable Allowance For Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	77	98	77	98
With no required reserve recorded, Average Carrying Value	79	99	80	102
With no required reserve recorded, Unpaid Principal Balance	112	128	112	128
Carrying Value	77	98	77	98
Average Carrying Value	79	99	80	102
Unpaid Principal Balance	$ 112	$ 128	$ 112	$ 128

Loans and the Allowance for L_9

Loans and the Allowance for Loan Losses - Summary of Changes in Allowance for Loan Losses Disaggregated by Loan Type (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance for loan losses, Beginning balance	$ 15,645	$ 15,303	$ 15,320	$ 15,261
Charge-offs	(10)	(1)	(42)	(15)
Recoveries	14	8	41	14
Provision for (Release of)	457	310	787	360
Allowance for loan losses, Ending balance	16,106	15,620	16,106	15,620
Residential Mortgage
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance for loan losses, Beginning balance	4,879	4,783	5,047	4,898
Provision for (Release of)	267	303	99	188
Allowance for loan losses, Ending balance	5,146	5,086	5,146	5,086
Commercial Mortgage
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance for loan losses, Beginning balance	8,674	8,449	8,289	8,451
Provision for (Release of)	149	(126)	534	(128)
Allowance for loan losses, Ending balance	8,823	8,323	8,823	8,323
Home Equity
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance for loan losses, Beginning balance	601	632	630	651
Provision for (Release of)	(56)	(4)	(85)	(23)
Allowance for loan losses, Ending balance	545	628	545	628
Commercial & Industrial
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance for loan losses, Beginning balance	1,195	758	946	807
Charge-offs			(11)	(1)
Recoveries	13	6	35	8
Provision for (Release of)	9	94	247	44
Allowance for loan losses, Ending balance	1,217	858	1,217	858
Consumer
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance for loan losses, Beginning balance	296	324	315	264
Charge-offs	(10)	(1)	(31)	(14)
Recoveries	1	2	6	6
Provision for (Release of)	11	43	8	112
Allowance for loan losses, Ending balance	298	368	298	368
Impaired
Financing Receivable Allowance For Credit Losses [Line Items]
Allowance for loan losses, Beginning balance		357	93	190
Provision for (Release of)	77		(16)	167
Allowance for loan losses, Ending balance	$ 77	$ 357	$ 77	$ 357

Loans and the Allowance for _10

Loans and the Allowance for Loan Losses - Summary of Allowance for Loan Losses and Related Loans Receivable Disaggregated by Impairment Method (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable Allowance For Credit Losses [Line Items]
Individually evaluated for impairment	$ 77	$ 93
Collectively evaluated for impairment	16,029	15,227
Allowance for loan losses	16,106	15,320
Individually evaluated for impairment	802	1,296
Collectively evaluated for impairment	1,450,979	1,349,603
Total loans	1,451,781	1,350,899
Residential Mortgage
Financing Receivable Allowance For Credit Losses [Line Items]
Individually evaluated for impairment		93
Collectively evaluated for impairment	5,146	5,047
Allowance for loan losses	5,146	5,140
Individually evaluated for impairment	578	968
Collectively evaluated for impairment	572,947	537,952
Total loans	573,525	538,920
Commercial Mortgage
Financing Receivable Allowance For Credit Losses [Line Items]
Collectively evaluated for impairment	8,823	8,289
Allowance for loan losses	8,823	8,289
Individually evaluated for impairment		213
Collectively evaluated for impairment	688,536	633,436
Total loans	688,536	633,649
Home Equity
Financing Receivable Allowance For Credit Losses [Line Items]
Collectively evaluated for impairment	545	630
Allowance for loan losses	545	630
Individually evaluated for impairment	64	86
Collectively evaluated for impairment	70,109	74,358
Total loans	70,173	74,444
Commercial & Industrial
Financing Receivable Allowance For Credit Losses [Line Items]
Individually evaluated for impairment	77
Collectively evaluated for impairment	1,217	946
Allowance for loan losses	1,294	946
Individually evaluated for impairment	160	29
Collectively evaluated for impairment	82,083	65,266
Total loans	82,243	65,295
Consumer
Financing Receivable Allowance For Credit Losses [Line Items]
Collectively evaluated for impairment	298	315
Allowance for loan losses	298	315
Collectively evaluated for impairment	37,304	38,591
Total loans	$ 37,304	$ 38,591

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Statutory federal tax rate			21.00%		35.00%
Effective tax rate	24.00%	35.00%	23.20%	33.50%
Net deferred tax assets	$ 8,990,000		$ 8,990,000		$ 8,273,000
Deferred tax assets, valuation allowance	$ 0		$ 0		$ 0

Income Taxes - Components of In

Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Current
Federal	$ 1,812	$ 2,651	$ 4,047	$ 5,966
State	799	690	1,731	1,552
Total current expense	2,611	3,341	5,778	7,518
Deferred
Federal	(344)	(504)	(106)	(414)
State	(162)	(143)	(50)	(117)
Total deferred	(506)	(647)	(156)	(531)
Total income tax expense	$ 2,105	$ 2,694	$ 5,622	$ 6,987

Pension and Retirement Plans -

Pension and Retirement Plans - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plan
Net periodic benefit cost
Service cost		$ 339		$ 1,162
Interest cost	$ 389	412	$ 1,167	1,413
Expected return on assets	(723)	(619)	(2,168)	(2,122)
Amortization of prior service cost (credit)	(1)	(1)	(3)	(3)
Amortization of net actuarial loss	27	179	80	614
Net periodic benefit cost (credit)	(308)	310	(924)	1,064
Supplemental Retirement Plan
Net periodic benefit cost
Service cost	88	66	265	200
Interest cost	78	91	232	273
Amortization of net actuarial loss	1		3
Net periodic benefit cost (credit)	167	157	500	473
Postretirement Healthcare Plan
Net periodic benefit cost
Service cost	6	4	17	12
Interest cost	6	5	17	15
Amortization of net actuarial loss		(2)		(6)
Net periodic benefit cost (credit)	$ 12	$ 7	$ 34	$ 21

Pension and Retirement Plans _2

Pension and Retirement Plans - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Profit Sharing Plan | 401(k)
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, maximum employee contribution, percent				100.00%
Profit Sharing Plan | 401(k) | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent				4.00%
Profit Sharing Plan | 401(k) | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent						3.00%
Profit Sharing Plan | Discretionary Contribution
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer matching contribution, percent				4.00%
Defined contribution plan, minimum number of hours of service per year required for eligibility				1000 hours
Employee Stock Ownership Plan (ESOP)
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, minimum number of hours of service per year required for eligibility				1000 hours
Defined contribution plan, minimum service period required for eligibility				12 months
Employee Stock Ownership Plan (ESOP) | Minimum
Defined Benefit Plan Disclosure [Line Items]
Employee eligibility age under the plan				21 years
Profit Sharing and ESOP Plans
Defined Benefit Plan Disclosure [Line Items]
Contribution expense related to Plans	$ 595,000	$ 238,000		$ 2,000,000	$ 713,000
Qualified Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to defined benefit pension plan	$ 0			$ 0
Qualified Defined Benefit Pension Plan | Scenario Forecast
Defined Benefit Plan Disclosure [Line Items]
Contributions to defined benefit pension plan			$ 0

Stock Based Compensation - Sche

Stock Based Compensation - Schedule of Pre-tax Expense Associated with All Outstanding Non-vested Restricted Stock Awards, Restricted Stock Units, Performance Based Restricted Stock Units and Related Tax Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Compensation [Abstract]
Stock based compensation expense	$ 705	$ 253	$ 2,052	$ 805
Related tax benefits	$ 198	$ 103	$ 577	$ 329

Financial Instruments with Of_3

Financial Instruments with Off-Balance-Sheet Risk - Summary of Off-Balance-Sheet Financial Instruments with Contractual Amounts Include Present Credit Risk (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Interest Rate Swap with Customers
Financial instruments whose notional amount exceeds the amount of credit risk:
Financial instrument notional amount exceeds credit risk	$ 96,450	$ 74,758
Mirror Swaps with Counterparties
Financial instruments whose notional amount exceeds the amount of credit risk:
Financial instrument notional amount exceeds credit risk	96,450	74,758
Standby Letters of Credit
Financial instruments whose contractual amount represents credit risk:
Financial instruments with off-balance-sheet risk	8,639	8,322
Unused Portion of Existing Lines of Credit
Financial instruments whose contractual amount represents credit risk:
Financial instruments with off-balance-sheet risk	359,798	304,298
Origination of New Loans
Financial instruments whose contractual amount represents credit risk:
Financial instruments with off-balance-sheet risk	30,283	45,061
Commitments to Sell Residential Mortgage Loans
Financial instruments whose notional amount exceeds the amount of credit risk:
Financial instrument notional amount exceeds credit risk	1,871	1,490
Risk Participation Agreements with Counterparties
Financial instruments whose notional amount exceeds the amount of credit risk:
Financial instrument notional amount exceeds credit risk	$ 59,812	$ 38,494

Shareholders' Equity - Minimum

Shareholders' Equity - Minimum Capital Requirements were Considered Well Capitalized by FRB and FDIC (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 185,522,000	$ 168,615,000
Tier I capital (to risk-weighted assets), Actual, Amount	169,366,000	153,281,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	169,366,000	153,281,000
Tier I capital (to average assets), Actual, Amount	$ 169,366,000	$ 153,281,000
Total capital (to risk-weighted assets), Actual, Ratio	14.00%	13.70%
Tier I capital (to risk-weighted assets), Actual, Ratio	12.80%	12.50%
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	12.80%	12.50%
Tier I capital (to average assets), Actual, Ratio	8.50%	8.10%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	$ 106,268,000	$ 98,136,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	79,701,000	73,602,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	59,776,000	55,202,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy, Amount	$ 79,576,000	$ 76,026,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	8.00%	8.00%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	6.00%	6.00%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	4.50%	4.50%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy, Ratio	4.00%	4.00%
Minimum
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount	$ 131,174,000	$ 113,470,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount	104,607,000	88,936,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount	84,682,000	70,535,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount	$ 79,576,000	$ 76,026,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio	9.875%	9.25%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio	7.875%	7.25%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio	6.375%	5.75%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio	4.00%	4.00%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount	$ 139,477,000	$ 128,804,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount	112,910,000	104,270,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount	92,984,000	85,869,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount	$ 79,576,000	$ 76,026,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio	10.50%	10.50%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio	8.50%	8.50%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio	7.00%	7.00%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio	4.00%	4.00%
Cambridge Trust Company
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Actual, Amount	$ 181,025,000	$ 164,880,000
Tier I capital (to risk-weighted assets), Actual, Amount	164,869,000	149,546,000
Common equity tier I capital (to risk-weighted assets), Actual, Amount	164,869,000	149,546,000
Tier I capital (to average assets), Actual, Amount	$ 164,869,000	$ 149,546,000
Total capital (to risk-weighted assets), Actual, Ratio	13.60%	13.40%
Tier I capital (to risk-weighted assets), Actual, Ratio	12.40%	12.20%
Common equity tier I capital (to risk-weighted assets), Actual, Ratio	12.40%	12.20%
Tier I capital (to average assets), Actual, Ratio	8.30%	7.90%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	$ 106,268,000	$ 98,136,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	79,701,000	73,602,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Amount	59,776,000	55,202,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy, Amount	$ 79,576,000	$ 76,026,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	8.00%	8.00%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	6.00%	6.00%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy, Ratio	4.50%	4.50%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy, Ratio	4.00%	4.00%
Cambridge Trust Company | Minimum
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount	$ 131,174,000	$ 113,470,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount	104,607,000	88,936,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount	84,682,000	70,535,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Amount	$ 79,576,000	$ 76,026,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio	9.875%	9.25%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio	7.875%	7.25%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio	6.375%	5.75%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Phase-In Schedule, Ratio	4.00%	4.00%
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount	$ 139,477,000	$ 128,804,000
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount	112,910,000	104,270,000
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount	92,984,000	85,869,000
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Amount	$ 79,576,000	$ 76,026,000
Total capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio	10.50%	10.50%
Tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio	8.50%	8.50%
Common equity tier I capital (to risk-weighted assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio	7.00%	7.00%
Tier I capital (to average assets), Minimum Capital Required For Capital Adequacy Plus Capital Conservation Buffer Basel III Fully Phased In, Ratio	4.00%	4.00%
Total capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	$ 132,835,000	$ 122,670,000
Tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	106,268,000	98,136,000
Common equity tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	86,343,000	79,736,000
Tier I capital (to average assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Amount	$ 99,470,000	$ 95,033,000
Total capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	10.00%	10.00%
Tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	8.00%	8.00%
Common equity tier I capital (to risk-weighted assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	6.50%	6.50%
Tier I capital (to average assets), Minimum To Be Well-Capitalized Under Prompt Corrective Action Provisions, Ratio	5.00%	5.00%

Other Comprehensive Income - Su

Other Comprehensive Income - Summary of Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Unrealized (losses)/gains on available for sale securities, before tax amount
Unrealized (losses)/ gains on available for sale securities, before tax amount	$ (228)	$ 452	$ (2,456)	$ 1,524
Reclassification adjustment for (gains)/losses recognized in net income, before tax amount			(2)	3
Defined benefit retirement plans, before tax amount
Change in retirement liability, before tax amount	14	225	43	673
Total Other Comprehensive (Loss)/Income, before tax amount	(214)	677	(2,415)	2,200
Unrealized (losses)/gains on available for sale securities, tax (expense) or benefit
Unrealized holding (losses)/gains arising during period, tax (expense) or benefit	56	(166)	509	(556)
Reclassification adjustment for (gains)/losses recognized in net income, tax (expense) or benefit				(2)
Defined benefit retirement plans, tax (expense) or benefit
Change in retirement liability, tax (expense) or benefit	(4)	(92)	(13)	(275)
Total Other Comprehensive (Loss)/Income, tax (expense) or benefit	52	(258)	496	(833)
Unrealized (losses)/gains on available for sale securities
Unrealized holding (losses)/gains arising during period, net-of-tax amount	(172)	286	(1,947)	968
Reclassification adjustment for (gains)/losses recognized in net income, net-of-tax amount			(2)	1
Defined benefit retirement plans
Change in retirement liability, net-of-tax amount	10	133	30	398
Other comprehensive (loss)/income	$ (162)	$ 419	$ (1,919)	$ 1,367

Other Comprehensive Income - _2

Other Comprehensive Income - Summary of Reclassifications out of Accumulated Other Comprehensive Income ("AOCI") (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains and losses on available for sale securities			$ 2	$ (3)
Tax benefit or (expense)	$ (2,105)	$ (2,694)	(5,622)	(6,987)
Net income	$ 6,659	$ 5,010	18,575	13,852
Reclassifications out of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains and losses on available for sale securities			2	(3)
Tax benefit or (expense)				2
Net income			$ 2	$ (1)

Earnings Per Share - Summary of

Earnings Per Share - Summary of Reconciliation Between Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 6,659	$ 5,010	$ 18,575	$ 13,852
Less dividends and undistributed earnings allocated to participating securities	(66)	(53)	(189)	(148)
Net income applicable to common shareholders	$ 6,593	$ 4,957	$ 18,386	$ 13,704
Denominator:
Weighted average common shares outstanding	4,064,620	4,037,026	4,059,608	4,027,378
Earnings per common share - basic	$ 1.62	$ 1.23	$ 4.53	$ 3.40
Numerator:
Net income	$ 6,659	$ 5,010	$ 18,575	$ 13,852
Less dividends and undistributed earnings allocated to participating securities	(66)	(53)	(189)	(148)
Net income applicable to common shareholders	$ 6,593	$ 4,957	$ 18,386	$ 13,704
Denominator:
Weighted average common shares outstanding	4,064,620	4,037,026	4,059,608	4,027,378
Dilutive effect of common stock equivalents	36,000	33,000	35,000	36,000
Weighted average diluted common shares outstanding	4,101,378	4,070,332	4,095,447	4,062,743
Earnings per common share - diluted	$ 1.61	$ 1.22	$ 4.49	$ 3.37

Derivative Financial Instrume_3

Derivative Financial Instruments - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Loan related derivative income	$ 636,000	$ 284,000	$ 1,220,000	$ 647,000
Interest Rate Swap Contracts | Commercial Loan Borrowers
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative, notional amount	96,400,000		96,400,000		$ 74,800,000
Mirror Swap Contracts | Third-Party Financial Institutions
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative, notional amount	96,400,000		96,400,000		74,800,000
Risk Participation-In Agreements
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative, notional amount	59,800,000		59,800,000		38,500,000
Risk Participation-Out Agreements
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative, notional amount	$ 0		$ 0		$ 0

Derivative Financial Instrume_4

Derivative Financial Instruments - Summary of Fair Values of Derivative Instruments in the Company's Unaudited Consolidated Balance Sheets (Details) - Loan Related Derivative Contracts - Derivatives Not Designated as Hedging Instruments - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative assets	$ 3,866	$ 1,859
Derivative liabilities	3,960	1,940
Interest Rate Swaps with Customers
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative assets	3,866	1,859
Mirror Swaps with Counterparties
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative liabilities	3,866	1,859
Risk Participation Agreements
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Derivative liabilities	$ 94	$ 81

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Carrying Values and Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial assets
Securities available for sale	$ 180,363	$ 205,017
Securities held to maturity	266,685	233,554
FHLB Boston stock	6,666	4,242
Recurring Basis | Carrying Value
Financial assets
Cash and cash equivalents	16,431	103,591
Securities available for sale	180,363	205,017
Securities held to maturity	271,005	232,188
Loans, net	1,435,675	1,335,579
FHLB Boston stock	6,666	4,242
Accrued interest receivable	5,536	5,128
Mortgage servicing rights	701	793
Loan level interest rate swaps	3,866	1,859
Financial liabilities
Deposits	1,731,279	1,775,400
Short-term borrowings	66,700
Long-term borrowings	3,452	3,579
Loan level interest rate swaps	3,866	1,859
Risk participation agreements	94	81
Recurring Basis | Estimated Fair Value
Financial assets
Cash and cash equivalents	16,431	103,591
Securities available for sale	180,363	205,017
Securities held to maturity	266,685	233,554
Loans, net	1,377,456	1,304,719
FHLB Boston stock	6,666	4,242
Accrued interest receivable	5,536	5,128
Mortgage servicing rights	1,002	1,049
Loan level interest rate swaps	3,866	1,859
Financial liabilities
Deposits	1,728,606	1,772,838
Short-term borrowings	66,700
Long-term borrowings	3,387	3,559
Loan level interest rate swaps	3,866	1,859
Risk participation agreements	$ 94	$ 81

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Certain Assets Reported at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale	$ 180,363	$ 205,017
Mutual Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale		599
U.S. GSE Obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale	83,357	88,791
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale	4,951	5,001
Recurring Basis | Mutual Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale		599
Recurring Basis | U.S. GSE Obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale	83,357	88,791
Recurring Basis | Mortgage Backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale	92,055	110,626
Recurring Basis | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale	4,951	5,001
Recurring Basis | Interest Rate Swap Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other assets	3,866	1,859
Recurring Basis | Mirror Swaps with Counterparties
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other liabilities	3,866	1,859
Recurring Basis | Risk Participation Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other liabilities	94	81
Recurring Basis | Level 1 | Mutual Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale		599
Recurring Basis | Level 2 | U.S. GSE Obligations
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale	83,357	88,791
Recurring Basis | Level 2 | Mortgage Backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale	92,055	110,626
Recurring Basis | Level 2 | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Securities available for sale	4,951	5,001
Recurring Basis | Level 2 | Interest Rate Swap Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other assets	3,866	1,859
Recurring Basis | Level 2 | Mirror Swaps with Counterparties
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other liabilities	3,866	1,859
Recurring Basis | Level 2 | Risk Participation Agreements
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Other liabilities	$ 94	$ 81

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Carrying Value of Assets Measured at Fair Value on a Non-Recurring Basis (Details) - Non-recurring $ in Thousands	Sep. 30, 2018USD ($)
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value, non-recurring basis	$ 148
Collateral Dependent Impaired Loans
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value, non-recurring basis	148
Level 3
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value, non-recurring basis	148
Level 3 | Collateral Dependent Impaired Loans
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Assets, fair value, non-recurring basis	$ 148

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Transfers between levels	$ 0	$ 0	$ 0	$ 0