Century Bancorp (CNBKA) 10-K 13 Mar 14 2013 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2013	Jun. 28, 2013	Feb. 28, 2014	Feb. 28, 2014
			Class A Common Stock [Member]	Class B Common Stock [Member]
Document Information [Line Items]	'	'	'	'
Document Type	'10-K	'	'	'
Amendment Flag	'false	'	'	'
Document Period End Date	31-Dec-13	'	'	'
Document Fiscal Year Focus	'2013	'	'	'
Document Fiscal Period Focus	'FY	'	'	'
Entity Registrant Name	'CENTURY BANCORP INC	'	'	'
Entity Central Index Key	'0000812348	'	'	'
Current Fiscal Year End Date	'--12-31	'	'	'
Entity Well-known Seasoned Issuer	'No	'	'	'
Entity Current Reporting Status	'Yes	'	'	'
Entity Voluntary Filers	'No	'	'	'
Entity Filer Category	'Accelerated Filer	'	'	'
Entity Common Stock, Shares Outstanding	'	'	3,581,429	1,975,180
Entity Public Float	'	$125,231,085	'	'

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2013	Dec. 31, 2012
ASSETS	'	'
Cash and due from banks (Note 2)	$59,956,000	$53,646,000
Federal funds sold and interest-bearing deposits in other banks	34,722,000	98,637,000
Total cash and cash equivalents	94,678,000	152,283,000
Short-term investments	4,617,000	17,367,000
Securities available-for-sale, amortized cost $465,943 in 2013 and $1,414,595 in 2012 (Notes 3, 9 and 11)	464,245,000	1,434,801,000
Securities held-to-maturity, fair value $1,464,449 in 2013 and $281,924 in 2012 (Notes 4 and 11)	1,487,884,000	275,507,000
Federal Home Loan Bank of Boston, stock at cost	18,072,000	15,146,000
Loans, net (Note 5)	1,264,763,000	1,111,788,000
Less: allowance for loan losses (Note 6)	20,941,000	19,197,000
Net loans	1,243,822,000	1,092,591,000
Bank premises and equipment (Note 7)	23,400,000	23,899,000
Accrued interest receivable	6,539,000	5,811,000
Prepaid FDIC assessments	'	2,773,000
Other assets (Notes 8 and 16)	87,897,000	66,031,000
Total assets	3,431,154,000	3,086,209,000
LIABILITIES AND STOCKHOLDERS' EQUITY	'	'
Demand deposits	475,862,000	438,429,000
Savings and NOW deposits	992,796,000	933,316,000
Money market accounts	864,957,000	653,345,000
Time deposits (Note 10)	382,224,000	419,983,000
Total deposits	2,715,839,000	2,445,073,000
Securities sold under agreements to repurchase (Note 11)	214,440,000	191,390,000
Other borrowed funds (Note 12)	255,144,000	195,144,000
Subordinated debentures (Note 12)	36,083,000	36,083,000
Other liabilities	33,176,000	38,529,000
Total liabilities	3,254,682,000	2,906,219,000
Commitments and contingencies (Notes 7, 18 and 19)	'	'
Stockholders' equity (Note 15):	'	'
Preferred Stock - $1.00 par value; 100,000 shares authorized; no shares issued and outstanding	'	'
Additional paid-in capital	11,932,000	11,891,000
Retained earnings	180,747,000	162,892,000
Stockholders' equity before adjustment of accumulated other comprehensive (income) loss	198,235,000	180,337,000
Unrealized gains (losses) on securities available-for-sale, net of taxes	-1,045,000	12,330,000
Unrealized losses on securities transferred to held-to-maturity, net of taxes	-13,667,000	'
Pension liability, net of taxes	-7,051,000	-12,677,000
Total accumulated other comprehensive loss, net of taxes (Notes 3, 13 and 15)	-21,763,000	-347,000
Total stockholders' equity	176,472,000	179,990,000
Total liabilities and stockholders' equity	3,431,154,000	3,086,209,000
Class A Common Stock [Member]	'	'
Stockholders' equity (Note 15):	'	'
Common stock value	3,580,000	3,568,000
Total stockholders' equity	3,580,000	3,568,000
Class B Common Stock [Member]	'	'
Stockholders' equity (Note 15):	'	'
Common stock value	1,976,000	1,986,000
Total stockholders' equity	$1,976,000	$1,986,000

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Amortized cost	$465,943,000	$1,414,595,000
Held-to-maturity securities, fair value	$1,464,449,000	$281,924,000
Preferred stock, par value	$1	$1
Preferred stock, shares authorized	100,000	100,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Class A Common Stock [Member]	'	'
Common stock, par value	$1	$1
Common stock, shares authorized	10,000,000	10,000,000
Common stock, shares issued	3,580,404	3,568,079
Class B Common Stock [Member]	'	'
Common stock, par value	$1	$1
Common stock, shares authorized	5,000,000	5,000,000
Common stock, shares issued	1,976,180	1,986,880

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
INTEREST INCOME	'	'	'
Loans, taxable	$33,214	$34,983	$36,772
Loans, non-taxable	16,082	16,432	11,324
Securities available-for-sale, taxable	13,024	22,286	22,782
Securities available-for-sale, non-taxable	286	340	211
Federal Home Loan Bank of Boston dividends	59	77	46
Securities held-to-maturity	16,615	6,746	5,816
Federal funds sold, interest-bearing deposits in other banks and short-term investments	485	630	1,114
Total interest income	79,765	81,494	78,065
INTEREST EXPENSE	'	'	'
Savings and NOW deposits	2,585	2,250	2,539
Money market accounts	2,472	2,373	2,706
Time deposits	4,777	6,250	9,356
Securities sold under agreements to repurchase	361	367	379
Other borrowed funds and subordinated debentures	8,610	8,300	7,786
Total interest expense	18,805	19,540	22,766
Net interest income	60,960	61,954	55,299
Provision for loan losses (Note 6)	2,710	4,150	4,550
Net interest income after provision for loan losses	58,250	57,804	50,749
OTHER OPERATING INCOME	'	'	'
Service charges on deposit accounts	8,113	7,880	7,885
Lockbox fees	3,079	2,930	2,770
Brokerage commissions	257	364	441
Net gains on sales of securities	3,019	1,843	1,940
Net gains on sales of loans	1,564	297	660
Other income	2,583	2,551	2,544
Total other operating income	18,615	15,865	16,240
OPERATING EXPENSES	'	'	'
Salaries and employee benefits (Note 17)	35,244	32,943	29,630
Occupancy	5,000	4,695	4,411
Equipment	2,298	2,255	2,235
FDIC assessments	1,790	1,737	2,025
Other (Note 20)	11,480	11,608	10,441
Total operating expenses	55,812	53,238	48,742
Income before income taxes	21,053	20,431	18,247
Provision for income taxes (Note 16)	1,007	1,392	1,554
Net income	20,046	19,039	16,693
Class A Common Stock [Member]	'	'	'
OPERATING EXPENSES	'	'	'
Net income	15,698	14,877	13,023
SHARE DATA (Note 14)	'	'	'
Weighted average number of shares outstanding, basic	3,575,683	3,557,693	3,543,233
Weighted average number of shares outstanding, diluted	5,557,693	5,549,191	5,541,794
Basic earnings per share	$4.39	$4.18	$3.68
Diluted earnings per share	$3.61	$3.43	$3.01
Class B Common Stock [Member]	'	'	'
OPERATING EXPENSES	'	'	'
Net income	$4,348	$4,162	$3,670
SHARE DATA (Note 14)	'	'	'
Weighted average number of shares outstanding, basic	1,980,855	1,990,474	1,997,411
Weighted average number of shares outstanding, diluted	1,980,855	1,990,474	1,997,411
Basic earnings per share	$2.19	$2.09	$1.84
Diluted earnings per share	$2.19	$2.09	$1.84

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Statement Of Income And Comprehensive Income [Abstract]	'	'	'
Net income	$20,046	$19,039	$16,693
Unrealized (losses) gains on securities:	'	'	'
Unrealized holding (losses) gains arising during period	-25,909	5,854	6,666
Less: reclassification adjustment for gains included in net income	-3,019	-1,843	-1,940
Total unrealized (losses) gains on securities	-28,928	4,011	4,726
Accretion of net unrealized losses transferred during period	1,886	'	'
Pension liability adjustment:	'	'	'
Net gain (loss)	4,932	-2,488	-4,047
Amortization of prior service cost and loss included in net periodic benefit cost	694	649	380
Total pension liability adjustment	5,626	-1,839	-3,667
Other comprehensive (loss) income	-21,416	2,172	1,059
Comprehensive income (loss)	($1,370)	$21,211	$17,752

Consolidated_Statements_of_Cha

Consolidated Statements of Changes in Shareholders' Equity (USD $)	Total	Class A Common Stock [Member]	Class B Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Retained Earnings [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
In Thousands, unless otherwise specified						Class A Common Stock [Member]	Class B Common Stock [Member]
Beginning balance at Dec. 31, 2010	$145,025	$3,529	$2,011	$11,537	$131,526	'	'	($3,578)
Net income	16,693	13,023	3,670	'	16,693	'	'	'
Other comprehensive income, net of tax:	'	'	'	'	'	'	'	'
Unrealized holding (losses) gains arising during period, net of taxes and realized net gains	4,726	'	'	'	'	'	'	4,726
Pension liability adjustment, net of taxes	-3,667	'	'	'	'	'	'	-3,667
Conversion of Class B Common Stock to Class A Common Stock	'	17	-17	'	'	'	'	'
Stock options exercised	52	2	'	50	'	'	'	'
Cash dividends	'	-1,701	-479	'	'	-1,701	-479	'
Ending balance at Dec. 31, 2011	160,649	3,548	1,994	11,587	146,039	'	'	-2,519
Net income	19,039	14,877	4,162	'	19,039	'	'	'
Other comprehensive income, net of tax:	'	'	'	'	'	'	'	'
Unrealized holding (losses) gains arising during period, net of taxes and realized net gains	4,011	'	'	'	'	'	'	4,011
Pension liability adjustment, net of taxes	-1,839	'	'	'	'	'	'	-1,839
Conversion of Class B Common Stock to Class A Common Stock	'	8	-8	'	'	'	'	'
Stock options exercised	304	12	'	292	'	'	'	'
Cashless stock options exercised	12	'	'	12	'	'	'	'
Cash dividends	'	-1,708	-478	'	'	-1,708	-478	'
Ending balance at Dec. 31, 2012	179,990	3,568	1,986	11,891	162,892	'	'	-347
Net income	20,046	15,698	4,348	'	20,046	'	'	'
Other comprehensive income, net of tax:	'	'	'	'	'	'	'	'
Unrealized holding (losses) gains arising during period, net of taxes and realized net gains	-13,375	'	'	'	'	'	'	-13,375
Unrealized losses on securities transferred to held-to-maturity net of taxes	-15,553	'	'	'	'	'	'	-15,553
Accretion of net unrealized losses transferred during the period, net of taxes	1,886	'	'	'	'	'	'	1,886
Pension liability adjustment, net of taxes	5,626	'	'	'	'	'	'	5,626
Conversion of Class B Common Stock to Class A Common Stock	'	10	-10	'	'	'	'	'
Stock options exercised	43	2	'	41	'	'	'	'
Cash dividends	'	-1,716	-475	'	'	-1,716	-475	'
Ending balance at Dec. 31, 2013	$176,472	$3,580	$1,976	$11,932	$180,747	'	'	($21,763)

Consolidated_Statements_of_Cha1

Consolidated Statements of Changes in Shareholders' Equity (Parenthetical) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Unrealized holding gains arising during period, taxes	$8,527	$2,491	$3,143
Realized net gains	3,019	1,843	1,940
Unrealized losses on securities transferred to held-to-maturity, taxes	9,781	'	'
Accretion of net unrealized losses transferred during the period, taxes	1,191	'	'
Pension liability adjustment, taxes	$3,741	$1,223	$2,439
Conversion of class B common stock to class A common stock, shares	10,700	7,500	17,000
Stock options exercised, shares	1,625	12,262	2,450
Cashless stock options exercised, shares	'	6,750	'
Class A Common Stock [Member]	'	'	'
Cash dividends, per share	$0.48	$0.48	$0.48
Class B Common Stock [Member]	'	'	'
Cash dividends, per share	$0.24	$0.24	$0.24

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
CASH FLOWS FROM OPERATING ACTIVITIES:	'	'	'
Net income	$20,046	$19,039	$16,693
Adjustments to reconcile net income to net cash provided by operating activities:	'	'	'
Mortgage loans originated for sale	-82,395	-20,149	-22,664
Proceeds from mortgage loans sold	93,337	14,457	19,697
Gain on sales of mortgage loans held for sale	-1,564	-297	-422
Gain on sale of loans	'	'	-238
Gain on sale of fixed assets	-1	-1	'
Net gains on sales of securities	-3,019	-1,843	-1,940
Provision for loan losses	2,710	4,150	4,550
Deferred tax benefit	-2,929	-2,104	-953
Net depreciation and amortization	5,358	6,445	5,558
(Increase) decrease in accrued interest receivable	-728	211	579
Decrease in prepaid FDIC assessments	2,773	1,562	1,794
(Gain) loss on sales of other real estate owned	'	-1	8
Write down of other real estate owned	'	'	117
Increase in other assets	-5,693	-3,113	-4,456
Increase in other liabilities	4,043	1,070	503
Net cash provided by operating activities	31,938	19,426	18,826
CASH FLOWS FROM INVESTING ACTIVITIES:	'	'	'
Proceeds from maturities of short-term investments	22,367	38,397	121,106
Purchase of short-term investments	-9,617	-37,413	-25,539
Proceeds from call of Federal Home Loan Bank of Boston stock	284	385	'
Purchase of Federal Home Loan Bank of Boston stock	-3,210	'	'
Proceeds from calls/maturities of securities available-for-sale	256,420	532,734	722,403
Proceeds from sales of securities available-for-sale	224,045	294,881	75,615
Purchase of securities available-for-sale	-543,072	-998,955	-1,140,194
Proceeds from calls/maturities of securities held-to-maturity	121,121	88,628	119,315
Purchase of securities held-to-maturity	-344,455	-185,346	-68,863
Proceeds from sales of loans	'	'	4,000
Net increase in loans	-163,275	-123,183	-82,793
Proceeds from sales of other real estate owned	'	1,584	802
Proceeds from sales of fixed assets	'	1	'
Capital expenditures	-1,819	-4,300	-2,692
Net cash used in investing activities	-441,211	-392,587	-276,840
CASH FLOWS FROM FINANCING ACTIVITIES:	'	'	'
Net (decrease) increase in time deposit accounts	-37,759	-13,518	16,241
Net increase in demand, savings, money market and NOW deposits	308,525	334,007	206,320
Net proceeds from the exercise of stock options	43	304	52
Cash dividends	-2,191	-2,186	-2,180
Net increase in securities sold under agreements to repurchase	23,050	48,070	34,770
Net increase (decrease) in other borrowed funds	60,000	-48,999	22,025
Net cash provided by financing activities	351,668	317,678	277,228
Net (decrease) increase in cash and cash equivalents	-57,605	-55,483	19,214
Cash and cash equivalents at beginning of year	152,283	207,766	188,552
Cash and cash equivalents at end of year	94,678	152,283	207,766
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:	'	'	'
Cash paid during the period for interest	18,812	19,597	22,799
Cash paid during the period for income taxes	4,008	3,348	3,109
Change in unrealized gains on securities available-for-sale, net of taxes	-13,375	4,011	4,726
Change in unrealized losses on securities transferred to held-to-maturity, net of taxes	-15,553	'	'
Pension liability adjustment, net of taxes	5,626	-1,839	-3,667
Transfer of loans to other real estate owned	'	400	2,110
Transfer of securities available-for-sale to held-to-maturity	$987,037	'	'

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2013
Accounting Policies [Abstract]	'
Summary of Significant Accounting Policies	'
	1.     Summary of Significant Accounting Policies
	BASIS OF FINANCIAL STATEMENT PRESENTATION
	The consolidated financial statements include the accounts of Century Bancorp, Inc. (the “Company”) and its wholly owned subsidiary, Century Bank and Trust Company (the “Bank”). The consolidated financial statements also include the accounts of the Bank’s wholly owned subsidiaries, Century Subsidiary Investments, Inc. (“CSII”), Century Subsidiary Investments, Inc. II (“CSII II”), Century Subsidiary Investments, Inc. III (“CSII III”) and Century Financial Services Inc. (“CFSI”). CSII, CSII II, and CSII III are engaged in buying, selling and holding investment securities. CFSI has the power to engage in financial agency, securities brokerage, and investment and financial advisory services and related securities credit. The Company also owns 100% of Century Bancorp Capital Trust II (“CBCT II”). The entity is an unconsolidated subsidiary of the Company.
	All significant intercompany accounts and transactions have been eliminated in consolidation. The Company provides a full range of banking services to individual, business and municipal customers in Massachusetts. As a bank holding company, the Company is subject to the regulation and supervision of the Federal Reserve Board. The Bank, a state chartered financial institution, is subject to supervision and regulation by applicable state and federal banking agencies, including the Federal Reserve Board, the Federal Deposit Insurance Corporation (the “FDIC”) and the Commonwealth of Massachusetts Commissioner of Banks. The Bank is also subject to various requirements and restrictions under federal and state law, including requirements to maintain reserves against deposits, restrictions on the types and amounts of loans that may be granted and the interest that may be charged thereon, and limitations on the types of investments that may be made and the types of services that may be offered. Various consumer laws and regulations also affect the operations of the Bank. In addition to the impact of regulation, commercial banks are affected significantly by the actions of the Federal Reserve Board as it attempts to control the money supply and credit availability in order to influence the economy. All aspects of the Company’s business are highly competitive. The Company faces aggressive competition from other lending institutions and from numerous other providers of financial services. The Company has one reportable operating segment.
	The financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America and general practices within the banking industry. In preparing the financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the balance sheet and revenues and expenses for the period. Actual results could differ from those estimates.
	Material estimates that are susceptible to change in the near term relate to the allowance for loan losses. Management believes that the allowance for loan losses is adequate based on independent appraisals and review of other factors, including historical charge-off rates with additional allocations based on risk factors for each category and general economic factors. While management uses available information to recognize loan losses, future additions to the allowance for loan losses may be necessary based on changes in economic conditions. In addition, regulatory agencies periodically review the Company’s allowance for loan losses. Such agencies may require the Company to recognize additions to the allowance for loan losses based on their judgments about information available to them at the time of their examination. Certain reclassifications are made to prior-year amounts whenever necessary to conform with the current-year presentation.
	FAIR VALUE MEASUREMENTS
	The Company follows FASB ASC 820-10, Fair Value Measurements and Disclosures, (formerly SFAS 157, “Fair Value Measurements,”) which among other things, requires enhanced disclosures about assets and liabilities carried at fair value. ASC 820-10 establishes a hierarchal disclosure framework associated with the level of pricing observability utilized in measuring financial instruments at fair value. The three broad levels of the hierarchy are as follows:
	Level I — Quoted prices are available in active markets for identical assets or liabilities as of the reported date. The type of financial instruments included in Level I are highly liquid cash instruments with quoted prices, such as G-7 government, agency securities, listed equities and money market securities, as well as listed derivative instruments.
	Level II — Pricing inputs are other than quoted prices in active markets, which are either directly or indirectly observable as of the reported date. The nature of these financial instruments includes cash instruments for which quoted prices are available but traded less frequently, derivative instruments whose fair value has been derived using a model where inputs to the model are directly observable in the market or can be derived principally from or corroborated by observable market data, and instruments that are fair valued using other financial instruments, the parameters of which can be directly observed. Instruments that are generally included in this category are corporate bonds and loans, mortgage whole loans, municipal bonds and over the counter (“OTC”) derivatives.
	Level III — These instruments have little to no pricing observability as of the reported date. These financial instruments do not have two-way markets and are measured using management’s best estimate of fair value, where the inputs into the determination of fair value require significant management judgment or estimation. Instruments that are included in this category generally include certain commercial mortgage loans, certain private equity investments, distressed debt, and noninvestment grade residual interests in securitizations as well as certain highly structured OTC derivative contracts.
	CASH AND CASH EQUIVALENTS
	For purposes of reporting cash flows, cash equivalents include highly liquid assets with an original maturity of three months or less. Highly liquid assets include cash and due from banks, federal funds sold and certificates of deposit.
	SHORT-TERM INVESTMENTS
	As of December 31, 2013 and 2012, short-term investments include highly liquid certificates of deposit with original maturities of more than 90 days but less than one year.
	INVESTMENT SECURITIES
	Debt securities that the Company has the positive intent and ability to hold to maturity are classified as held-to-maturity and reported at amortized cost; debt and equity securities that are bought and held principally for the purpose of selling are classified as trading and reported at fair value, with unrealized gains and losses included in earnings; and debt and equity securities not classified as either held-to-maturity or trading are classified as available-for-sale and reported at fair value, with unrealized gains and losses excluded from earnings and reported as a separate component of stockholders’ equity, net of estimated related income taxes. The Company has no securities held for trading.
	Premiums and discounts on investment securities are amortized or accreted into income by use of the level-yield method. If a decline in fair value below the amortized cost basis of an investment is judged to be other-than-temporary, the cost basis of the investment is written down to fair value. The total amount of the impairment charge is recognized in earnings, with an offset for the noncredit component, which is recognized as other comprehensive income. Gains and losses on the sale of investment securities are recognized on the trade date on a specific identification basis.
	The transfer of a security between categories of investments shall be accounted for at fair value. For a debt security transferred into the held-to-maturity category from the available-for-sale category, the unrealized holding gain or loss at the date of the transfer shall continue to be reported in a separate component of shareholders’ equity but shall be amortized over the remaining life of the security as an adjustment of yield in a manner consistent with the amortization of any premium or discount. The amortization of an unrealized holding gain or loss reported in equity will offset or mitigate the effect on interest income of the amortization of the premium or discount for that held-to-maturity security.
	FEDERAL HOME LOAN BANK STOCK
	The Bank, as a member of the Federal Home Loan Bank of Boston (“FHLBB”) system, is required to maintain an investment in capital stock of the FHLBB. Based on redemption provisions, the stock has no quoted market value and is carried at cost. At its discretion, the FHLBB may declare dividends on the stock. The Company reviews for impairment based on the ultimate recoverability of the cost basis of the stock. For the year ended December 31, 2013, the FHLBB reported preliminary net income of $212.3 million. The FHLBB also declared a dividend equal to an annual yield of 1.49%. As of December 31, 2013, no impairment has been recognized.
	LOANS HELD FOR SALE
	Loans originated and intended for sale in the secondary market are carried at the lower of cost or estimated fair value in the aggregate. Net unrealized losses, if any, are recognized through a valuation allowance by charges to income.
	LOANS
	Interest on loans is recognized based on the daily principal amount outstanding. Accrual of interest is discontinued when loans become ninety days delinquent unless the collateral is sufficient to cover both principal and interest and the loan is in the process of collection. Past-due status is based on contractual terms of the loan. Loans, including impaired loans, on which the accrual of interest has been discontinued, are designated nonaccrual loans. When a loan is placed on nonaccrual, all income that has been accrued but remains unpaid is reversed against current period income, and all amortization of deferred loan costs and fees is discontinued. Nonaccrual loans may be returned to an accrual status when principal and interest payments are not delinquent or the risk characteristics of the loan have improved to the extent that there no longer exists a concern as to the collectibility of principal and interest. Income received on nonaccrual loans is either recorded in income or applied to the principal balance of the loan, depending on management’s evaluation as to the collectibility of principal.
	Loan origination fees and related direct loan origination costs are offset, and the resulting net amount is deferred and amortized over the life of the related loans using the level-yield method. Prepayments are not initially considered when amortizing premiums and discounts.
	The Bank measures impairment for impaired loans at either the fair value of the loan, the present value of the expected future cash flows discounted at the loan’s effective interest rate or the fair value of the collateral if the loan is collateral dependent. This method applies to all loans, uncollateralized as well as collateralized, except large groups of smaller-balance homogeneous loans such as residential real estate and consumer loans that are collectively evaluated for impairment and loans that are measured at fair value. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off against the allowance for loan losses in lieu of an allocation of a specific allowance when such an amount has been identified definitively as uncollectible. Management considers the payment status, net worth and earnings potential of the borrower, and the value and cash flow of the collateral as factors to determine if a loan will be paid in accordance with its contractual terms. Management does not set any minimum delay of payments as a factor in reviewing for impaired classification. Loans are charged-off when management believes that the collectibility of the loan’s principal is not probable. The specific factors that management considers in making the determination that the collectibility of the loan’s principal is not probable include the delinquency status of the loan, the fair value of the collateral, if secured, and, the financial strength of the borrower and/or guarantors. In addition, criteria for classification of a loan as in-substance foreclosure has been modified so that such classification need be made only when a lender is in possession of the collateral. The Bank measures the impairment of troubled debt restructurings using the pre-modification rate of interest.
	TRANSFERS OF FINANCIAL ASSETS
	Transfers of financial assets, typically residential mortgages and loan participations for the Company, are accounted for as sales when control over the assets has been surrendered. Control over transferred assets is deemed to be surrendered when (1) the assets have been isolated from the Company, (2) the transferee obtains the right (free of conditions that constrain it from taking advantage of that right) to pledge or exchange the transferred assets, and (3) the Company does not maintain effective control over the transferred assets.
	ACQUIRED LOANS
	In accordance with FASB ASC 310-30, Loans and Debt Securities Acquired with Deteriorated Credit Quality (formerly Statement of Position (“SOP”) No. 03-3, “Accounting for Certain Loans or Debt Securities Acquired in a Transfer”) the Company reviews acquired loans for differences between contractual cash flows and cash flows expected to be collected from the Company’s initial investment in the acquired loans to determine if those differences are attributable, at least in part, to credit quality. If those differences are attributable to credit quality, the loan’s contractually required payments received in excess of the amount of its cash flows expected at acquisition, or nonaccretable discount, is not accreted into income. FASB ASC 310-30 requires that the Company recognize the excess of all cash flows expected at acquisition over the Company’s initial investment in the loan as interest income using the interest method over the term of the loan. This excess is referred to as accretable discount and is recorded as a reduction of the loan balance.
	Loans which, at acquisition, do not have evidence of deterioration of credit quality since origination are outside the scope of FASB ASC 310-30. For such loans, the discount, if any, representing the excess of the amount of reasonably estimable and probable discounted future cash collections over the purchase price, is accreted into interest income using the interest method over the term of the loan. Prepayments are not considered in the calculation of accretion income. Additionally, the discount is not accreted on nonperforming loans.
	When a loan is paid off, the excess of any cash received over the net investment is recorded as interest income. In addition to the amount of purchase discount that is recognized at that time, income may include interest owed by the borrower prior to the Company’s acquisition of the loan, interest collected if on nonperforming status, prepayment fees and other loan fees.
	NONPERFORMING ASSETS
	In addition to nonperforming loans, nonperforming assets include other real estate owned. Other real estate owned is comprised of properties acquired through foreclosure or acceptance of a deed in lieu of foreclosure. Other real estate owned is recorded initially at estimated fair value less costs to sell. When such assets are acquired, the excess of the loan balance over the estimated fair value of the asset is charged to the allowance for loan losses. An allowance for losses on other real estate owned is established by a charge to earnings when, upon periodic evaluation by management, further declines in the estimated fair value of properties have occurred. Such evaluations are based on an analysis of individual properties as well as a general assessment of current real estate market conditions. Holding costs and rental income on properties are included in current operations, while certain costs to improve such properties are capitalized. Gains and losses from the sale of other real estate owned are reflected in earnings when realized.
	ALLOWANCE FOR LOAN LOSSES
	The allowance for loan losses is based on management’s evaluation of the quality of the loan portfolio and is used to provide for losses resulting from loans that ultimately prove uncollectible. In determining the level of the allowance, periodic evaluations are made of the loan portfolio, which takes into account such factors as the character of the loans, loan status, financial posture of the borrowers, value of collateral securing the loans and other relevant information sufficient to reach an informed judgment. The allowance is increased by provisions charged to income and reduced by loan charge-offs, net of recoveries. Management maintains an allowance for loan losses to absorb losses inherent in the loan portfolio. The allowance is based on assessments of the probable estimated losses inherent in the loan portfolio. Management’s methodology for assessing the appropriateness of the allowance consists of several key elements, which include the formula allowance, specific allowances, if appropriate, for identified problem loans and the unallocated allowance. Arriving at an appropriate level of allowance for loan losses necessarily involves a high degree of judgment.
	While management uses available information in establishing the allowance for loan losses, future adjustments to the allowance may be necessary if economic conditions differ substantially from the assumptions used in making the evaluations. Loans are charged-off in whole or in part when, in management’s opinion, collectibility is not probable. The specific factors that management considers in making the determination that the collectibility of the loan’s principal is not probable include the delinquency status of the loan, the fair value of the collateral and the financial strength of the borrower and/or guarantors. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off against the allowance for loan losses in lieu of an allocation of a specific allowance when such an amount has been identified definitively as uncollectible.
	The formula allowance evaluates groups of loans to determine the allocation appropriate within each portfolio segment. Individual loans within the commercial and industrial, commercial real estate and real estate construction loan portfolio segments are assigned internal risk ratings to group them with other loans possessing similar risk characteristics. Changes in risk grades affect the amount of the formula allowance. Risk grades are determined by reviewing current collateral value, financial information, cash flow, payment history and other relevant facts surrounding the particular credit. Provisions for losses on the remaining commercial and commercial real estate loans are based on pools of similar loans using a combination of historical net loss experience and qualitative adjustments. For the residential real estate and consumer loan portfolios, the reserves are calculated by applying historical charge-off and recovery experience and qualitative adjustments to the current outstanding balance in each loan category. Loss factors are based on the Company’s historical net loss experience as well as regulatory guidelines.
	Specific allowances for loan losses entail the assignment of allowance amounts to individual loans on the basis of loan impairment. Certain loans are evaluated individually and are judged to be impaired when management believes it is probable that the Company will not collect all the contractual interest and principal payments as scheduled in the loan agreement. Under this method, loans are selected for evaluation based upon a change in internal risk rating, occurrence of delinquency, loan classification or nonaccrual status. A specific allowance amount is allocated to an individual loan when such loan has been deemed impaired and when the amount of a probable loss is able to be estimated on the basis of: (a) present value of anticipated future cash flows, (b) the loan’s observable fair market price or (c) fair value of collateral if the loan is collateral dependent.
	The formula allowance and specific allowances also include management’s evaluation of various conditions, including business and economic conditions, delinquency trends, charge-off experience and other quality factors.
	An unallocated component is maintained to cover uncertainties that could affect management’s estimate of probable losses. The unallocated component of the allowance reflects the margin of imprecision inherent in the underlying assumptions used in the methodologies for estimating allocated and general reserves in the portfolio.
	Management has identified certain risk factors, which could impact the degree of loss sustained within the portfolio. These include: (a) market risk factors, such as the effects of economic variability on the entire portfolio and (b) unique portfolio risk factors that are inherent characteristics of the Company’s loan portfolio. Market risk factors may consist of changes to general economic and business conditions that may impact the Company’s loan portfolio customer base in terms of ability to repay and that may result in changes in value of underlying collateral. Unique portfolio risk factors may include industry concentrations and geographic concentrations or trends that may exacerbate losses resulting from economic events which the Company may not be able to fully diversify out of its portfolio.
	The qualitative factors are determined based on the various risk characteristics of each loan segment. Risk characteristics relevant to each portfolio segment are as follows:
	Residential real estate — The Company generally does not originate loans with a loan-to-value ratio greater than 80 percent. All loans in this segment are collateralized by owner-occupied residential real estate and repayment is dependent on the credit quality of the individual borrower. The overall health of the economy, including unemployment rates, will have an effect on the credit quality in the segment.
	Commercial real estate — Loans in this segment are primarily income-producing properties. Also included are loans to educational institutions, hospitals and other non-profit organizations. The underlying cash flows generated by the properties are adversely impacted by a downturn in the economy as evidenced by increased vacancy rates, which in turn, will have an effect on the credit quality in this segment. Management monitors the cash flows of these loans.
	Construction loans — Loans in this segment primarily include real estate development loans for which payment is derived from sale of the property as well as construction projects in which the property will ultimately be used by the borrower. Credit risk is affected by cost overruns, time to sell at an adequate price and market conditions.
	Commercial and industrial loans — Loans in this segment are made to businesses and are generally secured by assets of the business. Repayment is expected from the cash flows of the business. A weakened economy, and resultant decreased consumer spending, will have an effect on the credit quality in this segment.
	BANK PREMISES AND EQUIPMENT
	Bank premises and equipment are stated at cost less accumulated depreciation and amortization. Land is stated at cost. Depreciation is computed using the straight-line method over the estimated useful lives of the assets or the terms of leases, if shorter. It is general practice to charge the cost of maintenance and repairs to operations when incurred; major expenditures for improvements are capitalized and depreciated.
	GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS
	Goodwill represents the excess of the cost of an acquisition over the fair value of the net assets acquired. Goodwill is not subject to amortization. Identifiable intangible assets consist of core deposit intangibles and are assets resulting from acquisitions that are being amortized over their estimated useful lives. Goodwill and identifiable intangible assets are included in other assets on the consolidated balance sheets. The Company tests goodwill for impairment on an annual basis, or more often if events or circumstances indicate there may be impairment. Goodwill impairment testing is performed at the segment (or “reporting unit”) level. Currently, the Company’s goodwill is evaluated at the entity level as there is only one reporting unit. Goodwill is assigned to reporting units at the date the goodwill is initially recorded. Once goodwill has been assigned to reporting units, it no longer retains its association with a particular acquisition, and all of the activities within a reporting unit, whether acquired or organically grown, are available to support the value of the goodwill.
	Goodwill impairment is evaluated by first assessing qualitative factors (events and circumstances) to determine whether it is more likely than not (meaning a likelihood of more than 50 percent) that the fair value of a reporting unit is less than its carrying amount. If, after considering all relevant events and circumstances, an entity determines it is not more likely than not that the fair value of a reporting unit is less than its carrying amount, then performing the two-step impairment test will be unnecessary.
	The first step, in the two-step impairment test, used to identify potential impairment, involves comparing each reporting unit’s fair value to its carrying value including goodwill. If the fair value of a reporting unit exceeds its carrying value, applicable goodwill is considered not to be impaired. If the carrying value exceeds fair value, there is an indication of impairment and the second step is performed to measure the amount of impairment.
	SERVICING
	The Company services mortgage loans for others. Mortgage servicing assets are recognized as separate assets when rights are acquired through purchase or through sale of financial assets. Fair value is determined using prices for similar assets with similar characteristics, when available, or based upon discounted cash flows using market-based assumptions. The valuation model incorporates assumptions that market participants would use in estimating future net servicing income, such as the cost to service, the discount rate, an inflation rate, ancillary income, prepayment speeds and default rates and losses. Capitalized servicing rights are reported in other assets and are amortized into loan servicing fee income in proportion to, and over the period of, the estimated future net servicing income of the underlying financial assets. Servicing assets are evaluated for impairment based upon the fair value of the rights as compared to amortized cost. Impairment is determined by stratifying rights by predominant risk characteristics, such as interest rates and terms. Impairment is recognized through a valuation allowance for an individual stratum, to the extent that fair value is less than the capitalized amount for the stratum. Changes in the valuation allowance are reported in loan servicing fee income.
	STOCK OPTION ACCOUNTING
	The Company follows the fair value recognition provisions of FASB ASC 718, Compensation – Stock Compensation (formerly SFAS 123R) for all share-based payments, using the modified-prospective transition method. The Company’s method of valuation for share-based awards granted utilizes the Black-Scholes option-pricing model, which was also previously used for the Company’s pro forma information required under FASB ASC 718. The Company will recognize compensation expense for its awards on a straight-line basis over the requisite service period for the entire award (straight-line attribution method), ensuring that the amount of compensation cost recognized at any date at least equals the portion of the grant-date fair value of the award that is vested at that time.
	During 2000 and 2004, common stockholders of the Company approved stock option plans (the “Option Plans”) that provide for granting of options to purchase up to 150,000 shares of Class A common stock per plan. Under the Option Plans, all officers and key employees of the Company are eligible to receive nonqualified or incentive stock options to purchase shares of Class A common stock. The Option Plans are administered by the Compensation Committee of the Board of Directors, whose members are ineligible to participate in the Option Plans. Based on management’s recommendations, the Committee submits its recommendations to the Board of Directors as to persons to whom options are to be granted, the number of shares granted to each, the option price (which may not be less than 85% of the fair market value for nonqualified stock options, or the fair market value for incentive stock options, of the shares on the date of grant) and the time period over which the options are exercisable (not more than ten years from the date of grant). There were options to purchase an aggregate of 20,375 shares of Class A common stock exercisable at December 31, 2013.
	On December 30, 2005, the Board of Directors approved the acceleration and immediate vesting of all unvested options with an exercise price of $31.60 or greater per share. As a consequence, options to purchase 23,950 shares of Class A common stock became exercisable immediately. The average of the high and low price at which the Class A common stock traded on December 30, 2005, the date of the acceleration and vesting, was $29.28 per share. In connection with this acceleration, the Board of Directors approved a technical amendment to each of the Option Plans to eliminate the possibility that the terms of any outstanding or future stock option would require a cash settlement on the occurrence of any circumstance outside the control of the Company.
	The Company uses the fair value method to account for stock options. All of the Company’s stock options are vested, and there were no options granted during 2013 and 2012.
	INCOME TAXES
	The Company uses the asset and liability method in accounting for income taxes. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which temporary differences are expected to be recovered or settled. Under this method, the effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date.
The Company accounts for uncertain tax positions in accordance with FASB ASC 740.
The Company classifies interest resulting from underpayment of income taxes as income tax expense in the first period the interest would begin accruing according to the provisions of the relevant tax law.
The Company classifies penalties resulting from underpayment of income taxes as income tax expense in the period for which the Company claims or expects to claim an uncertain tax position or in the period in which the Company’s judgment changes regarding an uncertain tax position.
TREASURY STOCK
Effective July 1, 2004, companies incorporated in Massachusetts became subject to Chapter 156D of the Massachusetts Business Corporation Act, provisions of which eliminate the concept of treasury stock and provide that shares reacquired by a company are to be treated as authorized but unissued shares.
PENSION
The Company provides pension benefits to its employees under a noncontributory, defined benefit plan, which is funded on a current basis in compliance with the requirements of the Employee Retirement Income Security Act of 1974 (“ERISA”) and recognizes costs over the estimated employee service period.
The Company also has a Supplemental Executive Insurance/Retirement Plan (“the Supplemental Plan”), which is limited to certain officers and employees of the Company. The Supplemental Plan is accrued on a current basis and recognizes costs over the estimated employee service period.
Executive officers of the Company or its subsidiaries who have at least one year of service may participate in the Supplemental Plan. The Supplemental Plan is voluntary. Individual life insurance policies, which are owned by the Company, are purchased covering the life of each participant.
REVISION OF EPS PRESENTATION
The Company has determined that although the Class A and Class B common stock have different dividend rates, the Company had not applied the two-class method when calculating earnings per share (“EPS”) separately for the Class A and Class B common stock. This resulted in immaterial revisions to previously reported basic EPS for Class A and Class B common stock and diluted EPS for the Class B common stock as summarized below:
For the year ended December 31, 2011:	As previously		As revised
	reported
Basic EPS – Class A common	$	3.01	$	3.68
Basic EPS – Class B common	$	3.01	$	1.84
Diluted EPS – Class A common	$	3.01	$	3.01
Diluted EPS – Class B common	$	3.01	$	1.84
RECENT ACCOUNTING DEVELOPMENTS
In February 2013, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2013-01, Clarifying the Scope of Disclosures about Offsetting Assets and Liabilities. This ASU clarifies the scope of offsetting disclosure requirements in ASU 2011-11, Balance Sheet (Topic 210): Disclosures about Offsetting Assets and Liabilities. Under ASU 2013-01, the disclosure requirements would apply to derivative instruments accounted for in accordance with ASC 815, including bifurcated embedded derivatives, repurchase agreements and reverse repurchase agreements, and securities borrowing and securities lending arrangements that are either offset on the balance sheet or subject to an enforceable master netting arrangement or similar agreement. Entities with other types of financial assets and financial liabilities subject to a master netting arrangement or similar agreement also are affected because these amendments make them no longer subject to the disclosure requirements in ASU No. 2011-11. Effective January 1, 2013, companies are required to disclose (a) gross amounts of recognized assets and liabilities; (b) gross amounts offset in the statement of financial position; (c) net amounts of assets and liabilities presented in the statement of financial position; (d) gross amount subject to enforceable master netting agreement not offset in the statements of financial position; and (e) net amounts after deducting (d) from (c). The disclosure should be presented in tabular format (unless another format is more appropriate) separately for assets and liabilities. The intent of the new disclosure is to enable users of financial statements to understand the effect of those arrangements on its financial position and to allow investors to better compare financial statements prepared under GAAP with financial statements prepared under International Financial Reporting Standards. The Company implemented the provisions of ASU 2011-11 as of January 1, 2013. The adoption of this pronouncement did not have a material effect on the consolidated financial statements.
In February 2013, the FASB issued ASU 2013-02, Comprehensive Income (Topic 220) Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income. ASU 2013-02 requires an entity to report the effect of significant reclassifications out of accumulated other comprehensive income on the respective line items in net income or as a separate disclosure in the notes to the financial statements. The new standard is effective for annual periods beginning January 1, 2013, and interim periods within those annual periods. The Company has presented a separate footnote (Note 13) as a result of this pronouncement.
In July 2013, the FASB issued ASU 2013-10, Inclusion of the Fed Funds Effective Swap Rate (or Overnight Index Swap Rate) as a Benchmark Interest Rate for Hedge Accounting Purposes. This ASU amends ASC 815 to allow entities to use the Fed Funds Effective Swap Rate, in addition to U.S. Treasury rates and LIBOR, as a benchmark interest rate in accounting for fair value and cash flow hedges in the United States. This ASU also eliminates the provision from ASC 815-20-25-6 that prohibits the use of different benchmark rates for similar hedges except in rate and justifiable circumstances. This ASU is effective prospectively for qualifying new hedging relationship entered into on or after July 17, 2013, and for hedging relationship redesignated on or after that day. As of December 31, 2013, the Company did not have any fair value and cash flow hedges. The adoption of ASU No. 2013-10 did not have a material impact on the Company’s financial statements.
In July 2013, the FASB issued ASU No. 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists. This ASU provides guidance on financial statement presentation of unrecognized tax benefits (“UTBs”) when a net operating loss (“NOL”) carryforward, a similar tax loss, or a tax credit carryforward exists. The FASB’s objective in issuing this ASU is to eliminate diversity in practice resulting from a lack of guidance on this topic in current U.S. GAAP. Under this ASU, an entity must present a UTB, or a portion of a UTB, in the financial statements as a reduction to a deferred tax asset (“DTA”) for an NOL carryforward, a similar tax loss, or a tax credit carryforward except when: (a) an NOL carryforward, a similar tax loss, or a tax credit carryforward is not available as of the reporting date under the governing tax law to settle taxes that would result from the disallowance of the tax position; (b) the entity does not intend to use the DTA for this purpose (provided that the tax law permits a choice). If either of these conditions exists, an entity should present a UTB in the financial statements as a liability and should not net the UTB with a DTA. New recurring disclosures are not required because the ASU does not affect the recognition or measurement of uncertain tax positions under ASC 740. This amendment does not affect the amounts public entities disclose in the tabular reconciliation of the total amounts of UTBs because the tabular reconciliation presents the gross amount of UTBs. This ASU is effective for fiscal years beginning after December 15, 2013, and interim periods within those years. The amendments should be applied to all UTBs that exist as of the effective date. Entities may choose to apply the amendments retrospectively to each prior reporting period presented. As of December 31, 2013, the Company did not have a UTB. Management will assess the applicability of this ASU after it becomes effective in the first quarter of 2014.

Cash_and_Due_from_Banks

Cash and Due from Banks	12 Months Ended
	Dec. 31, 2013
Cash And Cash Equivalents [Abstract]	'
Cash and Due from Banks	'
	2.     Cash and Due from Banks
	The Company is required to maintain a portion of its cash and due from banks as a reserve balance under the Federal Reserve Act. Such reserve is calculated based upon deposit levels and amounted to $0 at December 31, 2013, and $9,608,000 at December 31, 2012.

Securities_AvailableforSale

Securities Available-for-Sale	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Securities Available-for-Sale	'
	3.     Securities Available-for-Sale
		December 31, 2013								December 31, 2012
		Amortized		Gross		Gross		Estimated		Amortized		Gross		Gross		Estimated
		Cost		Unrealized		Unrealized		Fair		Cost		Unrealized		Unrealized		Fair
				Gains		Losses		Value				Gains		Losses		Value
	(dollars in thousands)
	U.S. Treasury	$	1,997	$	1	$	—	$	1,998	$	2,000	$	4	$	—	$	2,004
	U.S. Government Sponsored Enterprises		9,995		9		—		10,004		130,048		360		68		130,340
	SBA Backed Securities		7,270		32		—		7,302		8,043		113		—		8,156
	U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities		404,103		588		1,501		403,190		1,212,953		20,816		412		1,233,357
	Privately Issued Residential Mortgage-Backed Securities		2,294		6		23		2,277		2,938		31		22		2,947
	Obligations Issued by States and Political Subdivisions		37,578		15		870		36,723		55,855		41		722		55,174
	Other Debt Securities		2,300		—		125		2,175		2,300		—		47		2,253
	Equity Securities		406		170		—		576		458		112		—		570
	Total	$	465,943	$	821	$	2,519	$	464,245	$	1,414,595	$	21,477	$	1,271	$	1,434,801
	Included in U.S. Government Sponsored Enterprise Securities and U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities are securities at fair value pledged to secure public deposits and repurchase agreements amounting to $368,137,000 and $665,028,000 at December 31, 2013 and 2012, respectively. Also included in securities available-for-sale at fair value are securities pledged for borrowing at the Federal Home Loan Bank amounting to $12,214,000 and $220,313,000 at December 31, 2013 and 2012, respectively. The Company realized gains on sales of securities of $3,019,000, $1,843,000 and $1,940,000 from the proceeds of sales of available-for-sale securities of $224,045,000, $294,881,000 and $75,615,000 for the years ended December 31, 2013, 2012, and 2011, respectively.
	Debt securities of Government Sponsored Enterprises primarily refer to debt securities of Fannie Mae and Freddie Mac.
	The following table shows the estimated maturity distribution of the Company’s securities available-for-sale at December 31, 2013.
		Amortized		Fair Value
		Cost
	(dollars in thousands)
	Within one year	$	36,942	$	36,964
	After one but within five years		180,676		180,540
	After five but within ten years		238,822		238,018
	More than ten years		7,597		6,771
	Nonmaturing		1,906		1,952
	Total	$	465,943	$	464,245
	The weighted average remaining life of investment securities available-for-sale at December 31, 2013, was 5.1 years. The contractual maturities, which were used in the table above, of mortgage-backed securities, will differ from the actual maturities due to the ability of the issuers to prepay underlying obligations. Also, $415,692,000 of the securities are floating rate or adjustable rate and reprice prior to maturity.
	The Company transferred $987,037,000 of securities with unrealized losses of $25,333,000 from available-for-sale to held-to-maturity during 2013.
	As of December 31, 2013 and December 31, 2012, management concluded that the unrealized losses of its investment securities are temporary in nature since they are not related to the underlying credit quality of the issuers, and the Company does not intend to sell these debt securities and it is not more likely than not that it will be required to sell these debt securities before the anticipated recovery of its remaining amortized cost. In making its other-than-temporary impairment evaluation, the Company considered the fact that the principal and interest on these securities are from issuers that are investment grade. The change in the unrealized losses on the state and municipal securities and the nonagency mortgage-backed securities was primarily caused by changes in credit spreads and liquidity issues in the marketplace.
	The unrealized loss on U.S. Government Sponsored Enterprises and U.S. Government Sponsored Enterprises Mortgage Backed Securities related primarily to interest rates and not credit quality and because the Company has the ability and intent to hold these investments until recovery of fair value, which may be maturity. The Company does not consider these investments to be other-than-temporarily impaired at December 31, 2013 and December 31, 2012.
	In evaluating the underlying credit quality of a security, management considers several factors such as the credit rating of the obligor and the issuer, if applicable. Internal reviews of issuer financial statements are performed as deemed necessary. In the case of privately issued mortgage-backed securities, the performance of the underlying loans is analyzed as deemed necessary to determine the estimated future cash flows of the securities. Factors considered include the level of subordination, current and estimated future default rates, current and estimated prepayment rates, estimated loss severity rates, geographic concentrations and origination dates of underlying loans. In the case of marketable equity securities, the severity of the unrealized loss, the length of time the unrealized loss has existed, and the issuer’s financial performance are considered.
	The following table shows the temporarily impaired securities of the Company’s available-for-sale portfolio at December 31, 2013. This table shows the unrealized market loss of securities that have been in a continuous unrealized loss position for 12 months or less and a continuous loss position for 12 months and longer. There are 47 and 7 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 234 holdings at December 31, 2013.
		December 31, 2013
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair		Unrealized		Fair Value		Unrealized
				Losses		Value		Losses				Losses
	(dollars in thousands)
	U.S. Government Sponsored Enterprise	$	—	$	—	$	—	$	—	$	—	$	—
	U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities		289,709		1,352		24,557		149		314,266		1,501
	Privately Issued Residential Mortgage-Backed Securities		1,486		23		—		—		1,486		23
	Obligations Issued by States and Political Subdivisions		—		—		3,820		870		3,820		870
	Other Debt Securities		—		—		1,376		125		1,376		125
	Total temporarily impaired securities	$	291,195	$	1,375	$	29,753	$	1,144	$	320,948	$	2,519
	The following table shows the temporarily impaired securities of the Company’s available-for-sale portfolio at December 31, 2012. This table shows the unrealized market loss of securities that have been in a continuous unrealized loss position for 12 months or less and a continuous loss position for 12 months and longer. There are 20 and 7 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 458 holdings at December 31, 2012.
		December 31, 2012
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair		Unrealized		Fair Value		Unrealized
				Losses		Value		Losses				Losses
	(dollars in thousands)
	U.S. Government Sponsored Enterprise	$	34,967	$	68	$	—	$	—	$	34,967	$	68
	U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities		93,006		383		10,169		29		103,175		412
	Privately Issued Residential Mortgage-Backed Securities		—		—		1,863		22		1,863		22
	Obligations Issued by States and Political Subdivisions		—		—		3,963		722		3,963		722
	Other Debt Securities		—		—		1,453		47		1,453		47
	Total temporarily impaired securities	$	127,973	$	451	$	17,448	$	820	$	145,421	$	1,271

Investment_Securities_HeldtoMa

Investment Securities Held-to-Maturity	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Investment Securities Held-to-Maturity	'
	4.     Investment Securities Held-to-Maturity
		December 31, 2013								December 31, 2012
		Amortized		Gross		Gross		Estimated		Amortized		Gross		Gross		Estimated
		Cost		Unrealized		Unrealized		Fair Value		Cost		Unrealized		Unrealized		Fair
				Gains		Losses						Gains		Losses		Value
	(dollars in thousands)
	U.S. Government Sponsored Enterprise	$	291,779	$	185	$	5,043	$	286,921	$	17,747	$	19	$	8	$	17,758
	U.S. Government Sponsored Enterprise Mortgage-Backed Securities		1,196,105		2,239		20,816		1,177,528		257,760		6,480		74		264,166
	Total	$	1,487,884	$	2,424	$	25,859	$	1,464,449	$	275,507	$	6,499	$	82	$	281,924
	Included in U.S. Government and Agency Securities are securities pledged to secure public deposits and repurchase agreements at fair value amounting to $732,144,000 and $149,366,000 at December 31, 2013, and 2012, respectively. Also included are securities pledged for borrowing at the Federal Home Loan Bank at fair value amounting to $510,060,000 and $103,617,000 at December 31, 2013, and 2012, respectively.
	At December 31, 2013 and 2012, all mortgage-backed securities are obligations of U.S. Government Sponsored Enterprises. Government Sponsored Enterprises primarily refer to debt securities of Fannie Mae and Freddie Mac.
	The following table shows the maturity distribution of the Company’s securities held-to-maturity at December 31, 2013.
		Amortized		Fair Value
		Cost
	(dollars in thousands)
	Within one year	$	5,689	$	5,672
	After one but within five years		827,333		818,936
	After five but within ten years		653,784		638,750
	More than ten years		1,078		1,091
	Total	$	1,487,884	$	1,464,449
	The weighted average remaining life of investment securities held-to-maturity at December 31, 2013, was 5.2 years. Included in the weighted average remaining life calculation at December 31, 2013, were $224,663,000 of U.S. Government Sponsored Enterprises obligations that are callable at the discretion of the issuer. The actual maturities, which were used in the table above, of mortgage-backed securities, will differ from the contractual maturities due to the ability of the issuers to prepay underlying obligations.
	The Company transferred $987,037,000 of securities with unrealized losses of $25,333,000 from available-for-sale to held-to-maturity during 2013.
	As of December 31, 2013 and December 31, 2012, management concluded that the unrealized losses of its investment securities are temporary in nature since they are not related to the underlying credit quality of the issuers, and the Company does not intend to sell these debt securities and it is not more likely than not that it will be required to sell these debt securities before the anticipated recovery of their remaining amortized costs. In making its other-than-temporary impairment evaluation, the Company considered the fact that the principal and interest on these securities are from issuers that are investment grade.
	The unrealized loss on U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities related primarily to interest rates and not credit quality, and because the Company does not intend to sell any of these securities and it is not more likely than not that it will be required to sell these securities before the anticipated recovery of the remaining amortized cost, the Company does not consider these investments to be other-than-temporarily impaired at December 31, 2013 and December 31, 2012.
	In evaluating the underlying credit quality of a security, management considers several factors such as the credit rating of the obligor and the issuer, if applicable. Internal reviews of issuer financial statements are performed as deemed necessary.
	The following table shows the temporarily impaired securities of the Company’s held-to-maturity portfolio at December 31, 2013. This table shows the unrealized market loss of securities that have been in a continuous unrealized loss position for 12 months or less and a continuous loss position for 12 months and longer. There are 191 and 13 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 300 holdings at December 31, 2013.
		December 31, 2013
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair		Unrealized		Fair Value		Unrealized
				Losses		Value		Losses				Losses
	(dollars in thousands)
	U.S. Government Sponsored Enterprises	$	232,535	$	5,043	$	—	$	—	$	232,535	$	5,043
	U.S. Government Agency and Sponsored
	Enterprise Mortgage-Backed Securities		931,180		18,654		80,362		2,162		1,011,542		20,816
	Total temporarily impaired securities	$	1,163,715	$	23,697	$	80,362	$	2,162	$	1,244,077	$	25,859
	The following table shows the temporarily impaired securities of the Company’s held-to-maturity portfolio at December 31, 2012. This table shows the unrealized market loss of securities that have been in a continuous unrealized loss position for 12 months or less and a continuous loss position for 12 months and longer. There are 3 and 1 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 96 holdings at December 31, 2012.
		December 31, 2012
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
		Value		Losses		Value		Losses		Value		Losses
	(dollars in thousands)
	U.S. Government Sponsored Enterprises	$	9,994	$	8	$	—	$	—	$	9,994	$	8
	U.S. Government Agency and Sponsored
	Enterprise Mortgage-Backed Securities		8,936		50		5,371		24		14,307		74
	Total temporarily impaired securities	$	18,930	$	58	$	5,371	$	24	$	24,301	$	82

Loans

Loans	12 Months Ended
	Dec. 31, 2013
Debt Disclosure [Abstract]	'
Loans	'
	5.     Loans
	The majority of the Bank’s lending activities are conducted in Massachusetts. The Bank originates construction, commercial and residential real estate loans, commercial and industrial loans, consumer, home equity and other loans for its portfolio.
	The following summary shows the composition of the loan portfolio at the dates indicated.
	December 31,	2013		2012
	(dollars in thousands)
	Construction and land development	$	33,058	$	38,618
	Commercial and industrial		92,402		88,475
	Commercial real estate		713,327		576,465
	Residential real estate		286,041		281,857
	Consumer		8,824		6,823
	Home equity		130,277		118,923
	Overdrafts		834		627
	Total	$	1,264,763	$	1,111,788
	At December 31, 2013, and December 31, 2012, loans were carried net of discounts of $454,000 and $498,000, respectively. Net deferred fees included in loans at December 31, 2013, and December 31, 2012, were $174,000 and $369,000, respectively.
	The Company was servicing mortgage loans sold to others without recourse of approximately $109,301,000 and $26,786,000 at December 31, 2013, and December 31, 2012, respectively. The Company had no residential real estate loans held for sale at December 31, 2013 and had $9,378,000 at December 31, 2012.
	As of December 31, 2013 and 2012, the Company’s recorded investment in impaired loans was $7,788,000 and $5,925,000, respectively. If an impaired loan is placed on nonaccrual, the loan may be returned to an accrual status when principal and interest payments are not delinquent and the risk characteristics have improved to the extent that there no longer exists a concern as to the collectibility of principal and interest. At December 31, 2013, there were $6,723,000 of impaired loans with a specific reserve of $1,019,000. At December 31, 2012, there were $5,223,000 of impaired loans with a specific reserve of $1,732,000.
	Loans are designated as troubled debt restructures when a concession is made on a credit as a result of financial difficulties of the borrower. Typically, such concessions consist of a reduction in interest rate to a below-market rate, taking into account the credit quality of the note, or a deferment of payments, principal or interest, which materially alters the Bank’s position or significantly extends the note’s maturity date, such that the present value of cash flows to be received is materially less than those contractually established at the loan’s origination. Restructured loans are included in the impaired loan category.
	The composition of nonaccrual loans and impaired loans is as follows:
	December 31,	2013		2012		2011
	(dollars in thousands)
	Loans on nonaccrual	$	2,549	$	4,471	$	5,827
	Loans 90 days past due and still accruing		—		—		18
	Impaired loans on nonaccrual included above		1,819		2,878		3,468
	Total recorded investment in impaired loans		7,788		5,925		8,102
	Average recorded investment of impaired loans		6,776		7,043		10,284
	Accruing troubled debt restructures		5,969		3,048		4,634
	Interest income not recorded on nonaccrual loans according to their original terms		711		753		846
	Interest income on nonaccrual loans actually recorded		—		—		—
	Interest income recognized on impaired loans		161		180		155
	Directors and officers of the Company and their associates are customers of, and have other transactions with, the Company in the normal course of business. All loans and commitments included in such transactions were made on substantially the same terms, including interest rates and collateral, as those prevailing at the time for comparable transactions with other persons and do not involve more than normal risk of collection or present other unfavorable features.
	The following table shows the aggregate amount of loans to directors and officers of the Company and their associates during 2013.
	Balance at	Additions		Repayments		Balance at
	December 31, 2012			and Deletions		December 31, 2013
	(dollars in thousands)
	$4,663	$	310	$	269	$	4,704

Allowance_for_Loan_Losses

Allowance for Loan Losses	12 Months Ended
	Dec. 31, 2013
Receivables [Abstract]	'
Allowance for Loan Losses	'
	6.     Allowance for Loan Losses
	The Company maintains an allowance for loan losses in an amount determined by management on the basis of the character of the loans, loan performance, financial condition of borrowers, the value of collateral securing loans and other relevant factors. The following table summarizes the changes in the Company’s allowance for loan losses for the years indicated.
	An analysis of the allowance for loan losses for each of the three years ending December 31, 2013, 2012 and 2011 is as follows:
		2013			2012			2011
	(dollars in thousands)
	Allowance for loan losses, beginning of year	$	19,197		$	16,574		$	14,053
	Loans charged-off		(1,813	)		(2,301	)		(2,824	)
	Recoveries on loans previously charged-off		847			774			795
	Net charge-offs		(966	)		(1,527	)		(2,029	)
	Provision charged to expense		2,710			4,150			4,550
	Allowance for loan losses, end of year	$	20,941		$	19,197		$	16,574
	ALLOWANCE FOR LOAN LOSSES AND AMOUNT OF INVESTMENTS IN LOANS
	Further information pertaining to the allowance for loan losses at December 31, 2013 follows:
		Construction			Commercial			Commercial			Residential			Consumer			Home			Unallocated			Total
		and Land			and			Real Estate			Real						Equity
		Development			Industrial						Estate
	(dollars in thousands)
	Allowance for Loan Losses:
	Balance at December 31, 2012	$	3,041		$	3,118		$	9,065		$	1,994		$	333		$	886		$	760		$	19,197
	Charge-offs		(1,000	)		(234	)		—			—			(579	)		—			—			(1,813	)
	Recoveries		—			389			19			11			427			1			—			847
	EAProvision		133			(284	)		2,134			1			251			72			403			2,710
	Ending balance at December 31, 2013	$	2,174		$	2,989		$	11,218		$	2,006		$	432		$	959		$	1,163		$	20,941
	Amount of allowance for loan losses for loans deemed to be impaired	$	12		$	367		$	417		$	129		$	—		$	94		$	—		$	1,019
	Amount of allowance for loan losses for loans not deemed to be impaired	$	2,162		$	2,622		$	10,801		$	1,877		$	432		$	865		$	1,163		$	19,922
	Loans:
	Ending balance	$	33,058		$	92,402		$	713,327		$	286,041		$	9,658		$	130,277		$	—		$	1,264,763
	Loans deemed to be impaired	$	608		$	1,367		$	4,520		$	1,199		$	—		$	94		$	—		$	7,788
	Loans not deemed to be impaired	$	32,450		$	91,035		$	708,807		$	284,842		$	9,658		$	130,183		$	—		$	1,256,975
	Further information pertaining to the allowance for loan losses at December 31, 2012 follows:
		Construction			Commercial			Commercial			Residential			Consumer			Home			Unallocated			Total
		and Land			and			Real Estate			Real						Equity
		Development			Industrial						Estate
	(dollars in thousands)
	Allowance for Loan Losses:
	Balance at December 31, 2011	$	2,893		$	3,139		$	6,566		$	1,886		$	356		$	704		$	1,030		$	16,574
	Charge-offs		—			(1,253	)		—			(192	)		(697	)		(159	)		—			(2,301	)
	Recoveries		—			307			9			17			422			19			—			774
	Provision		148			925			2,490			283			252			322			(270	)		4,150
	Ending balance at December 31, 2012	$	3,041		$	3,118		$	9,065		$	1,994		$	333		$	886		$	760		$	19,197
	Amount of allowance for loan losses or loans deemed to be impaired	$	1,000		$	104		$	415		$	117		$	—		$	96		$	—		$	1,732
	Amount of allowance for loan losses for loans not deemed to be impaired	$	2,041		$	3,014		$	8,650		$	1,877		$	333		$	790		$	760		$	17,465
	Loans:
	Ending balance	$	38,618		$	88,475		$	576,465		$	281,857		$	7,450		$	118,923		$	—		$	1,111,788
	Loans deemed to be impaired	$	1,500		$	1,282		$	2,281		$	766		$	—		$	96		$	—		$	5,925
	Loans not deemed to be impaired	$	37,118		$	87,193		$	574,184		$	281,091		$	7,450		$	118,827		$	—		$	1,105,863
	CREDIT QUALITY INFORMATION
	The Company utilizes a six-grade internal loan rating system for commercial real estate, construction and commercial loans as follows:
	Loans rated 1-3 (Pass) — Loans in this category are considered “pass” rated loans with low to average risk.
	Loans rated 4 (Monitor) — These loans represent classified loans that management is closely monitoring for credit quality. These loans have had or may have minor credit quality deterioration as of December 31, 2013.
	Loans rated 5 (Substandard) — Substandard loans represent classified loans that management is closely monitoring for credit quality. These loans have had more significant credit quality deterioration as of December 31, 2013.
	Loans rated 6 (Doubtful) — Doubtful loans represent classified loans that management is closely monitoring for credit quality. These loans had more significant credit quality deterioration as of December 31, 2013, and are doubtful for full collection.
	Impaired — Impaired loans represent classified loans that management is closely monitoring for credit quality. A loan is classified as impaired when it is probable that the Company will be unable to collect all amounts due.
	The following table presents the Company’s loans by risk rating at December 31, 2013.
		Construction			Commercial			Commercial
		and Land			and			Real Estate
		Development			Industrial
	(dollars in thousands)
	Grade:
	1-3 (Pass)	$	25,138		$	90,563		$	707,461
	4 (Monitor)		7,312			472			1,346
	5 (Substandard)		—			—			—
	6 (Doubtful)		—			—			—
Impaired		608			1,367			4,520
Total	$	33,058		$	92,402		$	713,327
The following table presents the Company’s loans by risk rating at December 31, 2012.
	Construction			Commercial			Commercial
	and Land			and			Real Estate
	Development			Industrial
(dollars in thousands)
Grade:
1-3 (Pass)	$	29,719		$	86,587		$	569,760
4 (Monitor)		7,399			606			4,424
5 (Substandard)		—			—			—
6 (Doubtful)		—			—			—
Impaired		1,500			1,282			2,281
Total	$	38,618		$	88,475		$	576,465
The Company utilized payment performance as credit quality indicators for residential real state, consumer and overdrafts, and the home equity portfolio. The indicators are depicted in the table “aging of past-due loans,” below.
AGING OF PAST-DUE LOANS
Further information pertaining to the allowance for loan losses at December 31, 2013 follows:
	Accruing			Non			Accruing			Total			Current			Total
	30-89 Days			Accrual			Greater			Past Due			Loans
	Past Due						Than
							90 Days
(dollars in thousands)
Construction and land development	$	—		$	500		$	—		$	500		$	32,558		$	33,058
Commercial and industrial		112			706			—			818			91,584			92,402
Commercial real estate		1,496			306			—			1,802			711,525			713,327
Residential real estate		2,232			1,034			—			3,266			282,775			286,041
Consumer and overdrafts		11			3			—			14			9,644			9,658
Home equity		1,710			—			—			1,710			128,567			130,277
Total	$	5,561		$	2,549		$	—		$	8,110		$	1,256,653		$	1,264,763
Further information pertaining to the allowance for loan losses at December 31, 2012 follows:
	Accruing			Non			Accruing			Total			Current			Total
	30-89 Days			Accrual			Greater			Past Due			Loans
	Past Due						Than
							90 Days
(dollars in thousands)
Construction and land development	$	—		$	1,500		$	—		$	1,500		$	37,118		$	38,618
Commercial and industrial		1,256			676			—			1,932			86,543			88,475
Commercial real estate		3,450			674			—			4,124			572,341			576,465
Residential real estate		864			1,597			—			2,461			279,396			281,857
Consumer and overdrafts		32			24			—			56			7,394			7,450
Home equity		1,088			—			—			1,088			117,835			118,923
Total	$	6,690		$	4,471		$	—		$	11,161		$	1,100,627		$	1,111,788
IMPAIRED LOANS
A loan is impaired when, based on current information and events, it is probable that a creditor will be unable to collect all amounts due according to the contractual terms of the loan agreement. When a loan is impaired, the Company measures impairment based on the present value of expected future cash flows discounted at the loan’s effective interest rate, except that as a practical expedient, the Company measures impairment based on a loan’s observable market price or the fair value of the collateral if the loan is collateral dependent. Loans are charged-off when management believes that the collectibility of the loan’s principal is not probable. The specific factors that management considers in making the determination that the collectibility of the loan’s principal is not probable include; the delinquency status of the loan, the fair value of the collateral, if secured, and the financial strength of the borrower and/or guarantors. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off against the allowance for loan losses in lieu of an allocation of a specific allowance amount when such an amount has been identified definitively as uncollectible. The Company’s policy for recognizing interest income on impaired loans is contained within Note 1 of the “Notes to Consolidated Financial Statements.”
The following is information pertaining to impaired loans at December 31, 2013:
	Carrying			Unpaid			Required			Average			Interest
	Value			Balance			Reserve			Carrying			Income
				Principal						Value
										Recognized
(dollars in thousands)
With no required reserve recorded:
Construction and land development	$	500		$	3,292		$	—		$	—		$	—
Commercial and industrial		238			268			—			361			1
Commercial real estate		82			82			—			132			—
Residential real estate		246			259			—			169			—
Consumer		—			—			—			—			—
Home equity		—			—			—			—			—
Total	$	1,066		$	3,901		$	—		$	662		$	1
With required reserve recorded:
Construction and land development	$	108		$	108		$	12		$	1,371		$	1
Commercial and industrial		1,129			1,371			367			902			37
Commercial real estate		4,438			4,527			417			2,868			120
Residential real estate		953			1,035			129			878			2
Consumer		—			—			—			—			—
Home equity		94			94			94			95			—
Total	$	6,722		$	7,135		$	1,019		$	6,114		$	160
Total
Construction and land development	$	608		$	3,400		$	12		$	1,371		$	1
Commercial and industrial		1,367			1,639			367			1,263			38
Commercial real estate		4,520			4,609			417			3,000			120
Residential real estate		1,199			1,294			129			1,047			2
Consumer		—			—			—			—			—
Home equity		94			94			94			95			—
Total	$	7,788		$	11,036		$	1,019		$	6,776		$	161
The following is information pertaining to impaired loans at December 31, 2012:
	Carrying			Unpaid			Required			Average			Interest
	Value			Balance			Reserve			Carrying			Income
				Principal						Value			Recognized
(dollars in thousands)
With no required reserve recorded:
Construction and land development	$	—		$	—		$	—		$	346		$	—
Commercial and industrial		503			994			—			425			1
Commercial real estate		169			199			—			176			—
Residential real estate		30			31			—			124			—
Consumer		—			—			—			—			—
Home equity		—			—			—			—			—
Total	$	702		$	1,224		$	—		$	1,071		$	1
With required reserve recorded:
Construction and land development	$	1,500		$	3,292		$	1,000		$	1,154		$	—
Commercial and industrial		779			995			104			1,317			40
Commercial real estate		2,112			2,158			415			2,817			138
Residential real estate		736			736			117			640			1
Consumer		—			—			—			—			—
Home equity		96			96			96			44			—
Total	$	5,223		$	7,277		$	1,732		$	5,972		$	179
Total
Construction and land development	$	1,500		$	3,292		$	1,000		$	1,500		$	—
Commercial and industrial		1,282			1,989			104			1,742			41
Commercial real estate		2,281			2,357			415			2,993			138
Residential real estate		766			767			117			764			1
Consumer		—			—			—			—			—
Home equity		96			96			96			44			—
Total	$	5,925		$	8,501		$	1,732		$	7,043		$	180
Troubled Debt Restructurings are identified as a modification in which a concession was granted to a customer who was having financial difficulties. This concession may be below market rate, longer amortization/term, or a lower payment amount. The present value calculation of the modification did not result in an increase in the allowance for these loans beyond any previously established allocations.
The following is information pertaining to troubled debt restructurings occurring during the year ended December 31, 2013:
	Number			Pre-modification			Post-modification
	of			Outstanding			Outstanding
	Contracts			Recorded			Recorded
				Investment			Investment
(dollars in thousands)
Construction and land development		1		$	108		$	108
Commercial and industrial		2			67			64
Commercial real estate		3			2,376			2,356
Residential real estate		1			285			162
Total		7		$	2,836		$	2,690
There was one commercial and industrial troubled debt restructuring, that subsequently defaulted amounting to $6,000 during 2013. The loans were modified for 2013, by reducing interest rates as well as extending term on the commercial and industrial loan. The financial impact of the modifications for performing commercial and industrial loans were $808 reduction in principal and $606 reduction in interest payments for the year ended December 31, 2013. The financial impact of the modifications for performing commercial real estate loans were $5,246 increase in principal and $23,227 reduction in interest payments for the year ended December 31, 2013. The financial impact of the modifications for performing construction and land development loans were $1,515 reduction in principal and $1,098 reduction in interest payments for the year ended December 31, 2013. The financial impact of the modifications for performing residential real estate loans were $4,704 reduction in principal and $4,104 reduction in interest payments for the year ended December 31, 2013.
The following is information pertaining to troubled debt restructurings occurring during the year ended December 31, 2012:
	Number of			Pre-modification			Post-modification
	Contracts			Outstanding			Outstanding
				Recorded Investment			Recorded  Investment
(dollars in thousands)
Construction and land development		1		$	750		$	736
Commercial and industrial		1			6			6
Commercial real estate		1			98			96
Total		3		$	854		$	838
There were no troubled debt restructurings, that subsequently defaulted, during 2012. The loans were modified during 2012, for the commercial and industrial, and residential real estate loans, by reducing interest rates as well as extending the terms of the loans. The financial impact of the modification for the performing commercial and industrial loan was a $6,000 reduction in principal payments for the year ended December 31, 2012. The financial impact of the modification for performing residential real estate loan was an $8,000 reduction in interest payments for the year ended December 31, 2012.

Bank_Premises_and_Equipment

Bank Premises and Equipment	12 Months Ended
	Dec. 31, 2013
Property Plant And Equipment [Abstract]	'
Bank Premises and Equipment	'
	7.     Bank Premises and Equipment
	December 31,	2013			2012			Estimated Useful Life
	(dollars in thousands)
	Land	$	3,478		$	3,478			—
	Bank premises		19,235			18,353			30-39 years
	Furniture and equipment		31,965			31,319			3-10 years
	Leasehold improvements		10,010			9,930			30-39 years or lease term
			64,688			63,080
	Accumulated depreciation and amortization		(41,288	)		(39,181	)
	Total	$	23,400		$	23,899
	The Company and its subsidiaries are obligated under a number of non-cancelable operating leases for premises and equipment expiring in various years through 2026. Total lease expense approximated $2,094,000, $2,055,000 and $2,007,000 for the years ended December 31, 2013, 2012 and 2011, respectively. Rental income approximated $299,000, $329,000 and $455,000 in 2013, 2012 and 2011, respectively.
	Future minimum rental commitments for non-cancelable operating leases with initial or remaining terms of one year or more at December 31, 2013, were as follows:
		Year			Amount
		(dollars in thousands)
			2014		$	2,070
			2015			1,824
			2016			1,675
			2017			1,235
			2018			1,035
			Thereafter			2,816
					$	10,655

Goodwill_and_Identifiable_Inta

Goodwill and Identifiable Intangible Assets	12 Months Ended
	Dec. 31, 2013
Goodwill And Intangible Assets Disclosure [Abstract]	'
Goodwill and Identifiable Intangible Assets	'
	8.     Goodwill and Identifiable Intangible Assets
	At December 31, 2013 and 2012, the Company concluded that it is not more likely than not that fair value of the reporting unit is less than its carrying value, and goodwill is not considered to be impaired.
	During the full year of 2011, the Company’s Class A common stock traded close to or above book value per share. Accordingly, at December 31, 2011, management measured for impairment utilizing the fair value of the reporting unit based on the recent stock price of the Company.
	The changes in goodwill and identifiable intangible assets for the years ended December 31, 2013 and 2012 are shown in the table below.
	Carrying Amount of Goodwill and Intangibles	Goodwill		Core Deposit			Mortgage			Total
				Intangibles			Servicing
							Rights
	(dollars in thousands)
	Balance at December 31, 2011	$	2,714	$	120		$	123		$	2,957
	Additions		—		—			48			48
	Amortization Expense		—		(120	)		(34	)		(154	)
	Balance at December 31, 2012	$	2,714	$	—			137		$	2,851
	Additions		—		—			653			653
	Amortization Expense		—		—			(87	)		(87	)
	Balance at December 31, 2013	$	2,714	$	—			703		$	3,417

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2013
Fair Value Disclosures [Abstract]	'
Fair Value Measurements	'
	9.     Fair Value Measurements
	The Company follows FASB ASC 820-10, Fair Value Measurements and Disclosures (formerly SFAS 157, “Fair Value Measurements”), which among other things, requires enhanced disclosures about assets and liabilities carried at fair value. ASC 820-10 establishes a hierarchal disclosure framework associated with the level of pricing observability utilized in measuring financial instruments at fair value. The three broad levels of the hierarchy are as follows:
	Level I — Quoted prices are available in active markets for identical assets or liabilities as of the reported date. The type of financial instruments included in Level I are highly liquid cash instruments with quoted prices such as G-7 government, agency securities, listed equities and money market securities, as well as listed derivative instruments.
	Level II — Pricing inputs are other than quoted prices in active markets, which are either directly or indirectly observable as of the reported date. The nature of these financial instruments include cash instruments for which quoted prices are available but traded less frequently, derivative instruments whose fair value have been derived using a model where inputs to the model are directly observable in the market, or can be derived principally from or corroborated by observable market data, and instruments that are fair valued using other financial instruments, the parameters of which can be directly observed. Instruments which are generally included in this category are corporate bonds and loans, mortgage whole loans, municipal bonds and OTC derivatives.
	Level III — These instruments have little to no pricing observability as of the reported date. These financial instruments do not have two-way markets and are measured using management’s best estimate of fair value, where the inputs into the determination of fair value require significant management judgment or estimation. Instruments that are included in this category generally include certain commercial mortgage loans, certain private equity investments, distressed debt, non-investment grade residual interests in securitizations, as well as certain highly structured OTC derivative contracts.
	The results of the fair value hierarchy as of December 31, 2013, are as follows:
						Fair Value Measurements Using
			Carrying			Quoted Prices			Significant			Significant
			Value			in Active Markets			Observable			Other
						for Identical Assets			Inputs			Unobservable
						(Level 1)			(Level 2)			Inputs (Level 3)
	(dollars in thousands)
	Financial Instruments Measured at Fair Value on a Recurring Basis — Securities AFS
	U.S. Treasury		$	1,998		$	—		$	1,998		$	—
	U.S. Government Sponsored Enterprises			10,004			—			10,004			—
	SBA Backed Securities			7,302			—			7,302			—
	U.S. Government Agency and Sponsored Enterprises
	Mortgage-Backed Securities			403,189			—			403,189			—
	Privately Issued Residential Mortgage-Backed Securities			2,277			—			2,277			—
	Obligations Issued by States and Political Subdivisions			36,723			—			416			36,307
	Other Debt Securities			2,176			—			2,176			—
	Equity Securities			576			286			—			290
	Total		$	464,245		$	286		$	427,362		$	36,597
	Financial Instruments Measured at Fair Value on a Non-recurring Basis
	Impaired Loans		$	1,747		$	—		$	—		$	1,747
	Impaired loan balances in the table above represent those collateral dependent loans where management has estimated the credit loss by comparing the loan’s carrying value against the expected realizable fair value of the collateral. Fair value is generally determined through a review process that includes independent appraisals, discounted cash flows, or other external assessments of the underlying collateral, which generally include various Level 3 inputs which are not identifiable. The Company discounts the fair values, as appropriate, based on management’s observations of the local real estate market for loans in this category.
	Appraisals, discounted cash flows and real estate tax assessments are reviewed quarterly. There is no specific policy regarding how frequently appraisals will be updated. Adjustments are made to appraisals and real estate tax assessments based on management’s estimate of changes in real estate values. Within the past twelve months there have been no updated appraisals, however, all impaired loans have been reviewed during the past quarter using either a discounted cash flow analysis or other type of real estate tax assessment. The types of adjustments that are made to specific provisions (credits) relate to impaired loans recognized for 2013 for the estimated credit loss amounted to $48,000.
	There were no transfers between level 1 and 2 for the year ended December 31, 2013. There were no liabilities measured at fair value on a recurring or nonrecurring basis during the year ended December 31, 2013.
	The following table presents additional information about assets measured at fair value on a recurring and nonrecurring basis for which the Company has utilized Level 3 inputs to determine fair value (dollars in thousands) at December 31, 2013. Management continues to monitor the assumptions used to value the assets listed below.
	Asset		Fair Value			Valuation Technique		Unobservable Input	Unobservable Input
									Value or Range
	Securities AFS(1)		$	36,597		Discounted cash flow		Discount rate	0%-1%(2)
	Impaired Loans			1,747		Appraisal of collateral(3)		Appraisal adjustments(4)	0%-25% discount
	-1	Municipal securities generally have maturities of one year or less and, therefore, the amortized cost equates to the fair value.
	-2	Weighted averages.
	-3	Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various Level 3 inputs which are not identifiable.
	-4	Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated expenses.
	The changes in Level 3 securities for the year ended December 31, 2013 are as shown in the table below:
			Auction Rate			Obligations			Equity			Total
			Securities			Issued by States			Securities
						and Political
						Subdivisions
	(dollars in thousands)
	Balance at December 31, 2012		$	3,963		$	49,477		$	342		$	53,782
	Purchases			—			50,012			—			50,012
	Maturities			—			(66,976	)		(52	)		(67,028	)
	Amortization			—			(26	)		—			(26	)
	Change in fair value			(143	)		—			—			(143	)
	Balance at December 31, 2013		$	3,820		$	32,487		$	290		$	36,597
	The amortized cost of Level 3 securities was $37,463,000 with an unrealized loss of $866,000 at December 31, 2013. The securities in this category are generally equity investments, municipal securities with no readily determinable fair value or failed auction rate securities. Management evaluated the fair value of these securities based on an evaluation of the underlying issuer, prevailing rates and market liquidity.
	The results of the fair value hierarchy as of December 31, 2012, are as follows:
			Fair Value Measurements Using
			Carrying			Quoted Prices			Significant			Significant
			Value			in Active Markets			Observable			Other Unobservable
						for Identical Assets			Inputs			Inputs
						(Level 1)			(Level 2)			(Level 3)
	(dollars in thousands)
	Financial Instruments Measured at Fair Value on a Recurring Basis — Securities AFS
	U.S. Treasury		$	2,004		$	—		$	2,004		$	—
	U.S. Government Sponsored Enterprises			130,340			—			130,340			—
	SBA Backed Securities			8,156			—			8,156			—
	U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities			1,233,357			—			1,233,357			—
	Privately Issued Residential Mortgage-Backed Securities			2,947			—			2,947			—
Privately Issued Commercial Mortgage-Backed Securities			—			—			—			—
Obligations Issued by States and Political Subdivisions			55,174			—			1,734			53,440
Other Debt Securities			2,253			—			2,253			—
Equity Securities			570			228			—			342
Total		$	1,434,801		$	228		$	1,380,791		$	53,782
Financial Instruments Measured at Fair Value on a Non-recurring Basis
Impaired Loans		$	3,587		$	—		$	—		$	3,587
Appraisals, discounted cash flows and real estate tax assessments are reviewed quarterly. There is no specific policy regarding how frequently appraisals will be updated. Adjustments are made to appraisals and real estate tax assessments based on management’s estimate of changes in real estate values. Within the past twelve months there have been no updated appraisals, however, all impaired loans have been reviewed during the past quarter using either a discounted cash flow analysis or other type of real estate tax assessment. Specific provisions relate to impaired loans recognized for 2012 for the estimated credit loss amounted to $1,909,000.
There were no transfers between level 1 and 2 for the year ended December 31, 2012. There were no liabilities measured at fair value on a recurring or nonrecurring basis during the year ended December 31, 2012.
The following table presents additional information about assets measured at fair value on a recurring and nonrecurring basis for which the Company has utilized Level 3 inputs to determine fair value (dollars in thousands) at December 31, 2012. Management continues to monitor the assumptions used to value the assets listed below.
Asset		Fair Value			Valuation Technique		Unobservable Input	Unobservable Input
								Unobservable Input
								Value or Range
Securities AFS(1)		$	53,782		Discounted cash flow		Discount rate	0%-1%(2)
Impaired Loans			3,587		Appraisal of collateral(3)		Appraisal adjustments(4)	0%-25% discount
-1	Municipal securities generally have maturities of one year or less and, therefore, the amortized cost equates to the fair value.
(2)	Weighted averages.
(3)	Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various Level 3 inputs which are not identifiable.
(4)	Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated expenses.
The changes in Level 3 securities for the year ended December 31, 2012 are as shown in the table below:
		Auction Rate			Obligations			Equity			Total
		Securities			Issued by States			Securities
					and Political
					Subdivisions
(dollars in thousands)
Balance at December 31, 2011		$	3,725		$	14,772		$	417		$	18,914
Purchases			—			90,960			—			90,960
Maturities			—			(56,214	)		(75	)		(56,289	)
Amortization			—			(41	)		—			(41	)
Change in fair value			238			—			—			238
Balance at December 31, 2012		$	3,963		$	49,477		$	342		$	53,782
The amortized cost of Level 3 securities was $54,504,000 with an unrealized loss of $722,000 at December 31, 2012. The securities in this category are generally equity investments, municipal securities with no readily determinable fair value or failed auction rate securities. Management evaluated the fair value of these securities based on an evaluation of the underlying issuer, prevailing rates and market liquidity.

Deposits

Deposits	12 Months Ended
	Dec. 31, 2013
Banking And Thrift [Abstract]	'
Deposits	'
	10.     Deposits
	The following is a summary of remaining maturities or re-pricing of time deposits as of December 31,
		2013		Percent			2012		Percent
	(dollars in thousands)
	Within one year	$	248,758		65	%	$	299,456		71	%
	Over one year to two years		41,533		11	%		67,918		16	%
	Over two years to three years		48,357		13	%		19,834		5	%
	Over three years to five years		43,576		11	%		32,775		8	%
	Total	$	382,224		100	%	$	419,983		100	%
	Time deposits of $100,000 or more totaled $259,665,000 and $287,048,000 in 2013 and 2012, respectively.

Securities_Sold_under_Agreemen

Securities Sold under Agreements to Repurchase	12 Months Ended
	Dec. 31, 2013
Banking And Thrift [Abstract]	'
Securities Sold under Agreements to Repurchase	'
	11.     Securities Sold Under Agreements to Repurchase
	The following is a summary of securities sold under agreements to repurchase as of December 31,
		2013			2012			2011
	(dollars in thousands)
	Amount outstanding at December 31	$	214,440		$	191,390		$	143,320
	Weighted average rate at December 31		0.18	%		0.17	%		0.24	%
	Maximum amount outstanding at any month end	$	214,440		$	213,730		$	152,267
	Daily average balance outstanding during the year	$	203,888		$	174,624		$	129,137
	Weighted average rate during the year		0.18	%		0.21	%		0.29	%
	Amounts outstanding at December 31, 2013, 2012 and 2011 carried maturity dates of the next business day. U.S. Government Sponsored Enterprise securities with a total amortized cost of $216,747,000, $187,995,000 and $140,891,000 were pledged as collateral and held by custodians to secure the agreements at December 31, 2013, 2012 and 2011, respectively. The approximate fair value of the collateral at those dates was $213,350,000, $191,704,000, and $143,212,000, respectively.

Other_Borrowed_Funds_and_Subor

Other Borrowed Funds and Subordinated Debentures	12 Months Ended
	Dec. 31, 2013
Banking And Thrift [Abstract]	'
Other Borrowed Funds and Subordinated Debentures	'
	12.     Other Borrowed Funds and Subordinated Debentures
	The following is a summary of other borrowed funds and subordinated debentures as of December 31,
		2013			2012			2011
	(dollars in thousands)
	Amount outstanding at December 31	$	291,227		$	231,227		$	280,226
	Weighted average rate at December 31		3.04	%		3.54	%		2.85	%
	Maximum amount outstanding at any month end	$	291,227		$	277,226		$	280,226
	Daily average balance outstanding during the year	$	231,032		$	217,542		$	202,209
	Weighted average rate during the year		3.73	%		3.82	%		3.85	%
	FEDERAL HOME LOAN BANK BORROWINGS
	Federal Home Loan Bank of Boston (“FHLBB”) borrowings are collateralized by a blanket pledge agreement on the Bank’s FHLBB stock, certain qualified investment securities, deposits at the FHLBB and residential mortgages held in the Bank’s portfolios. The Bank’s remaining term borrowing capacity at the FHLBB at December 31, 2013, was approximately $423,143,000. In addition, the Bank has a $14,500,000 line of credit with the FHLBB. A schedule of the maturity distribution of FHLBB advances with the weighted average interest rates is as follows:
		2013						2012						2011
	December 31,	Amount			Weighted			Amount			Weighted			Amount		Weighted
					Average						Average					Average
					Rate						Rate					Rate
	(dollars in thousands)
	Within one year	$	53,000			0.4	%	$	46,000			1.86	%	$	81,500		0.42	%
	Over one year to two years		19,500			2.42	%		17,500			3.01	%		23,500		3.34	%
	Over two years to three years		55,000			3.07	%		19,500			2.42	%		17,500		3.01	%
	Over three years to five years		77,000			3.05	%		90,000			3.33	%		74,500		2.9	%
	Over five years		50,500			3.39	%		22,000			4.19	%		47,000		4.38	%
	Total	$	255,000			2.52	%	$	195,000			2.96	%	$	244,000		2.41	%
	Included in the table above are $35,000,000 of FHLBB advances for each of the years at December 31, 2013, 2012 and 2011, respectively, that are putable at the discretion of FHLBB. These put dates were not utilized in the table above.
	During 2013, the Company restructured $14,500,000 of FHLBB advances. Prior to restructure, the weighted average rate on these advances was 3.16% and the weighted average remaining maturity was 12 months. Subsequent to restructure, the weighted average rate was 3.24% and the weighted average maturity was 68 months. The restructures were accounted for as modifications.
	During 2011, the Company restructured $18,000,000 of FHLBB advances. Prior to restructure, the weighted average rate on these advances was 4.45% and the weighted average remaining maturity was 25 months. Subsequent to restructure, the weighted average rate was 3.50% and the weighted average maturity was 60 months. The restructures were accounted for as modifications.
	SUBORDINATED DEBENTURES
	Subordinated debentures totaled $36,083,000 at December 31, 2013 and 2012. In May 1998, the Company consummated the sale of a trust preferred securities offering, in which it issued $29,639,000 of subordinated debt securities due 2029 to its newly formed unconsolidated subsidiary Century Bancorp Capital Trust. Century Bancorp Capital Trust then issued 2,875,000 shares of Cumulative Trust Preferred Securities with a liquidation value of $10 per share. These securities pay dividends at an annualized rate of 8.30%. The Company redeemed through its subsidiary, Century Bancorp Capital Trust, its 8.30% Trust Preferred Securities on January 10, 2005.
	In December 2004, the Company consummated the sale of a trust preferred securities offering, in which it issued $36,083,000 of subordinated debt securities due 2034 to its newly formed unconsolidated subsidiary Century Bancorp Capital Trust II.
	Century Bancorp Capital Trust II then issued 35,000 shares of Cumulative Trust Preferred Securities with a liquidation value of $1,000 per share. These securities pay dividends at an annualized rate of 6.65% for the first ten years and then convert to the three-month LIBOR rate plus 1.87% for the remaining 20 years.
	OTHER BORROWED FUNDS
	There were no overnight federal funds purchased at December 31, 2013 and 2012.
	The Bank also has an outstanding loan in the amount of $144,000 at December 31, 2013 and 2012, borrowed against the cash value of a whole life insurance policy for a key executive of the Bank.

Reclassifications_Out_of_Accum

Reclassifications Out of Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Reclassifications Out of Accumulated Other Comprehensive Income	'
	13.     Reclassifications Out of Accumulated Other Comprehensive Income(a)
			Amount Reclassified from Accumulated
			Other Comprehensive Income
	Details about Accumulated Other		Year ended			Year ended			Affected line item in the Statement
	Comprehensive Income Components		December 31, 2013(a)			December 31, 2012(a)			Where Net Income is Presented
	Unrealized gains and losses on available-for-sale securities		$	3,019		$	1,843		Net gains on sales of investments
				(1,179	)		(728	)	Provision for income taxes
			$	1,840		$	1,115		Net income
	Accretion of unrealized losses transferred		$	3,077		$	—		Securities held-to-maturity
				(1,191	)		—		Provision for income taxes
			$	1,886		$	—		Net Income
	Amortization of defined benefit pension items
	Prior-service costs		$	(10	)	$	(10	)	Salaries and employee benefits(b)
	Actuarial gains (losses)			(1,144	)		(1,071	)	Salaries and employee benefits(b)
	Total before tax			(1,154	)		(1,081	)	Income before taxes
	Tax (expense) or benefit			460			432		Provision for income taxes
	Net of tax		$	(694	)	$	(649	)	Net income
	Total reclassifications for the period		$	3,032		$	466		Net income
	(a)	Amounts in parentheses indicate debits to profit/loss.
	(b)	These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see employee benefits footnote (Note 17) for additional details).

Earnings_Per_Share_EPS

Earnings Per Share ("EPS")	12 Months Ended
	Dec. 31, 2013
Earnings Per Share [Abstract]	'
Earnings Per Share ("EPS")	'
	14.     Earnings per share (“EPS”)
	Class A and Class B shares participate equally in undistributed earnings. Under the Company’s Articles of Organization, the holders of Class A Common Stock are entitled to receive dividends per share equal to at least 200% of dividends paid, if any, from time to time, on each share of Class B Common Stock.
	Diluted EPS includes the dilutive effect of common stock equivalents; basic EPS excludes all common stock equivalents. The only common stock equivalents for the Company are the stock options discussed below. The dilutive effect of these stock options for 2013, 2012 and 2011 was an increase of 1,155, 1,024 and 1,149 shares, respectively.
	The following table is a reconciliation of basic EPS and diluted EPS:
	Year Ended December 31,	2013		2012		2011
	(in thousands except share and per share data)
	BASIC EPS COMPUTATION
	Numerator:
	Net income, Class A	$	15,698	$	14,877	$	13,023
	Net income, Class B		4,348		4,162		3,670
	Denominator:
	Weighted average shares outstanding, Class A		3,575,683		3,557,693		3,543,233
	Weighted average shares outstanding, Class B		1,980,855		1,990,474		1,997,411
	Basic EPS, Class A	$	4.39	$	4.18	$	3.68
	Basic EPS, Class B		2.19		2.09		1.84
	DILUTED EPS COMPUTATION
	Numerator:
	Net income, Class A	$	15,698	$	14,877	$	13,023
	Net income, Class B		4,348		4,162		3,670
	Total net income, for diluted EPS, Class A computation		20,046		19,039		16,693
	Denominator:
	Weighted average shares outstanding, basic, Class A		3,575,683		3,557,693		3,543,233
	Weighted average shares outstanding, Class B		1,980,855		1,990,474		1,997,411
	Dilutive effect of Class A stock options		1,155		1,024		1,150
	Weighted average shares outstanding diluted, Class A		5,557,693		5,549,191		5,541,794
	Weighted average shares outstanding, Class B		1,980,855		1,990,474		1,997,411
	Diluted EPS, Class A	$	3.61	$	3.43	$	3.01
	Diluted EPS, Class B		2.19		2.09		1.84

Stockholders_Equity_Dividends

Stockholders' Equity Dividends	12 Months Ended
	Dec. 31, 2013
Equity [Abstract]	'
Stockholders' Equity Dividends	'
	15.     Stockholders’ Equity DIVIDENDS
	Holders of the Class A common stock may not vote in the election of directors but may vote as a class to approve certain extraordinary corporate transactions. Holders of Class B common stock may vote in the election of directors. Class A common stockholders are entitled to receive dividends per share equal to at least 200% per share of that paid, if any, on each share of Class B common stock. Class A common stock is publicly traded. Class B common stock is not publicly traded; however, it can be converted on a per share basis to Class A common stock at any time at the option of the holder. Dividend payments by the Company are dependent in part on the dividends it receives from the Bank, which are subject to certain regulatory restrictions.
	STOCK REPURCHASE PLAN
	During 2013, the Board of Directors of the Company approved a reauthorization of the stock repurchase program. Under the program, the Company is reauthorized to repurchase up to 300,000, or less than 9%, of Century Bancorp Class A Common Stock outstanding. This vote supersedes the previous program voted by the Board of Directors during 2012, which also authorized the Company to repurchase up to 300,000, or less than 9%, of Century Bancorp Class A Common Stock. The stock buyback is authorized to take place from time-to-time, subject to prevailing market conditions. The purchases are made on the open market and are funded from available cash.
	STOCK OPTION PLAN
	During 2000 and 2004, common stockholders of the Company approved stock option plans (the “Option Plans”) that provide for granting of options for not more than 150,000 shares of Class A common stock per plan. Under the Option Plans, all officers and key employees of the Company are eligible to receive nonqualified and incentive stock options to purchase shares of Class A common stock. The Option Plans are administered by the Compensation Committee of the Board of Directors, whose members are ineligible to participate in the Option Plans. Based on management’s recommendations, the Committee submits its recommendations to the Board of Directors as to persons to whom options are to be granted, the number of shares granted to each, the option price (which may not be less than 85% of the fair market value for nonqualified stock options, or the fair market value for incentive stock options, of the shares on the date of grant) and the time period over which the options are exercisable (not more than ten years from the date of grant). There were 20,375 options exercisable at December 31, 2013.
	Stock option activity under the plan is as follows:
		December 31, 2013						December 31, 2012						December 31, 2011
		Amount			Weighted			Amount			Weighted			Amount			Weighted
					Average						Average						Average
					Exercise Price						Exercise Price						Exercise Price
	Shares under option:
	Outstanding at beginning of year		23,350		$	31.17			36,062		$	28.9			38,712		$	28.36
	Forfeited		(1,350	)		26.68			(450	)		22.5			(200	)		15.06
	Exercised		(1,625	)		26.76			(12,262	)		24.82			(2,450	)		21.44
	Outstanding at end of year		20,375		$	31.82			23,350		$	31.17			36,062		$	28.9
	Exercisable at end of year		20,375		$	31.82			23,350		$	31.17			36,062		$	28.9
	Available to be granted at end of year		224,884						223,534						223,084
	At December 31, 2013, 2012 and 2011, the options outstanding have exercise prices between $15.06 and $31.83, and a weighted average remaining contractual life of one year for 2013 and two years for 2012 and 2011. The weighted average intrinsic value of options exercised for the period ended December 31, 2013, was $6.49 per share with an aggregate value of $10,548. The average intrinsic value of options exercisable at December 31, 2013, 2012 and 2011 had an aggregate value of $29,136, $41,549 and $49,145, respectively.
	CAPITAL RATIOS
	The Bank and the Company are subject to various regulatory requirements administered by federal banking agencies. Failure to meet minimum capital requirements can initiate certain mandatory and possibly additional discretionary actions by regulators that, if undertaken, could have a direct material effect on the Bank and Company’s financial statements. Under capital adequacy guidelines and regulatory framework for prompt corrective action, the Bank and Company must meet specific capital guidelines that involve quantitative measures of the Bank and Company’s assets and liabilities, and certain off-balance-sheet items as calculated under regulatory accounting practices. The Bank and Company’s capital amounts and classification are also subject to qualitative judgments by the regulators about components, risk weightings and other factors.
	Quantitative measures established by regulation to ensure capital adequacy require the Bank and the Company to maintain minimum amounts and ratios (set forth in the table below) of total and Tier 1 capital (as defined in the regulation) to risk-weighted assets (as defined) and Tier 1 capital (as defined) to average assets (as defined). Management believes, as of December 31, 2013, that the Bank and the Company meet all capital adequacy requirements to which they are subject.
	As of December 31, 2013, the most recent notification from the FDIC categorized the Bank as well capitalized under the regulatory framework for prompt corrective action. To be categorized as well capitalized, the Bank must maintain minimum total risk-based, Tier risk-based, and Tier 1 leverage ratios as set forth in the table below. There are no conditions or events since that notification that management believes would cause a change in the Bank’s categorization.
	The Bank’s actual capital amounts and ratios are presented in the following table:
		Actual			Ratio			For  Capital						To Be Well
		Amount						Adequacy						Capitalized  Under
								Purposes						Prompt Corrective
														Action Provisions
								Amount			Ratio			Amount			Ratio
	As of December 31, 2013
	Total Capital (to Risk-Weighted Assets)	$	230,038			13.91	%	$	132,338			8	%	$	165,422			10	%
	Tier 1 Capital (to Risk-Weighted Assets)		209,360			12.66	%		66,169			4	%		99,253			6	%
	Tier 1 Capital (to 4th Qtr. Average Assets)		209,360			6	%		139,467			4	%		174,334			5	%
	As of December 31, 2012
	Total Capital (to Risk-Weighted Assets)	$	206,464			14.15	%	$	116,726			8	%	$	145,907			10	%
	Tier 1 Capital (to Risk-Weighted Assets)		188,226			12.9	%		58,363			4	%		87,544			6	%
	Tier 1 Capital (to 4th Qtr. Average Assets)		188,226			6.11	%		123,202			4	%		154,002			5	%
	The Company’s actual capital amounts and ratios are presented in the following table:
								For  Capital						To Be Well
								Adequacy						Capitalized  Under
								Purposes						Prompt Corrective
														Action Provisions
		Actual			Ratio			Amount			Ratio			Amount			Ratio
		Amount
	As of December 31, 2013
	Total Capital (to Risk-Weighted Assets)	$	247,761			14.92	%	$	132,870			8	%	$	166,088			10	%
	Tier 1 Capital (to Risk-Weighted Assets)		227,000			13.67	%		66,435			4	%		99,653			6	%
	Tier 1 Capital (to 4th Qtr. Average Assets)		227,000			6.5	%		139,769			4	%		174,711			5	%
	As of December 31, 2012
	Total Capital (to Risk-Weighted Assets)	$	227,945			15.59	%	$	116,976			8	%	$	146,220			10	%
	Tier 1 Capital (to Risk-Weighted Assets)		209,668			14.34	%		58,488			4	%		87,732			6	%
	Tier 1 Capital (to 4th Qtr. Average Assets)		209,668			6.8	%		123,377			4	%		154,221			5	%

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	16.     Income Taxes
	The current and deferred components of income tax expense for the years ended December 31 are as follows:
		2013			2012			2011
	(dollars in thousands)
	Current expense:
	Federal	$	3,520		$	3,181		$	2,198
	State		416			315			309
	Total current expense		3,936			3,496			2,507
	Deferred (benefit) expense:
	Federal		(2,564	)		(1,833	)		(961	)
	State		(365	)		(271	)		8
	Total deferred benefit		(2,929	)		(2,104	)		(953	)
	Provision for income taxes	$	1,007		$	1,392		$	1,554
	There were no penalties during 2011, 2012, or 2013. There was approximately $2,000 paid to the Internal Revenue Service for interest during 2012.
	Income tax accounts included in other assets/liabilities at December 31 are as follows:
		2013			2012
	(dollars in thousands)
	Currently receivable	$	702		$	630
	Deferred income tax asset, net		30,857			14,551
	Total	$	31,559		$	15,181
	Differences between income tax expense at the statutory federal income tax rate and total income tax expense are summarized as follows:
		2013			2012			2011
	(dollars in thousands)
	Federal income tax expense at statutory rates	$	7,158		$	6,946		$	6,204
	State income tax, net of federal income tax benefit		34			29			209
	Insurance income		(380	)		(396	)		(396	)
	Effect of tax-exempt interest		(5,348	)		(4,628	)		(3,801	)
	Net tax credit		(572	)		(633	)		(683	)
	Other		115			74			21
	Total	$	1,007		$	1,392		$	1,554
	Effective tax rate		4.8	%		6.8	%		8.5	%
	The following table sets forth the Company’s gross deferred income tax assets and gross deferred income tax liabilities at December 31:
		2013			2012
	(dollars in thousands)
	Deferred income tax assets:
	Allowance for loan losses	$	9,199		$	8,103
	Unrealized losses on securities transferred to held-to-maturity		8,590			—
	Deferred compensation		6,515			5,643
	Pension and SERP liability		4,880			8,621
	AMT credit		3,164			1,908
	Unrealized losses (gains) on securities available-for-sale		652			(7,875	)
	Acquisition premium		438			541
	Nonaccrual interest		136			151
	Depreciation		109			(36	)
	Investments write down		26			26
	Deferred gain		—			11
	Other		198			235
	Gross deferred income tax asset		33,907			17,328
	Deferred income tax liabilities:
	Limited partnerships		(2,769	)		(2,722	)
	Mortgage servicing rights		(281	)		(55	)
	Gross deferred income tax liability		(3,050	)		(2,777	)
	Deferred income tax asset net	$	30,857		$	14,551
	Based on the Company’s historical and current pre-tax earnings, management believes it is more likely than not that the Company will realize the deferred income tax asset existing at December 31, 2013. Management believes that existing net deductible temporary differences which give rise to the deferred tax asset will reverse during periods in which the Company generates net taxable income. In addition, gross deductible temporary differences are expected to reverse in periods during which offsetting gross taxable temporary differences are expected to reverse. Factors beyond management’s control, such as the general state of the economy and real estate values, can affect future levels of taxable income, and no assurance can be given that sufficient taxable income will be generated to fully absorb gross deductible temporary differences. The Company is in an Alternative Minimum Tax (“AMT”) credit position. The AMT credit is carried as a deferred asset and has an indefinite life. The Company has potential tax planning strategies available which support the deferred AMT credit and, at this time, no valuation allowance is needed.
	The Company and its subsidiaries file a consolidated federal tax return. For the tax year beginning in 2009, the Commonwealth of Massachusetts requires a combined state tax return, except for security corporations, which file separate tax returns. The Company is subject to federal and state examinations for tax years after December 31, 2009.

Employee_Benefits

Employee Benefits	12 Months Ended
	Dec. 31, 2013
Compensation And Retirement Disclosure [Abstract]	'
Employee Benefits	'
	17	Employee Benefits
	The Company has a Qualified Defined Benefit Pension Plan (the “Plan”), which had been offered to all employees reaching minimum age and service requirements. In 2006, the Bank became a member of the Savings Bank Employees Retirement Association (“SBERA”) within which it then began maintaining the Qualified Defined Benefit Pension Plan. SBERA offers a common and collective trust as the underlying investment structure for its retirement plans. The target allocation mix for the common and collective trust portfolio calls for an equity-based investment deployment range of 40% to 64% of total portfolio assets. The remainder of the portfolio is allocated to fixed income securities with target range of 15% to 25% and other investments including global asset allocation and hedge funds from 20% to 36%.
	The Trustees of SBERA, through its Investment Committee, select investment managers for the common and collective trust portfolio. A professional investment advisory firm is retained by the Investment Committee to provide allocation analysis, performance measurement and to assist with manager searches. The overall investment objective is to diversify investments across a spectrum of investment types to limit risks from large market swings. The Company closed the plan to employees hired after March 31, 2006.
	The measurement date for the Plan is December 31 for each year. The benefits expected to be paid in each year from 2014 to 2018 are $1,092,000, $1,176,000, $1,247,000, $1,322,000, and $1,325,000, respectively. The aggregate benefits expected to be paid in the five years from 2019 to 2023 are $8,011,000. The Company plans to contribute $1,000,000 to the Plan in 2014.
	The fair value of plan assets and major categories as of December 31, 2013, is as follows:
	Asset Category		Percent			Total			Level 1			Level 2			Level 3
	(dollars in thousands)
	Collective funds			52.9	%	$	17,092		$	856		$	16,236		$	—
	Equity securities			25.9	%		8,355			8,355			—			—
	Mutual funds			13.1	%		4,250			3,832			418			—
	Hedge funds			7	%		2,256			—			—			2,256
	Short-term investments			1.1	%		369			249			120			—
				100	%	$	32,322		$	13,292		$	16,774		$	2,256
	The fair value of plan assets and major categories as of December 31, 2012, is as follows:
	Asset Category		Percent			Total			Level 1			Level 2			Level 3
	(dollars in thousands)
	Collective funds			52.5	%	$	12,593		$	767		$	11,826		$	—
	Equity securities			24.6	%		5,892			5,892			—			—
	Mutual funds			14.1	%		3,370			3,086			284			—
	Hedge funds			7	%		1,691			—			—			1,691
	Short-term investments			1.8	%		437			62			375			—
				100	%	$	23,983		$	9,807		$	12,485		$	1,691
	LEVEL 1
	The plan assets measured at fair value in Level 1 are based on quoted market prices in an active exchange market.
	LEVEL 2
	Plan assets measured at fair value in Level 2 are based on pricing models that consider standard input factors, such as observable market data, benchmark yields, interest rate volatilities, broker/dealer quotes, credit spreads and new issue data.
	LEVEL 3
	Plan assets measured at fair value in Level 3 are based on unobservable inputs, which includes SBERA’s assumptions and the best information available under the circumstance. Level 3 assets consist of hedge funds. The underlying assets are valued based upon quoted exchange prices, over-the-counter trades, bid/ask prices, relative value assessments based on market conditions, and other information, as available. Further adjustments may be made based on factors impacting liquidity.
	The asset or liability’s fair value measurement level within fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Valuation techniques used need to maximize the use of observable inputs and minimize the use of unobservable inputs. Below is a description of the valuation methodologies used for assets measured at fair value.
	The Trust reports bonds and other obligations, short-term investments and equity securities at fair values based on published quotations, Collective funds and hedge funds (Funds) are valued in accordance with valuations provided by such Funds, which generally value marketable securities at the last reported sales price on the valuation date and other investments at fair value, as determined by each Fund’s manager.
	The preceding methods described may produce a fair value calculation that may not be indicative of net realizable value or reflective of future values. Furthermore, although the Trust believes its valuation methods are appropriate and consistent with other market participants, the use of different methodologies to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date.
	The changes in Level 3 securities are shown in the table below:
	Year Ended December 31,		2013			2012
	(dollars in thousands)
	Balance at beginning of year		$	1,691		$	1,522
	Purchases			55			152
	Actual return – assets still being held			510			17
	Balance at end of year		$	2,256		$	1,691
	The performance of the plan assets is dependent upon general market conditions and specific conditions related to the issuers of the underlying securities.
	The Company has a Supplemental Executive Insurance/Retirement Plan (the Supplemental Plan), which is limited to certain officers and employees of the Company. The Supplemental Plan is voluntary and participants are required to contribute to its cost. Under the Supplemental Plan, each participant will receive a retirement benefit based on compensation and length of service. Life insurance policies, which are owned by the Company, are purchased covering the lives of each participant.
	The benefits expected to be paid in each year from 2014 to 2018 are $1,108,000, $1,088,000, $1,515,000, $1,973,000 and $1,981,000, respectively. The aggregate benefits expected to be paid in the five years from 2019 to 2023 are $10,341,000.
			Defined Benefit						Supplemental Insurance/
			Pension  Plan						Retirement Plan
			2013			2012			2013			2012
	(dollars in thousands)
	Change projected in benefit obligation
	Benefit obligation at beginning of year		$	31,910		$	28,784		$	25,835		$	21,097
	Service cost			1,196			1,097			1,554			1,425
	Interest cost			1,257			1,295			1,072			923
	Actuarial (gain)/loss			(3,734	)		1,401			(1,916	)		3,482
	Benefits paid			(750	)		(667	)		(1,043	)		(1,092	)
	Projected benefit obligation at end of year		$	29,879		$	31,910		$	25,502		$	25,835
	Change in plan assets
	Fair value of plan assets at beginning of year		$	23,983		$	20,517
	Actual (loss) return on plan assets			4,619			2,333
	Employer contributions			4,470			1,800
	Benefits paid			(750	)		(667	)
	Fair value of plan assets at end of year		$	32,322		$	23,983
	(Unfunded) Funded status		$	2,443		$	(7,927	)	$	(25,502	)	$	(25,835	)
	Accumulated benefit obligation		$	29,747		$	31,773		$	22,278		$	22,181
	Weighted-average assumptions as of December 31
	Discount rate — Liability			5	%		4	%		5	%		4	%
	Discount rate — Expense			4	%		4.5	%		4	%		4.5	%
Expected return on plan assets			8	%		8	%		NA			NA
Rate of compensation increase			4	%		4	%		4	%		4	%
Components of net periodic benefit cost
Service cost		$	1,196		$	1,097		$	1,554		$	1,425
Interest cost			1,257			1,295			1,072			923
Expected return on plan assets			(1,880	)		(1,641	)		—			—
Recognized prior service cost			(104	)		(104	)		114			114
Recognized net losses			630			735			514			336
Net periodic cost		$	1,099		$	1,382		$	3,254		$	2,798
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Amortization of prior service cost		$	104		$	104		$	(114	)	$	(114	)
Net (gain) loss			(6,956	)		(25	)		(2,401	)		3,098
Total recognized in other comprehensive income			(6,852	)		79			(2,515	)		2,984
Total recognized in net periodic benefit cost and other comprehensive income		$	(5,753	)	$	1,461		$	739		$	5,782
		December 31, 2013									December 31, 2012
		Plan			Supplemental			Total			Plan			Supplemental			Total
					Plan									Plan
(dollars in thousands)
Prior service cost		$	516		$	(991	)	$	(475	)	$	620		$	(1,105	)	$	(485	)
Net actuarial loss			(3,361	)		(7,901	)		(11,262	)		(10,317	)		(10,302	)		(20,619	)
Total		$	(2,845	)	$	(8,892	)	$	(11,737	)	$	(9,697	)	$	(11,407	)	$	(21,104	)
The following table summarizes the amounts included in Accumulated Other Comprehensive Loss at December 31, 2013, expected to be recognized as components of net periodic benefit cost in the next year:
		Plan			Supplemental
					Plan
Amortization of prior service cost to be recognized in 2014		$	(104	)	$	114
Amortization of loss to be recognized in 2014			12			355
Assumptions for the expected return on plan assets and discount rates in the Company’s Plan and Supplemental Plan are periodically reviewed. As part of the review, management in consultation with independent consulting actuaries performs an analysis of expected returns based on the plan’s asset allocation. This forecast reflects the Company’s and actuarial firm’s expected return on plan assets for each significant asset class or economic indicator. The range of returns developed relies on forecasts and on broad market historical benchmarks for expected return, correlation and volatility for each asset class. Also, as a part of the review, the Company’s management in consultation with independent consulting actuaries performs an analysis of discount rates based on expected returns of high-grade fixed income debt securities.
The Company offers a 401(k) defined contribution plan for all employees reaching minimum age and service requirements. The plan is voluntary and employee contributions are matched by the Company at a rate of 33.3% for the first 6% of compensation contributed by each employee. The Company’s match totaled $322,000 for 2013, $308,000 for 2012 and $266,000 for 2011. Administrative costs associated with the plan are absorbed by the Company.
The Company has a cash incentive plan that is designed to reward our executives and officers for the achievement of annual financial performance goals of the Company as well as business line, department and individual performance. The plan supports the philosophy that management be measured for their performance as a team in the attainment of these goals. Discretionary bonus expense amounted to $1,313,000, $1,289,000 and $1,100,000 in 2013, 2012, and 2011, respectively.
The Company does not offer any postretirement programs other than pensions.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2013
Commitments And Contingencies Disclosure [Abstract]	'
Commitments and Contingencies	'
	18.     Commitments and Contingencies
	A number of legal claims against the Company arising in the normal course of business were outstanding at December 31, 2013. Management, after reviewing these claims with legal counsel, is of the opinion that their resolution will not have a material adverse effect on the Company’s consolidated financial position or results of operations.

Financial_Instruments_with_Off

Financial Instruments with Off-Balance-Sheet Risk	12 Months Ended
	Dec. 31, 2013
Risks And Uncertainties [Abstract]	'
Financial Instruments with Off-Balance-Sheet Risk	'
	19.     Financial Instruments with Off-Balance-Sheet Risk
	The Company is party to financial instruments with off-balance-sheet risk in the normal course of business to meet the financing needs of its customers.
	These financial instruments primarily include commitments to originate and sell loans, standby letters of credit, unused lines of credit and unadvanced portions of construction loans. The instruments involve, to varying degrees, elements of credit and interest rate risk in excess of the amount recognized in the consolidated balance sheet. The contract or notional amounts of those instruments reflect the extent of involvement the Company has in these particular classes of financial instruments.
	The Company’s exposure to credit loss in the event of nonperformance by the other party to the financial instrument for loan commitments, standby letters of credit and unadvanced portions of construction loans is represented by the contractual amount of those instruments. The Company uses the same credit policies in making commitments and conditional obligations as it does for on-balance-sheet instruments. Financial instruments with off-balance-sheet risk at December 31 are as follows:
	Contract or Notional Amount
		2013		2012
	(dollars in thousands)
	Financial instruments whose contract amount represents credit risk:
	Commitments to originate 1–4 family mortgages	$	3,373	$	13,580
	Standby and commercial letters of credit		7,930		8,411
	Unused lines of credit		249,941		217,246
	Unadvanced portions of construction loans		7,026		17,609
	Unadvanced portions of other loans		17,750		4,872
	Commitments to originate loans, unadvanced portions of construction loans, unused lines of credit and unused letters of credit are generally agreements to lend to a customer, provided there is no violation of any condition established in the contract. Commitments generally have fixed expiration dates or other termination clauses and may require payment of a fee. Since many of the commitments are expected to expire without being drawn upon, the total commitment amounts do not necessarily represent future cash requirements. The Company evaluates each customer’s creditworthiness on a case-by-case basis. The amount of collateral obtained, if deemed necessary by the Company upon extension of credit, is based on management’s credit evaluation of the borrower.
	Standby letters of credit are conditional commitments issued by the Company to guarantee the performance by a customer to a third party. The credit risk involved in issuing letters of credit is essentially the same as that involved in extending loan facilities to customers.

Other_Operating_Expenses

Other Operating Expenses	12 Months Ended
	Dec. 31, 2013
Other Income And Expenses [Abstract]	'
Other Operating Expenses	'
	20.     Other Operating Expenses
	Year ended December 31,	2013		2012		2011
	(dollars in thousands)
	Marketing	$	1,749	$	1,853	$	1,575
	Software maintenance/amortization		1,417		1,256		951
	Legal and audit		1,281		1,179		1,140
	Contributions		673		1,074		479
	Processing services		812		921		865
	Consulting		874		890		796
	Postage and delivery		939		877		773
	Supplies		848		849		868
	Telephone		719		750		742
	Directors’ fees		373		330		309
	Insurance		295		279		275
	Core deposit intangible amortization		—		120		388
	Other		1,500		1,230		1,280
	Total	$	11,480	$	11,608	$	10,441

Fair_Values_of_Financial_Instr

Fair Values of Financial Instruments	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Fair Values of Financial Instruments	'
	21.     Fair Values of Financial Instruments
	The following methods and assumptions were used by the Company in estimating fair values of its financial instruments. Excluded from this disclosure are all non financial instruments. Accordingly, the aggregate fair value amounts presented do not represent the underlying value of the Company.
	The assumptions used below are expected to approximate those that market participants would use in valuing these financial instruments.
	Fair value estimates are made at a specific point in time, based on available market information and judgments about the financial instrument, including estimates of timing, amount of expected future cash flows and the credit standing of the issuer. Such estimates do not consider the tax impact of the realization of unrealized gains or losses. In some cases, the fair value estimates cannot be substantiated by comparison to independent markets. In addition, the disclosed fair value may not be realized in the immediate settlement of the financial instrument. Care should be exercised in deriving conclusions about our business, its value or financial position based on the fair value information of financial instruments presented below.
	SECURITIES HELD-TO-MATURITY
	The fair values of these securities were based on quoted market prices, where available, as provided by third-party investment portfolio pricing vendors. If quoted market prices were not available, fair values provided by the vendors were based on quoted market prices of comparable instruments in active markets and/or based on a matrix pricing methodology which employs The Bond Market Association’s standard calculations for cash flow and price/yield analysis, live benchmark bond pricing and terms/condition data available from major pricing sources. Management regards the inputs and methods used by third party pricing vendors to be “Level 2 inputs and methods” as defined in the “fair value hierarchy” provided by FASB.
	LOANS
	For variable-rate loans, that reprice frequently and with no significant change in credit risk, fair values are based on carrying amounts. The fair value of other loans is estimated using discounted cash flow analysis, based on interest rates currently being offered for loans with similar terms to borrowers of similar credit quality. Incremental credit risk for nonperforming loans has been considered.
	TIME DEPOSITS
	The fair value of time deposits was estimated using a discounted cash flow approach that applies prevailing market interest rates for similar maturity instruments. The fair values of the Company’s time deposit liabilities do not take into consideration the value of the Company’s long-term relationships with depositors, which may have significant value.
	OTHER BORROWED FUNDS
	The fair value of other borrowed funds is based on the discounted value of contractual cash flows. The discount rate used is estimated based on the rates currently offered for other borrowed funds of similar remaining maturities.
	SUBORDINATED DEBENTURES
	The fair value of subordinated debentures is based on the discounted value of contractual cash flows. The discount rate used is estimated based on the rates currently offered for other subordinated debentures of similar remaining maturities.
	The following presents (in thousands) the carrying amount, estimated fair value, and placement in the fair value hierarchy of the Company’s financial instruments as of December 31, 2013 and December 31, 2012. This table excludes financial instruments for which the carrying amount approximates fair value. Financial assets for which the fair value approximates carrying value include cash and cash equivalents, short-term investments, FHLBB stock and accrued interest receivable. Financial liabilities for which the fair value approximates carrying value include non-maturity deposits, short-term borrowings and accrued interest payable.
			Carrying Amount		Estimated		Level 1 Inputs		Fair Value		Level 3 Inputs
					Fair Value				Measurements
									Level 2 Inputs
	(dollars in thousands)
	December 31, 2013
	Financial assets:
	Securities held-to-maturity		$	1,487,884	$	1,464,449	$	—	$	1,464,449	$	—
	Loans(1)			1,243,822		1,214,192		—		—		1,214,192
	Financial liabilities:
	Time deposits			382,224		386,742		—		386,742		—
	Other borrowed funds			255,144		254,736		—		254,736		—
	Subordinated debentures			36,083		39,503		—		—		39,503
	December 31, 2012
	Financial assets:
	Securities held-to-maturity		$	275,507	$	281,924	$	—	$	281,924	$	—
	Loans(1)			1,092,591		1,124,716		—		—		1,124,716
	Financial liabilities:
	Time deposits			419,983		424,253		—		424,253		—
	Other borrowed funds			195,144		205,481		—		205,481		—
	Subordinated debentures			36,083		43,423		—		—		43,423
	-1	Comprised of loans (including collateral dependent impaired loans), net of deferred loan costs and the allowance for loan losses.
	LIMITATIONS
	Fair value estimates are made at a specific point in time, based on relevant market information and information about the type of financial instrument. These estimates do not reflect any premium or discount that could result from offering for sale at one time the Bank’s entire holdings of a particular financial instrument. Because no active market exists for some of the Bank’s financial instruments, fair value estimates are based on judgments regarding future expected loss experience, cash flows, current economic conditions, risk characteristics and other factors. These estimates are subjective in nature and involve uncertainties and matters of significant judgment and therefore cannot be determined with precision. Changes in assumptions and changes in the loan, debt and interest rate markets could significantly affect the estimates. Further, the income tax ramifications related to the realization of the unrealized gains and losses can have a significant effect on the fair value estimates and have not been considered.

Quarterly_Results_of_Operation

Quarterly Results of Operations (Unaudited)	12 Months Ended
	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]	'
Quarterly Results of Operations (Unaudited)	'
	22.     Quarterly Results of Operations (unaudited)
	2013 Quarters	Fourth		Third		Second		First
	(in thousands, except share data)
	Interest income	$	20,947	$	20,549	$	19,132	$	19,137
	Interest expense		4,842		4,751		4,620		4,592
	Net interest income		16,105		15,798		14,512		14,545
	Provision for loan losses		460		750		750		750
	Net interest income after provision for loan losses		15,645		15,048		13,762		13,795
	Other operating income		4,186		4,774		5,221		4,434
	Operating expenses		14,690		13,995		13,662		13,465
	Income before income taxes		5,141		5,827		5,321		4,764
	Provision for income taxes		116		308		295		288
	Net income	$	5,025	$	5,519	$	5,026	$	4,476
	Share data:
	Average shares outstanding, basic
	Class A		3,580,404		3,578,400		3,574,379		3,569,546
	Class B		1,976,180		1,978,180		1,982,180		1,986,880
	Average shares outstanding, diluted
	Class A		5,557,419		5,558,031		5,557,354		5,557,365
	Class B		1,976,180		1,978,180		1,982,180		1,986,880
	Earnings per share, basic
	Class A	$	1.1	$	1.21	$	1.1	$	0.98
	Class B	$	0.55	$	0.6	$	0.55	$	0.49
	Earnings per share, diluted
	Class A	$	0.9	$	0.99	$	0.9	$	0.81
	Class B	$	0.55	$	0.6	$	0.55	$	0.49
	2012 Quarters	Fourth		Third		Second		First
	(in thousands, except share data)
	Interest income	$	19,738	$	22,079	$	20,312	$	19,365
	Interest expense		4,795		4,859		4,923		4,963
	Net interest income		14,943		17,220		15,389		14,402
	Provision for loan losses		900		1,250		900		1,100
	Net interest income after provision for loan losses		14,043		15,970		14,489		13,302
	Other operating income		4,153		4,105		3,988		3,619
	Operating expenses		13,279		13,708		13,451		12,800
	Income before income taxes		4,917		6,367		5,026		4,121
	Provision for income taxes		139		685		255		313
	Net income	$	4,778	$	5,682	$	4,771	$	3,808
	Share data:
	Average shares outstanding, basic
	Class A		3,564,145		3,559,125		3,556,474		3,550,993
	Class B		1,986,880		1,989,380		1,991,880		1,993,755
	Average shares outstanding, diluted
	Class A		5,552,121		5,549,810		5,548,830		5,545,711
	Class B		1,986,880		1,989,380		1,991,880		1,993,755
	Earnings per share, basic
	Class A	$	1.05	$	1.25	$	1.05	$	0.84
	Class B	$	0.52	$	0.62	$	0.52	$	0.42
	Earnings per share, diluted
	Class A	$	0.86	$	1.02	$	0.86	$	0.69
	Class B	$	0.52	$	0.62	$	0.52	$	0.42

Parent_Company_Financial_State

Parent Company Financial Statements	12 Months Ended
	Dec. 31, 2013
Condensed Financial Information Of Parent Company Only Disclosure [Abstract]	'
Parent Company Financial Statements	'
	23.     Parent Company Financial Statements
	The balance sheets of Century Bancorp, Inc. (“Parent Company”) as of December 31, 2013 and 2012 and the statements of income and cash flows for each of the years in the three-year period ended December 31, 2013, are presented below. The statements of changes in stockholders’ equity are identical to the consolidated statements of changes in stockholders’ equity and are therefore not presented here.
	BALANCE SHEETS
	December 31,	2013			2012
	(dollars in thousands)
	ASSETS:
	Cash	$	12,245		$	19,536
	Investment in subsidiary, at equity		193,783			193,499
	Other assets		6,634			3,145
	Total assets	$	212,662		$	216,180
	LIABILITIES AND STOCKHOLDERS’ EQUITY:
	Liabilities	$	107		$	107
	Subordinated debentures		36,083			36,083
	Stockholders’ equity		176,472			179,990
	Total liabilities and stockholders’ equity	$	212,662		$	216,180
	STATEMENTS OF INCOME
	Year Ended December 31,	2013			2012			2011
	(dollars in thousands)
	Income:
	Interest income from deposits in bank	$	28		$	33		$	100
	Other income		72			72			72
	Total income		100			105			172
	Interest expense		2,400			2,400			2,400
	Operating expenses		208			198			178
	Income before income taxes and equity in undistributed income of subsidiary		(2,508	)		(2,493	)		(2,406	)
	Benefit from income taxes		(853	)		(848	)		(818	)
	Income before equity in undistributed income of subsidiary		(1,655	)		(1,645	)		(1,588	)
	Equity in undistributed income of subsidiary		21,701			20,684			18,281
	Net income	$	20,046		$	19,039		$	16,693
	STATEMENTS OF CASH FLOWS
	December 31,	2013			2012			2011
	(dollars in thousands)
	CASH FLOWS FROM OPERATING ACTIVITIES:
	Net income	$	20,046		$	19,039		$	16,693
	Adjustments to reconcile net income to net cash provided by operating activities
	Undistributed income of subsidiary		(21,701	)		(20,684	)		(18,281	)
	Depreciation and amortization		12			12			12
	Increase in other assets		(3,500	)		(416	)		(182	)
	Net cash (used in) operating activities		(5,143	)		(2,049	)		(1,758	)
	CASH FLOWS FROM FINANCING ACTIVITIES:
	Net proceeds from the exercise of stock options		43			304			53
	Cash dividends paid		(2,191	)		(2,186	)		(2,180	)
	Net cash used in financing activities		(2,148	)		(1,882	)		(2,127	)
	Net (decrease) in cash		(7,291	)		(3,931	)		(3,885	)
	Cash at beginning of year		19,536			23,467			27,352
	Cash at end of year	$	12,245		$	19,536		$	23,467

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2013
Accounting Policies [Abstract]	'
Basis of Financial Statement Presentation	'
	BASIS OF FINANCIAL STATEMENT PRESENTATION
	The consolidated financial statements include the accounts of Century Bancorp, Inc. (the “Company”) and its wholly owned subsidiary, Century Bank and Trust Company (the “Bank”). The consolidated financial statements also include the accounts of the Bank’s wholly owned subsidiaries, Century Subsidiary Investments, Inc. (“CSII”), Century Subsidiary Investments, Inc. II (“CSII II”), Century Subsidiary Investments, Inc. III (“CSII III”) and Century Financial Services Inc. (“CFSI”). CSII, CSII II, and CSII III are engaged in buying, selling and holding investment securities. CFSI has the power to engage in financial agency, securities brokerage, and investment and financial advisory services and related securities credit. The Company also owns 100% of Century Bancorp Capital Trust II (“CBCT II”). The entity is an unconsolidated subsidiary of the Company.
	All significant intercompany accounts and transactions have been eliminated in consolidation. The Company provides a full range of banking services to individual, business and municipal customers in Massachusetts. As a bank holding company, the Company is subject to the regulation and supervision of the Federal Reserve Board. The Bank, a state chartered financial institution, is subject to supervision and regulation by applicable state and federal banking agencies, including the Federal Reserve Board, the Federal Deposit Insurance Corporation (the “FDIC”) and the Commonwealth of Massachusetts Commissioner of Banks. The Bank is also subject to various requirements and restrictions under federal and state law, including requirements to maintain reserves against deposits, restrictions on the types and amounts of loans that may be granted and the interest that may be charged thereon, and limitations on the types of investments that may be made and the types of services that may be offered. Various consumer laws and regulations also affect the operations of the Bank. In addition to the impact of regulation, commercial banks are affected significantly by the actions of the Federal Reserve Board as it attempts to control the money supply and credit availability in order to influence the economy. All aspects of the Company’s business are highly competitive. The Company faces aggressive competition from other lending institutions and from numerous other providers of financial services. The Company has one reportable operating segment.
	The financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America and general practices within the banking industry. In preparing the financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the balance sheet and revenues and expenses for the period. Actual results could differ from those estimates.
	Material estimates that are susceptible to change in the near term relate to the allowance for loan losses. Management believes that the allowance for loan losses is adequate based on independent appraisals and review of other factors, including historical charge-off rates with additional allocations based on risk factors for each category and general economic factors. While management uses available information to recognize loan losses, future additions to the allowance for loan losses may be necessary based on changes in economic conditions. In addition, regulatory agencies periodically review the Company’s allowance for loan losses. Such agencies may require the Company to recognize additions to the allowance for loan losses based on their judgments about information available to them at the time of their examination. Certain reclassifications are made to prior-year amounts whenever necessary to conform with the current-year presentation.
Fair Value Measurements	'
	FAIR VALUE MEASUREMENTS
	The Company follows FASB ASC 820-10, Fair Value Measurements and Disclosures, (formerly SFAS 157, “Fair Value Measurements,”) which among other things, requires enhanced disclosures about assets and liabilities carried at fair value. ASC 820-10 establishes a hierarchal disclosure framework associated with the level of pricing observability utilized in measuring financial instruments at fair value. The three broad levels of the hierarchy are as follows:
	Level I — Quoted prices are available in active markets for identical assets or liabilities as of the reported date. The type of financial instruments included in Level I are highly liquid cash instruments with quoted prices, such as G-7 government, agency securities, listed equities and money market securities, as well as listed derivative instruments.
	Level II — Pricing inputs are other than quoted prices in active markets, which are either directly or indirectly observable as of the reported date. The nature of these financial instruments includes cash instruments for which quoted prices are available but traded less frequently, derivative instruments whose fair value has been derived using a model where inputs to the model are directly observable in the market or can be derived principally from or corroborated by observable market data, and instruments that are fair valued using other financial instruments, the parameters of which can be directly observed. Instruments that are generally included in this category are corporate bonds and loans, mortgage whole loans, municipal bonds and over the counter (“OTC”) derivatives.
	Level III — These instruments have little to no pricing observability as of the reported date. These financial instruments do not have two-way markets and are measured using management’s best estimate of fair value, where the inputs into the determination of fair value require significant management judgment or estimation. Instruments that are included in this category generally include certain commercial mortgage loans, certain private equity investments, distressed debt, and noninvestment grade residual interests in securitizations as well as certain highly structured OTC derivative contracts.
Cash and Cash Equivalents	'
	CASH AND CASH EQUIVALENTS
	For purposes of reporting cash flows, cash equivalents include highly liquid assets with an original maturity of three months or less. Highly liquid assets include cash and due from banks, federal funds sold and certificates of deposit.
Short-Term Investments	'
	SHORT-TERM INVESTMENTS
	As of December 31, 2013 and 2012, short-term investments include highly liquid certificates of deposit with original maturities of more than 90 days but less than one year.
Investment Securities	'
	INVESTMENT SECURITIES
	Debt securities that the Company has the positive intent and ability to hold to maturity are classified as held-to-maturity and reported at amortized cost; debt and equity securities that are bought and held principally for the purpose of selling are classified as trading and reported at fair value, with unrealized gains and losses included in earnings; and debt and equity securities not classified as either held-to-maturity or trading are classified as available-for-sale and reported at fair value, with unrealized gains and losses excluded from earnings and reported as a separate component of stockholders’ equity, net of estimated related income taxes. The Company has no securities held for trading.
	Premiums and discounts on investment securities are amortized or accreted into income by use of the level-yield method. If a decline in fair value below the amortized cost basis of an investment is judged to be other-than-temporary, the cost basis of the investment is written down to fair value. The total amount of the impairment charge is recognized in earnings, with an offset for the noncredit component, which is recognized as other comprehensive income. Gains and losses on the sale of investment securities are recognized on the trade date on a specific identification basis.
	The transfer of a security between categories of investments shall be accounted for at fair value. For a debt security transferred into the held-to-maturity category from the available-for-sale category, the unrealized holding gain or loss at the date of the transfer shall continue to be reported in a separate component of shareholders’ equity but shall be amortized over the remaining life of the security as an adjustment of yield in a manner consistent with the amortization of any premium or discount. The amortization of an unrealized holding gain or loss reported in equity will offset or mitigate the effect on interest income of the amortization of the premium or discount for that held-to-maturity security.
Federal Home Loan Bank Stock	'
	FEDERAL HOME LOAN BANK STOCK
	The Bank, as a member of the Federal Home Loan Bank of Boston (“FHLBB”) system, is required to maintain an investment in capital stock of the FHLBB. Based on redemption provisions, the stock has no quoted market value and is carried at cost. At its discretion, the FHLBB may declare dividends on the stock. The Company reviews for impairment based on the ultimate recoverability of the cost basis of the stock. For the year ended December 31, 2013, the FHLBB reported preliminary net income of $212.3 million. The FHLBB also declared a dividend equal to an annual yield of 1.49%. As of December 31, 2013, no impairment has been recognized.
Loans Held for Sale	'
	LOANS HELD FOR SALE
	Loans originated and intended for sale in the secondary market are carried at the lower of cost or estimated fair value in the aggregate. Net unrealized losses, if any, are recognized through a valuation allowance by charges to income.
Loans	'
	LOANS
	Interest on loans is recognized based on the daily principal amount outstanding. Accrual of interest is discontinued when loans become ninety days delinquent unless the collateral is sufficient to cover both principal and interest and the loan is in the process of collection. Past-due status is based on contractual terms of the loan. Loans, including impaired loans, on which the accrual of interest has been discontinued, are designated nonaccrual loans. When a loan is placed on nonaccrual, all income that has been accrued but remains unpaid is reversed against current period income, and all amortization of deferred loan costs and fees is discontinued. Nonaccrual loans may be returned to an accrual status when principal and interest payments are not delinquent or the risk characteristics of the loan have improved to the extent that there no longer exists a concern as to the collectibility of principal and interest. Income received on nonaccrual loans is either recorded in income or applied to the principal balance of the loan, depending on management’s evaluation as to the collectibility of principal.
	Loan origination fees and related direct loan origination costs are offset, and the resulting net amount is deferred and amortized over the life of the related loans using the level-yield method. Prepayments are not initially considered when amortizing premiums and discounts.
	The Bank measures impairment for impaired loans at either the fair value of the loan, the present value of the expected future cash flows discounted at the loan’s effective interest rate or the fair value of the collateral if the loan is collateral dependent. This method applies to all loans, uncollateralized as well as collateralized, except large groups of smaller-balance homogeneous loans such as residential real estate and consumer loans that are collectively evaluated for impairment and loans that are measured at fair value. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off against the allowance for loan losses in lieu of an allocation of a specific allowance when such an amount has been identified definitively as uncollectible. Management considers the payment status, net worth and earnings potential of the borrower, and the value and cash flow of the collateral as factors to determine if a loan will be paid in accordance with its contractual terms. Management does not set any minimum delay of payments as a factor in reviewing for impaired classification. Loans are charged-off when management believes that the collectibility of the loan’s principal is not probable. The specific factors that management considers in making the determination that the collectibility of the loan’s principal is not probable include the delinquency status of the loan, the fair value of the collateral, if secured, and, the financial strength of the borrower and/or guarantors. In addition, criteria for classification of a loan as in-substance foreclosure has been modified so that such classification need be made only when a lender is in possession of the collateral. The Bank measures the impairment of troubled debt restructurings using the pre-modification rate of interest.
Transfers of Financial Assets	'
	TRANSFERS OF FINANCIAL ASSETS
	Transfers of financial assets, typically residential mortgages and loan participations for the Company, are accounted for as sales when control over the assets has been surrendered. Control over transferred assets is deemed to be surrendered when (1) the assets have been isolated from the Company, (2) the transferee obtains the right (free of conditions that constrain it from taking advantage of that right) to pledge or exchange the transferred assets, and (3) the Company does not maintain effective control over the transferred assets.
Acquired Loans	'
	ACQUIRED LOANS
	In accordance with FASB ASC 310-30, Loans and Debt Securities Acquired with Deteriorated Credit Quality (formerly Statement of Position (“SOP”) No. 03-3, “Accounting for Certain Loans or Debt Securities Acquired in a Transfer”) the Company reviews acquired loans for differences between contractual cash flows and cash flows expected to be collected from the Company’s initial investment in the acquired loans to determine if those differences are attributable, at least in part, to credit quality. If those differences are attributable to credit quality, the loan’s contractually required payments received in excess of the amount of its cash flows expected at acquisition, or nonaccretable discount, is not accreted into income. FASB ASC 310-30 requires that the Company recognize the excess of all cash flows expected at acquisition over the Company’s initial investment in the loan as interest income using the interest method over the term of the loan. This excess is referred to as accretable discount and is recorded as a reduction of the loan balance.
	Loans which, at acquisition, do not have evidence of deterioration of credit quality since origination are outside the scope of FASB ASC 310-30. For such loans, the discount, if any, representing the excess of the amount of reasonably estimable and probable discounted future cash collections over the purchase price, is accreted into interest income using the interest method over the term of the loan. Prepayments are not considered in the calculation of accretion income. Additionally, the discount is not accreted on nonperforming loans.
	When a loan is paid off, the excess of any cash received over the net investment is recorded as interest income. In addition to the amount of purchase discount that is recognized at that time, income may include interest owed by the borrower prior to the Company’s acquisition of the loan, interest collected if on nonperforming status, prepayment fees and other loan fees.
Nonperforming Assets	'
	NONPERFORMING ASSETS
	In addition to nonperforming loans, nonperforming assets include other real estate owned. Other real estate owned is comprised of properties acquired through foreclosure or acceptance of a deed in lieu of foreclosure. Other real estate owned is recorded initially at estimated fair value less costs to sell. When such assets are acquired, the excess of the loan balance over the estimated fair value of the asset is charged to the allowance for loan losses. An allowance for losses on other real estate owned is established by a charge to earnings when, upon periodic evaluation by management, further declines in the estimated fair value of properties have occurred. Such evaluations are based on an analysis of individual properties as well as a general assessment of current real estate market conditions. Holding costs and rental income on properties are included in current operations, while certain costs to improve such properties are capitalized. Gains and losses from the sale of other real estate owned are reflected in earnings when realized.
Allowance for Loan Losses	'
	ALLOWANCE FOR LOAN LOSSES
	The allowance for loan losses is based on management’s evaluation of the quality of the loan portfolio and is used to provide for losses resulting from loans that ultimately prove uncollectible. In determining the level of the allowance, periodic evaluations are made of the loan portfolio, which takes into account such factors as the character of the loans, loan status, financial posture of the borrowers, value of collateral securing the loans and other relevant information sufficient to reach an informed judgment. The allowance is increased by provisions charged to income and reduced by loan charge-offs, net of recoveries. Management maintains an allowance for loan losses to absorb losses inherent in the loan portfolio. The allowance is based on assessments of the probable estimated losses inherent in the loan portfolio. Management’s methodology for assessing the appropriateness of the allowance consists of several key elements, which include the formula allowance, specific allowances, if appropriate, for identified problem loans and the unallocated allowance. Arriving at an appropriate level of allowance for loan losses necessarily involves a high degree of judgment.
	While management uses available information in establishing the allowance for loan losses, future adjustments to the allowance may be necessary if economic conditions differ substantially from the assumptions used in making the evaluations. Loans are charged-off in whole or in part when, in management’s opinion, collectibility is not probable. The specific factors that management considers in making the determination that the collectibility of the loan’s principal is not probable include the delinquency status of the loan, the fair value of the collateral and the financial strength of the borrower and/or guarantors. For collateral dependent loans, the amount of the recorded investment in a loan that exceeds the fair value of the collateral is charged-off against the allowance for loan losses in lieu of an allocation of a specific allowance when such an amount has been identified definitively as uncollectible.
	The formula allowance evaluates groups of loans to determine the allocation appropriate within each portfolio segment. Individual loans within the commercial and industrial, commercial real estate and real estate construction loan portfolio segments are assigned internal risk ratings to group them with other loans possessing similar risk characteristics. Changes in risk grades affect the amount of the formula allowance. Risk grades are determined by reviewing current collateral value, financial information, cash flow, payment history and other relevant facts surrounding the particular credit. Provisions for losses on the remaining commercial and commercial real estate loans are based on pools of similar loans using a combination of historical net loss experience and qualitative adjustments. For the residential real estate and consumer loan portfolios, the reserves are calculated by applying historical charge-off and recovery experience and qualitative adjustments to the current outstanding balance in each loan category. Loss factors are based on the Company’s historical net loss experience as well as regulatory guidelines.
	Specific allowances for loan losses entail the assignment of allowance amounts to individual loans on the basis of loan impairment. Certain loans are evaluated individually and are judged to be impaired when management believes it is probable that the Company will not collect all the contractual interest and principal payments as scheduled in the loan agreement. Under this method, loans are selected for evaluation based upon a change in internal risk rating, occurrence of delinquency, loan classification or nonaccrual status. A specific allowance amount is allocated to an individual loan when such loan has been deemed impaired and when the amount of a probable loss is able to be estimated on the basis of: (a) present value of anticipated future cash flows, (b) the loan’s observable fair market price or (c) fair value of collateral if the loan is collateral dependent.
	The formula allowance and specific allowances also include management’s evaluation of various conditions, including business and economic conditions, delinquency trends, charge-off experience and other quality factors.
	An unallocated component is maintained to cover uncertainties that could affect management’s estimate of probable losses. The unallocated component of the allowance reflects the margin of imprecision inherent in the underlying assumptions used in the methodologies for estimating allocated and general reserves in the portfolio.
	Management has identified certain risk factors, which could impact the degree of loss sustained within the portfolio. These include: (a) market risk factors, such as the effects of economic variability on the entire portfolio and (b) unique portfolio risk factors that are inherent characteristics of the Company’s loan portfolio. Market risk factors may consist of changes to general economic and business conditions that may impact the Company’s loan portfolio customer base in terms of ability to repay and that may result in changes in value of underlying collateral. Unique portfolio risk factors may include industry concentrations and geographic concentrations or trends that may exacerbate losses resulting from economic events which the Company may not be able to fully diversify out of its portfolio.
	The qualitative factors are determined based on the various risk characteristics of each loan segment. Risk characteristics relevant to each portfolio segment are as follows:
	Residential real estate — The Company generally does not originate loans with a loan-to-value ratio greater than 80 percent. All loans in this segment are collateralized by owner-occupied residential real estate and repayment is dependent on the credit quality of the individual borrower. The overall health of the economy, including unemployment rates, will have an effect on the credit quality in the segment.
	Commercial real estate — Loans in this segment are primarily income-producing properties. Also included are loans to educational institutions, hospitals and other non-profit organizations. The underlying cash flows generated by the properties are adversely impacted by a downturn in the economy as evidenced by increased vacancy rates, which in turn, will have an effect on the credit quality in this segment. Management monitors the cash flows of these loans.
	Construction loans — Loans in this segment primarily include real estate development loans for which payment is derived from sale of the property as well as construction projects in which the property will ultimately be used by the borrower. Credit risk is affected by cost overruns, time to sell at an adequate price and market conditions.
	Commercial and industrial loans — Loans in this segment are made to businesses and are generally secured by assets of the business. Repayment is expected from the cash flows of the business. A weakened economy, and resultant decreased consumer spending, will have an effect on the credit quality in this segment.
Bank Premises and Equipment	'
	BANK PREMISES AND EQUIPMENT
	Bank premises and equipment are stated at cost less accumulated depreciation and amortization. Land is stated at cost. Depreciation is computed using the straight-line method over the estimated useful lives of the assets or the terms of leases, if shorter. It is general practice to charge the cost of maintenance and repairs to operations when incurred; major expenditures for improvements are capitalized and depreciated.
Goodwill and Identifiable Intangible Assets	'
	GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS
	Goodwill represents the excess of the cost of an acquisition over the fair value of the net assets acquired. Goodwill is not subject to amortization. Identifiable intangible assets consist of core deposit intangibles and are assets resulting from acquisitions that are being amortized over their estimated useful lives. Goodwill and identifiable intangible assets are included in other assets on the consolidated balance sheets. The Company tests goodwill for impairment on an annual basis, or more often if events or circumstances indicate there may be impairment. Goodwill impairment testing is performed at the segment (or “reporting unit”) level. Currently, the Company’s goodwill is evaluated at the entity level as there is only one reporting unit. Goodwill is assigned to reporting units at the date the goodwill is initially recorded. Once goodwill has been assigned to reporting units, it no longer retains its association with a particular acquisition, and all of the activities within a reporting unit, whether acquired or organically grown, are available to support the value of the goodwill.
	Goodwill impairment is evaluated by first assessing qualitative factors (events and circumstances) to determine whether it is more likely than not (meaning a likelihood of more than 50 percent) that the fair value of a reporting unit is less than its carrying amount. If, after considering all relevant events and circumstances, an entity determines it is not more likely than not that the fair value of a reporting unit is less than its carrying amount, then performing the two-step impairment test will be unnecessary.
	The first step, in the two-step impairment test, used to identify potential impairment, involves comparing each reporting unit’s fair value to its carrying value including goodwill. If the fair value of a reporting unit exceeds its carrying value, applicable goodwill is considered not to be impaired. If the carrying value exceeds fair value, there is an indication of impairment and the second step is performed to measure the amount of impairment.
Servicing	'
	SERVICING
	The Company services mortgage loans for others. Mortgage servicing assets are recognized as separate assets when rights are acquired through purchase or through sale of financial assets. Fair value is determined using prices for similar assets with similar characteristics, when available, or based upon discounted cash flows using market-based assumptions. The valuation model incorporates assumptions that market participants would use in estimating future net servicing income, such as the cost to service, the discount rate, an inflation rate, ancillary income, prepayment speeds and default rates and losses. Capitalized servicing rights are reported in other assets and are amortized into loan servicing fee income in proportion to, and over the period of, the estimated future net servicing income of the underlying financial assets. Servicing assets are evaluated for impairment based upon the fair value of the rights as compared to amortized cost. Impairment is determined by stratifying rights by predominant risk characteristics, such as interest rates and terms. Impairment is recognized through a valuation allowance for an individual stratum, to the extent that fair value is less than the capitalized amount for the stratum. Changes in the valuation allowance are reported in loan servicing fee income.
Stock Option Accounting	'
	STOCK OPTION ACCOUNTING
	The Company follows the fair value recognition provisions of FASB ASC 718, Compensation – Stock Compensation (formerly SFAS 123R) for all share-based payments, using the modified-prospective transition method. The Company’s method of valuation for share-based awards granted utilizes the Black-Scholes option-pricing model, which was also previously used for the Company’s pro forma information required under FASB ASC 718. The Company will recognize compensation expense for its awards on a straight-line basis over the requisite service period for the entire award (straight-line attribution method), ensuring that the amount of compensation cost recognized at any date at least equals the portion of the grant-date fair value of the award that is vested at that time.
	During 2000 and 2004, common stockholders of the Company approved stock option plans (the “Option Plans”) that provide for granting of options to purchase up to 150,000 shares of Class A common stock per plan. Under the Option Plans, all officers and key employees of the Company are eligible to receive nonqualified or incentive stock options to purchase shares of Class A common stock. The Option Plans are administered by the Compensation Committee of the Board of Directors, whose members are ineligible to participate in the Option Plans. Based on management’s recommendations, the Committee submits its recommendations to the Board of Directors as to persons to whom options are to be granted, the number of shares granted to each, the option price (which may not be less than 85% of the fair market value for nonqualified stock options, or the fair market value for incentive stock options, of the shares on the date of grant) and the time period over which the options are exercisable (not more than ten years from the date of grant). There were options to purchase an aggregate of 20,375 shares of Class A common stock exercisable at December 31, 2013.
	On December 30, 2005, the Board of Directors approved the acceleration and immediate vesting of all unvested options with an exercise price of $31.60 or greater per share. As a consequence, options to purchase 23,950 shares of Class A common stock became exercisable immediately. The average of the high and low price at which the Class A common stock traded on December 30, 2005, the date of the acceleration and vesting, was $29.28 per share. In connection with this acceleration, the Board of Directors approved a technical amendment to each of the Option Plans to eliminate the possibility that the terms of any outstanding or future stock option would require a cash settlement on the occurrence of any circumstance outside the control of the Company.
	The Company uses the fair value method to account for stock options. All of the Company’s stock options are vested, and there were no options granted during 2013 and 2012.
Income Taxes	'
	INCOME TAXES
	The Company uses the asset and liability method in accounting for income taxes. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which temporary differences are expected to be recovered or settled. Under this method, the effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date.
	The Company accounts for uncertain tax positions in accordance with FASB ASC 740.
	The Company classifies interest resulting from underpayment of income taxes as income tax expense in the first period the interest would begin accruing according to the provisions of the relevant tax law.
	The Company classifies penalties resulting from underpayment of income taxes as income tax expense in the period for which the Company claims or expects to claim an uncertain tax position or in the period in which the Company’s judgment changes regarding an uncertain tax position.
Treasury Stock	'
	TREASURY STOCK
	Effective July 1, 2004, companies incorporated in Massachusetts became subject to Chapter 156D of the Massachusetts Business Corporation Act, provisions of which eliminate the concept of treasury stock and provide that shares reacquired by a company are to be treated as authorized but unissued shares.
Pension	'
	PENSION
	The Company provides pension benefits to its employees under a noncontributory, defined benefit plan, which is funded on a current basis in compliance with the requirements of the Employee Retirement Income Security Act of 1974 (“ERISA”) and recognizes costs over the estimated employee service period.
	The Company also has a Supplemental Executive Insurance/Retirement Plan (“the Supplemental Plan”), which is limited to certain officers and employees of the Company. The Supplemental Plan is accrued on a current basis and recognizes costs over the estimated employee service period.
	Executive officers of the Company or its subsidiaries who have at least one year of service may participate in the Supplemental Plan. The Supplemental Plan is voluntary. Individual life insurance policies, which are owned by the Company, are purchased covering the life of each participant.
Revision of EPS Presentation	'
	REVISION OF EPS PRESENTATION
	The Company has determined that although the Class A and Class B common stock have different dividend rates, the Company had not applied the two-class method when calculating earnings per share (“EPS”) separately for the Class A and Class B common stock. This resulted in immaterial revisions to previously reported basic EPS for Class A and Class B common stock and diluted EPS for the Class B common stock as summarized below:
	For the year ended December 31, 2011:	As previously		As revised
		reported
	Basic EPS – Class A common	$	3.01	$	3.68
	Basic EPS – Class B common	$	3.01	$	1.84
	Diluted EPS – Class A common	$	3.01	$	3.01
	Diluted EPS – Class B common	$	3.01	$	1.84
Recent Accounting Developments	'
	RECENT ACCOUNTING DEVELOPMENTS
	In February 2013, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2013-01, Clarifying the Scope of Disclosures about Offsetting Assets and Liabilities. This ASU clarifies the scope of offsetting disclosure requirements in ASU 2011-11, Balance Sheet (Topic 210): Disclosures about Offsetting Assets and Liabilities. Under ASU 2013-01, the disclosure requirements would apply to derivative instruments accounted for in accordance with ASC 815, including bifurcated embedded derivatives, repurchase agreements and reverse repurchase agreements, and securities borrowing and securities lending arrangements that are either offset on the balance sheet or subject to an enforceable master netting arrangement or similar agreement. Entities with other types of financial assets and financial liabilities subject to a master netting arrangement or similar agreement also are affected because these amendments make them no longer subject to the disclosure requirements in ASU No. 2011-11. Effective January 1, 2013, companies are required to disclose (a) gross amounts of recognized assets and liabilities; (b) gross amounts offset in the statement of financial position; (c) net amounts of assets and liabilities presented in the statement of financial position; (d) gross amount subject to enforceable master netting agreement not offset in the statements of financial position; and (e) net amounts after deducting (d) from (c). The disclosure should be presented in tabular format (unless another format is more appropriate) separately for assets and liabilities. The intent of the new disclosure is to enable users of financial statements to understand the effect of those arrangements on its financial position and to allow investors to better compare financial statements prepared under GAAP with financial statements prepared under International Financial Reporting Standards. The Company implemented the provisions of ASU 2011-11 as of January 1, 2013. The adoption of this pronouncement did not have a material effect on the consolidated financial statements.
	In February 2013, the FASB issued ASU 2013-02, Comprehensive Income (Topic 220) Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income. ASU 2013-02 requires an entity to report the effect of significant reclassifications out of accumulated other comprehensive income on the respective line items in net income or as a separate disclosure in the notes to the financial statements. The new standard is effective for annual periods beginning January 1, 2013, and interim periods within those annual periods. The Company has presented a separate footnote (Note 13) as a result of this pronouncement.
	In July 2013, the FASB issued ASU 2013-10, Inclusion of the Fed Funds Effective Swap Rate (or Overnight Index Swap Rate) as a Benchmark Interest Rate for Hedge Accounting Purposes. This ASU amends ASC 815 to allow entities to use the Fed Funds Effective Swap Rate, in addition to U.S. Treasury rates and LIBOR, as a benchmark interest rate in accounting for fair value and cash flow hedges in the United States. This ASU also eliminates the provision from ASC 815-20-25-6 that prohibits the use of different benchmark rates for similar hedges except in rate and justifiable circumstances. This ASU is effective prospectively for qualifying new hedging relationship entered into on or after July 17, 2013, and for hedging relationship redesignated on or after that day. As of December 31, 2013, the Company did not have any fair value and cash flow hedges. The adoption of ASU No. 2013-10 did not have a material impact on the Company’s financial statements.
	In July 2013, the FASB issued ASU No. 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists. This ASU provides guidance on financial statement presentation of unrecognized tax benefits (“UTBs”) when a net operating loss (“NOL”) carryforward, a similar tax loss, or a tax credit carryforward exists. The FASB’s objective in issuing this ASU is to eliminate diversity in practice resulting from a lack of guidance on this topic in current U.S. GAAP. Under this ASU, an entity must present a UTB, or a portion of a UTB, in the financial statements as a reduction to a deferred tax asset (“DTA”) for an NOL carryforward, a similar tax loss, or a tax credit carryforward except when: (a) an NOL carryforward, a similar tax loss, or a tax credit carryforward is not available as of the reporting date under the governing tax law to settle taxes that would result from the disallowance of the tax position; (b) the entity does not intend to use the DTA for this purpose (provided that the tax law permits a choice). If either of these conditions exists, an entity should present a UTB in the financial statements as a liability and should not net the UTB with a DTA. New recurring disclosures are not required because the ASU does not affect the recognition or measurement of uncertain tax positions under ASC 740. This amendment does not affect the amounts public entities disclose in the tabular reconciliation of the total amounts of UTBs because the tabular reconciliation presents the gross amount of UTBs. This ASU is effective for fiscal years beginning after December 15, 2013, and interim periods within those years. The amendments should be applied to all UTBs that exist as of the effective date. Entities may choose to apply the amendments retrospectively to each prior reporting period presented. As of December 31, 2013, the Company did not have a UTB. Management will assess the applicability of this ASU after it becomes effective in the first quarter of 2014.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2013
Accounting Policies [Abstract]	'
Revision of EPS Presentation	'
	This resulted in immaterial revisions to previously reported basic EPS for Class A and Class B common stock and diluted EPS for the Class B common stock as summarized below:
	For the year ended December 31, 2011:	As previously		As revised
		reported
	Basic EPS – Class A common	$	3.01	$	3.68
	Basic EPS – Class B common	$	3.01	$	1.84
	Diluted EPS – Class A common	$	3.01	$	3.01
	Diluted EPS – Class B common	$	3.01	$	1.84

Securities_AvailableforSale_Ta

Securities Available-for-Sale (Tables)	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Summary of Securities Available-for-Sale	'
		December 31, 2013								December 31, 2012
		Amortized		Gross		Gross		Estimated		Amortized		Gross		Gross		Estimated
		Cost		Unrealized		Unrealized		Fair		Cost		Unrealized		Unrealized		Fair
				Gains		Losses		Value				Gains		Losses		Value
	(dollars in thousands)
	U.S. Treasury	$	1,997	$	1	$	—	$	1,998	$	2,000	$	4	$	—	$	2,004
	U.S. Government Sponsored Enterprises		9,995		9		—		10,004		130,048		360		68		130,340
	SBA Backed Securities		7,270		32		—		7,302		8,043		113		—		8,156
	U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities		404,103		588		1,501		403,190		1,212,953		20,816		412		1,233,357
	Privately Issued Residential Mortgage-Backed Securities		2,294		6		23		2,277		2,938		31		22		2,947
	Obligations Issued by States and Political Subdivisions		37,578		15		870		36,723		55,855		41		722		55,174
	Other Debt Securities		2,300		—		125		2,175		2,300		—		47		2,253
	Equity Securities		406		170		—		576		458		112		—		570
	Total	$	465,943	$	821	$	2,519	$	464,245	$	1,414,595	$	21,477	$	1,271	$	1,434,801
Estimated Maturity Distribution of Securities Available-for-Sale	'
	The following table shows the estimated maturity distribution of the Company’s securities available-for-sale at December 31, 2013.
		Amortized		Fair Value
		Cost
	(dollars in thousands)
	Within one year	$	36,942	$	36,964
	After one but within five years		180,676		180,540
	After five but within ten years		238,822		238,018
	More than ten years		7,597		6,771
	Nonmaturing		1,906		1,952
	Total	$	465,943	$	464,245
Continuous Unrealized Loss Position for 12 Months or Less and 12 Months or Longer	'
	The following table shows the temporarily impaired securities of the Company’s available-for-sale portfolio at December 31, 2013. This table shows the unrealized market loss of securities that have been in a continuous unrealized loss position for 12 months or less and a continuous loss position for 12 months and longer. There are 47 and 7 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 234 holdings at December 31, 2013.
		December 31, 2013
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair		Unrealized		Fair Value		Unrealized
				Losses		Value		Losses				Losses
	(dollars in thousands)
	U.S. Government Sponsored Enterprise	$	—	$	—	$	—	$	—	$	—	$	—
	U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities		289,709		1,352		24,557		149		314,266		1,501
	Privately Issued Residential Mortgage-Backed Securities		1,486		23		—		—		1,486		23
	Obligations Issued by States and Political Subdivisions		—		—		3,820		870		3,820		870
	Other Debt Securities		—		—		1,376		125		1,376		125
	Total temporarily impaired securities	$	291,195	$	1,375	$	29,753	$	1,144	$	320,948	$	2,519
	The following table shows the temporarily impaired securities of the Company’s available-for-sale portfolio at December 31, 2012. This table shows the unrealized market loss of securities that have been in a continuous unrealized loss position for 12 months or less and a continuous loss position for 12 months and longer. There are 20 and 7 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 458 holdings at December 31, 2012.
		December 31, 2012
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair		Unrealized		Fair Value		Unrealized
				Losses		Value		Losses				Losses
	(dollars in thousands)
	U.S. Government Sponsored Enterprise	$	34,967	$	68	$	—	$	—	$	34,967	$	68
	U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities		93,006		383		10,169		29		103,175		412
	Privately Issued Residential Mortgage-Backed Securities		—		—		1,863		22		1,863		22
	Obligations Issued by States and Political Subdivisions		—		—		3,963		722		3,963		722
	Other Debt Securities		—		—		1,453		47		1,453		47
	Total temporarily impaired securities	$	127,973	$	451	$	17,448	$	820	$	145,421	$	1,271

Investment_Securities_HeldtoMa1

Investment Securities Held-to-Maturity (Tables)	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Unrealized Gain (Losses) on Securities	'
		December 31, 2013								December 31, 2012
		Amortized		Gross		Gross		Estimated		Amortized		Gross		Gross		Estimated
		Cost		Unrealized		Unrealized		Fair Value		Cost		Unrealized		Unrealized		Fair
				Gains		Losses						Gains		Losses		Value
	(dollars in thousands)
	U.S. Government Sponsored Enterprise	$	291,779	$	185	$	5,043	$	286,921	$	17,747	$	19	$	8	$	17,758
	U.S. Government Sponsored Enterprise Mortgage-Backed Securities		1,196,105		2,239		20,816		1,177,528		257,760		6,480		74		264,166
	Total	$	1,487,884	$	2,424	$	25,859	$	1,464,449	$	275,507	$	6,499	$	82	$	281,924
Company's Securities Held-to-Maturity	'
	The following table shows the maturity distribution of the Company’s securities held-to-maturity at December 31, 2013.
		Amortized		Fair Value
		Cost
	(dollars in thousands)
	Within one year	$	5,689	$	5,672
	After one but within five years		827,333		818,936
	After five but within ten years		653,784		638,750
	More than ten years		1,078		1,091
	Total	$	1,487,884	$	1,464,449
Unrealized Market Loss of Securities	'
	The following table shows the temporarily impaired securities of the Company’s held-to-maturity portfolio at December 31, 2013. This table shows the unrealized market loss of securities that have been in a continuous unrealized loss position for 12 months or less and a continuous loss position for 12 months and longer. There are 191 and 13 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 300 holdings at December 31, 2013.
		December 31, 2013
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair		Unrealized		Fair Value		Unrealized
				Losses		Value		Losses				Losses
	(dollars in thousands)
	U.S. Government Sponsored Enterprises	$	232,535	$	5,043	$	—	$	—	$	232,535	$	5,043
	U.S. Government Agency and Sponsored
	Enterprise Mortgage-Backed Securities		931,180		18,654		80,362		2,162		1,011,542		20,816
	Total temporarily impaired securities	$	1,163,715	$	23,697	$	80,362	$	2,162	$	1,244,077	$	25,859
	The following table shows the temporarily impaired securities of the Company’s held-to-maturity portfolio at December 31, 2012. This table shows the unrealized market loss of securities that have been in a continuous unrealized loss position for 12 months or less and a continuous loss position for 12 months and longer. There are 3 and 1 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 96 holdings at December 31, 2012.
		December 31, 2012
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
		Value		Losses		Value		Losses		Value		Losses
	(dollars in thousands)
	U.S. Government Sponsored Enterprises	$	9,994	$	8	$	—	$	—	$	9,994	$	8
	U.S. Government Agency and Sponsored
	Enterprise Mortgage-Backed Securities		8,936		50		5,371		24		14,307		74
	Total temporarily impaired securities	$	18,930	$	58	$	5,371	$	24	$	24,301	$	82

Loans_Tables

Loans (Tables)	12 Months Ended
	Dec. 31, 2013
Debt Disclosure [Abstract]	'
Summary Shows the Composition of the Loan Portfolio	'
	The following summary shows the composition of the loan portfolio at the dates indicated.
	December 31,	2013		2012
	(dollars in thousands)
	Construction and land development	$	33,058	$	38,618
	Commercial and industrial		92,402		88,475
	Commercial real estate		713,327		576,465
	Residential real estate		286,041		281,857
	Consumer		8,824		6,823
	Home equity		130,277		118,923
	Overdrafts		834		627
	Total	$	1,264,763	$	1,111,788
Composition of Nonaccrual Loans and Impaired Loans	'
	The composition of nonaccrual loans and impaired loans is as follows:
	December 31,	2013		2012		2011
	(dollars in thousands)
	Loans on nonaccrual	$	2,549	$	4,471	$	5,827
	Loans 90 days past due and still accruing		—		—		18
	Impaired loans on nonaccrual included above		1,819		2,878		3,468
	Total recorded investment in impaired loans		7,788		5,925		8,102
	Average recorded investment of impaired loans		6,776		7,043		10,284
	Accruing troubled debt restructures		5,969		3,048		4,634
	Interest income not recorded on nonaccrual loans according to their original terms		711		753		846
	Interest income on nonaccrual loans actually recorded		—		—		—
	Interest income recognized on impaired loans		161		180		155
The Aggregate Amount of Loans to Directors and Officers of the Company and Their Associates	'
	The following table shows the aggregate amount of loans to directors and officers of the Company and their associates during 2013.
	Balance at	Additions		Repayments		Balance at
	December 31, 2012			and Deletions		December 31, 2013
	(dollars in thousands)
	$4,663	$	310	$	269	$	4,704

Allowance_for_Loan_Losses_Tabl

Allowance for Loan Losses (Tables)	12 Months Ended
	Dec. 31, 2013
Receivables [Abstract]	'
Summary of Changes in Allowance for Loan Losses	'
	An analysis of the allowance for loan losses for each of the three years ending December 31, 2013, 2012 and 2011 is as follows:
		2013			2012			2011
	(dollars in thousands)
	Allowance for loan losses, beginning of year	$	19,197		$	16,574		$	14,053
	Loans charged-off		(1,813	)		(2,301	)		(2,824	)
	Recoveries on loans previously charged-off		847			774			795
	Net charge-offs		(966	)		(1,527	)		(2,029	)
	Provision charged to expense		2,710			4,150			4,550
	Allowance for loan losses, end of year	$	20,941		$	19,197		$	16,574
Summary of Allowance for Loan Losses	'
	Further information pertaining to the allowance for loan losses at December 31, 2013 follows:
		Construction			Commercial			Commercial			Residential			Consumer			Home			Unallocated			Total
		and Land			and			Real Estate			Real						Equity
		Development			Industrial						Estate
	(dollars in thousands)
	Allowance for Loan Losses:
	Balance at December 31, 2012	$	3,041		$	3,118		$	9,065		$	1,994		$	333		$	886		$	760		$	19,197
	Charge-offs		(1,000	)		(234	)		—			—			(579	)		—			—			(1,813	)
	Recoveries		—			389			19			11			427			1			—			847
	EAProvision		133			(284	)		2,134			1			251			72			403			2,710
	Ending balance at December 31, 2013	$	2,174		$	2,989		$	11,218		$	2,006		$	432		$	959		$	1,163		$	20,941
	Amount of allowance for loan losses for loans deemed to be impaired	$	12		$	367		$	417		$	129		$	—		$	94		$	—		$	1,019
	Amount of allowance for loan losses for loans not deemed to be impaired	$	2,162		$	2,622		$	10,801		$	1,877		$	432		$	865		$	1,163		$	19,922
	Loans:
	Ending balance	$	33,058		$	92,402		$	713,327		$	286,041		$	9,658		$	130,277		$	—		$	1,264,763
	Loans deemed to be impaired	$	608		$	1,367		$	4,520		$	1,199		$	—		$	94		$	—		$	7,788
	Loans not deemed to be impaired	$	32,450		$	91,035		$	708,807		$	284,842		$	9,658		$	130,183		$	—		$	1,256,975
	Further information pertaining to the allowance for loan losses at December 31, 2012 follows:
		Construction			Commercial			Commercial			Residential			Consumer			Home			Unallocated			Total
		and Land			and			Real Estate			Real						Equity
		Development			Industrial						Estate
	(dollars in thousands)
	Allowance for Loan Losses:
	Balance at December 31, 2011	$	2,893		$	3,139		$	6,566		$	1,886		$	356		$	704		$	1,030		$	16,574
	Charge-offs		—			(1,253	)		—			(192	)		(697	)		(159	)		—			(2,301	)
	Recoveries		—			307			9			17			422			19			—			774
	Provision		148			925			2,490			283			252			322			(270	)		4,150
	Ending balance at December 31, 2012	$	3,041		$	3,118		$	9,065		$	1,994		$	333		$	886		$	760		$	19,197
	Amount of allowance for loan losses or loans deemed to be impaired	$	1,000		$	104		$	415		$	117		$	—		$	96		$	—		$	1,732
	Amount of allowance for loan losses for loans not deemed to be impaired	$	2,041		$	3,014		$	8,650		$	1,877		$	333		$	790		$	760		$	17,465
	Loans:
	Ending balance	$	38,618		$	88,475		$	576,465		$	281,857		$	7,450		$	118,923		$	—		$	1,111,788
	Loans deemed to be impaired	$	1,500		$	1,282		$	2,281		$	766		$	—		$	96		$	—		$	5,925
	Loans not deemed to be impaired	$	37,118		$	87,193		$	574,184		$	281,091		$	7,450		$	118,827		$	—		$	1,105,863
Loans by Risk Rating	'
	The following table presents the Company’s loans by risk rating at December 31, 2013.
		Construction			Commercial			Commercial
		and Land			and			Real Estate
		Development			Industrial
	(dollars in thousands)
	Grade:
	1-3 (Pass)	$	25,138		$	90,563		$	707,461
	4 (Monitor)		7,312			472			1,346
	5 (Substandard)		—			—			—
	6 (Doubtful)		—			—			—
	Impaired		608			1,367			4,520
	Total	$	33,058		$	92,402		$	713,327
	The following table presents the Company’s loans by risk rating at December 31, 2012.
		Construction			Commercial			Commercial
		and Land			and			Real Estate
		Development			Industrial
	(dollars in thousands)
	Grade:
	1-3 (Pass)	$	29,719		$	86,587		$	569,760
	4 (Monitor)		7,399			606			4,424
	5 (Substandard)		—			—			—
	6 (Doubtful)		—			—			—
	Impaired		1,500			1,282			2,281
	Total	$	38,618		$	88,475		$	576,465
Aging of Past-Due Loans	'
	Further information pertaining to the allowance for loan losses at December 31, 2013 follows:
		Accruing			Non			Accruing			Total			Current			Total
		30-89 Days			Accrual			Greater			Past Due			Loans
		Past Due						Than
								90 Days
	(dollars in thousands)
	Construction and land development	$	—		$	500		$	—		$	500		$	32,558		$	33,058
	Commercial and industrial		112			706			—			818			91,584			92,402
	Commercial real estate		1,496			306			—			1,802			711,525			713,327
	Residential real estate		2,232			1,034			—			3,266			282,775			286,041
	Consumer and overdrafts		11			3			—			14			9,644			9,658
	Home equity		1,710			—			—			1,710			128,567			130,277
	Total	$	5,561		$	2,549		$	—		$	8,110		$	1,256,653		$	1,264,763
	Further information pertaining to the allowance for loan losses at December 31, 2012 follows:
		Accruing			Non			Accruing			Total			Current			Total
		30-89 Days			Accrual			Greater			Past Due			Loans
		Past Due						Than
								90 Days
	(dollars in thousands)
	Construction and land development	$	—		$	1,500		$	—		$	1,500		$	37,118		$	38,618
	Commercial and industrial		1,256			676			—			1,932			86,543			88,475
	Commercial real estate		3,450			674			—			4,124			572,341			576,465
	Residential real estate		864			1,597			—			2,461			279,396			281,857
	Consumer and overdrafts		32			24			—			56			7,394			7,450
	Home equity		1,088			—			—			1,088			117,835			118,923
	Total	$	6,690		$	4,471		$	—		$	11,161		$	1,100,627		$	1,111,788
Information Pertaining to Impaired Loans	'
	The following is information pertaining to impaired loans at December 31, 2013:
		Carrying			Unpaid			Required			Average			Interest
		Value			Balance			Reserve			Carrying			Income
					Principal						Value
											Recognized
	(dollars in thousands)
	With no required reserve recorded:
	Construction and land development	$	500		$	3,292		$	—		$	—		$	—
	Commercial and industrial		238			268			—			361			1
	Commercial real estate		82			82			—			132			—
	Residential real estate		246			259			—			169			—
	Consumer		—			—			—			—			—
	Home equity		—			—			—			—			—
	Total	$	1,066		$	3,901		$	—		$	662		$	1
	With required reserve recorded:
	Construction and land development	$	108		$	108		$	12		$	1,371		$	1
	Commercial and industrial		1,129			1,371			367			902			37
	Commercial real estate		4,438			4,527			417			2,868			120
	Residential real estate		953			1,035			129			878			2
	Consumer		—			—			—			—			—
	Home equity		94			94			94			95			—
	Total	$	6,722		$	7,135		$	1,019		$	6,114		$	160
	Total
	Construction and land development	$	608		$	3,400		$	12		$	1,371		$	1
	Commercial and industrial		1,367			1,639			367			1,263			38
	Commercial real estate		4,520			4,609			417			3,000			120
	Residential real estate		1,199			1,294			129			1,047			2
	Consumer		—			—			—			—			—
	Home equity		94			94			94			95			—
	Total	$	7,788		$	11,036		$	1,019		$	6,776		$	161
	The following is information pertaining to impaired loans at December 31, 2012:
		Carrying			Unpaid			Required			Average			Interest
		Value			Balance			Reserve			Carrying			Income
					Principal						Value			Recognized
	(dollars in thousands)
	With no required reserve recorded:
	Construction and land development	$	—		$	—		$	—		$	346		$	—
	Commercial and industrial		503			994			—			425			1
	Commercial real estate		169			199			—			176			—
	Residential real estate		30			31			—			124			—
	Consumer		—			—			—			—			—
	Home equity		—			—			—			—			—
	Total	$	702		$	1,224		$	—		$	1,071		$	1
	With required reserve recorded:
	Construction and land development	$	1,500		$	3,292		$	1,000		$	1,154		$	—
	Commercial and industrial		779			995			104			1,317			40
	Commercial real estate		2,112			2,158			415			2,817			138
	Residential real estate		736			736			117			640			1
	Consumer		—			—			—			—			—
	Home equity		96			96			96			44			—
	Total	$	5,223		$	7,277		$	1,732		$	5,972		$	179
	Total
	Construction and land development	$	1,500		$	3,292		$	1,000		$	1,500		$	—
	Commercial and industrial		1,282			1,989			104			1,742			41
	Commercial real estate		2,281			2,357			415			2,993			138
	Residential real estate		766			767			117			764			1
	Consumer		—			—			—			—			—
	Home equity		96			96			96			44			—
	Total	$	5,925		$	8,501		$	1,732		$	7,043		$	180
Troubled Debt Restructurings	'
	The following is information pertaining to troubled debt restructurings occurring during the year ended December 31, 2013:
		Number			Pre-modification			Post-modification
		of			Outstanding			Outstanding
		Contracts			Recorded			Recorded
					Investment			Investment
	(dollars in thousands)
	Construction and land development		1		$	108		$	108
	Commercial and industrial		2			67			64
	Commercial real estate		3			2,376			2,356
	Residential real estate		1			285			162
	Total		7		$	2,836		$	2,690
	The following is information pertaining to troubled debt restructurings occurring during the year ended December 31, 2012:
		Number of			Pre-modification			Post-modification
		Contracts			Outstanding			Outstanding
					Recorded Investment			Recorded  Investment
	(dollars in thousands)
	Construction and land development		1		$	750		$	736
	Commercial and industrial		1			6			6
	Commercial real estate		1			98			96
	Total		3		$	854		$	838

Bank_Premises_and_Equipment_Ta

Bank Premises and Equipment (Tables)	12 Months Ended
	Dec. 31, 2013
Property Plant And Equipment [Abstract]	'
Schedule of Bank Premises and Equipment	'
	December 31,	2013			2012			Estimated Useful Life
	(dollars in thousands)
	Land	$	3,478		$	3,478			—
	Bank premises		19,235			18,353			30-39 years
	Furniture and equipment		31,965			31,319			3-10 years
	Leasehold improvements		10,010			9,930			30-39 years or lease term
			64,688			63,080
	Accumulated depreciation and amortization		(41,288	)		(39,181	)
	Total	$	23,400		$	23,899
Summary of Future Minimum Rental Commitments for Non-Cancelable Operating Leases	'
	Future minimum rental commitments for non-cancelable operating leases with initial or remaining terms of one year or more at December 31, 2013, were as follows:
		Year			Amount
		(dollars in thousands)
			2014		$	2,070
			2015			1,824
			2016			1,675
			2017			1,235
			2018			1,035
			Thereafter			2,816
					$	10,655

Goodwill_and_Identifiable_Inta1

Goodwill and Identifiable Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2013
Goodwill And Intangible Assets Disclosure [Abstract]	'
Carrying Amount of Goodwill and Intangibles	'
	The changes in goodwill and identifiable intangible assets for the years ended December 31, 2013 and 2012 are shown in the table below.
	Carrying Amount of Goodwill and Intangibles	Goodwill		Core Deposit			Mortgage			Total
				Intangibles			Servicing
							Rights
	(dollars in thousands)
	Balance at December 31, 2011	$	2,714	$	120		$	123		$	2,957
	Additions		—		—			48			48
	Amortization Expense		—		(120	)		(34	)		(154	)
	Balance at December 31, 2012	$	2,714	$	—			137		$	2,851
	Additions		—		—			653			653
	Amortization Expense		—		—			(87	)		(87	)
	Balance at December 31, 2013	$	2,714	$	—			703		$	3,417

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2013
Fair Value Disclosures [Abstract]	'
Financial Instruments Measured at Fair Value on a Recurring and Non-recurring Basis	'
	The results of the fair value hierarchy as of December 31, 2013, are as follows:
						Fair Value Measurements Using
			Carrying			Quoted Prices			Significant			Significant
			Value			in Active Markets			Observable			Other
						for Identical Assets			Inputs			Unobservable
						(Level 1)			(Level 2)			Inputs (Level 3)
	(dollars in thousands)
	Financial Instruments Measured at Fair Value on a Recurring Basis — Securities AFS
	U.S. Treasury		$	1,998		$	—		$	1,998		$	—
	U.S. Government Sponsored Enterprises			10,004			—			10,004			—
	SBA Backed Securities			7,302			—			7,302			—
	U.S. Government Agency and Sponsored Enterprises
	Mortgage-Backed Securities			403,189			—			403,189			—
	Privately Issued Residential Mortgage-Backed Securities			2,277			—			2,277			—
	Obligations Issued by States and Political Subdivisions			36,723			—			416			36,307
	Other Debt Securities			2,176			—			2,176			—
	Equity Securities			576			286			—			290
	Total		$	464,245		$	286		$	427,362		$	36,597
	Financial Instruments Measured at Fair Value on a Non-recurring Basis
	Impaired Loans		$	1,747		$	—		$	—		$	1,747
	The results of the fair value hierarchy as of December 31, 2012, are as follows:
			Fair Value Measurements Using
			Carrying			Quoted Prices			Significant			Significant
			Value			in Active Markets			Observable			Other Unobservable
						for Identical Assets			Inputs			Inputs
						(Level 1)			(Level 2)			(Level 3)
	(dollars in thousands)
	Financial Instruments Measured at Fair Value on a Recurring Basis — Securities AFS
	U.S. Treasury		$	2,004		$	—		$	2,004		$	—
	U.S. Government Sponsored Enterprises			130,340			—			130,340			—
	SBA Backed Securities			8,156			—			8,156			—
	U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities			1,233,357			—			1,233,357			—
	Privately Issued Residential Mortgage-Backed Securities			2,947			—			2,947			—
	Privately Issued Commercial Mortgage-Backed Securities			—			—			—			—
	Obligations Issued by States and Political Subdivisions			55,174			—			1,734			53,440
	Other Debt Securities			2,253			—			2,253			—
	Equity Securities			570			228			—			342
	Total		$	1,434,801		$	228		$	1,380,791		$	53,782
	Financial Instruments Measured at Fair Value on a Non-recurring Basis
	Impaired Loans		$	3,587		$	—		$	—		$	3,587
Assets Measured at Fair Value	'
	The following table presents additional information about assets measured at fair value on a recurring and nonrecurring basis for which the Company has utilized Level 3 inputs to determine fair value (dollars in thousands) at December 31, 2013. Management continues to monitor the assumptions used to value the assets listed below.
	Asset		Fair Value			Valuation Technique		Unobservable Input	Unobservable Input
									Value or Range
	Securities AFS(1)		$	36,597		Discounted cash flow		Discount rate	0%-1%(2)
	Impaired Loans			1,747		Appraisal of collateral(3)		Appraisal adjustments(4)	0%-25% discount
	-1	Municipal securities generally have maturities of one year or less and, therefore, the amortized cost equates to the fair value.
	-2	Weighted averages.
	-3	Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various Level 3 inputs which are not identifiable.
	-4	Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated expenses.
	The following table presents additional information about assets measured at fair value on a recurring and nonrecurring basis for which the Company has utilized Level 3 inputs to determine fair value (dollars in thousands) at December 31, 2012. Management continues to monitor the assumptions used to value the assets listed below.
	Asset		Fair Value			Valuation Technique		Unobservable Input	Unobservable Input
									Unobservable Input
									Value or Range
	Securities AFS(1)		$	53,782		Discounted cash flow		Discount rate	0%-1%(2)
	Impaired Loans			3,587		Appraisal of collateral(3)		Appraisal adjustments(4)	0%-25% discount
	-1	Municipal securities generally have maturities of one year or less and, therefore, the amortized cost equates to the fair value.
	(2)	Weighted averages.
	(3)	Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various Level 3 inputs which are not identifiable.
	(4)	Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated expenses.
Changes in Level 3 Securities	'
	The changes in Level 3 securities for the year ended December 31, 2013 are as shown in the table below:
			Auction Rate			Obligations			Equity			Total
			Securities			Issued by States			Securities
						and Political
						Subdivisions
	(dollars in thousands)
	Balance at December 31, 2012		$	3,963		$	49,477		$	342		$	53,782
	Purchases			—			50,012			—			50,012
	Maturities			—			(66,976	)		(52	)		(67,028	)
	Amortization			—			(26	)		—			(26	)
	Change in fair value			(143	)		—			—			(143	)
	Balance at December 31, 2013		$	3,820		$	32,487		$	290		$	36,597
	The changes in Level 3 securities for the year ended December 31, 2012 are as shown in the table below:
			Auction Rate			Obligations			Equity			Total
			Securities			Issued by States			Securities
						and Political
						Subdivisions
	(dollars in thousands)
	Balance at December 31, 2011		$	3,725		$	14,772		$	417		$	18,914
	Purchases			—			90,960			—			90,960
	Maturities			—			(56,214	)		(75	)		(56,289	)
	Amortization			—			(41	)		—			(41	)
	Change in fair value			238			—			—			238
	Balance at December 31, 2012		$	3,963		$	49,477		$	342		$	53,782

Deposits_Tables

Deposits (Tables)	12 Months Ended
	Dec. 31, 2013
Banking And Thrift [Abstract]	'
Summary of Remaining Maturities or Re-pricing of Time Deposits	'
	The following is a summary of remaining maturities or re-pricing of time deposits as of December 31,
		2013		Percent			2012		Percent
	(dollars in thousands)
	Within one year	$	248,758		65	%	$	299,456		71	%
	Over one year to two years		41,533		11	%		67,918		16	%
	Over two years to three years		48,357		13	%		19,834		5	%
	Over three years to five years		43,576		11	%		32,775		8	%
	Total	$	382,224		100	%	$	419,983		100	%

Securities_Sold_under_Agreemen1

Securities Sold under Agreements to Repurchase (Tables)	12 Months Ended
	Dec. 31, 2013
Banking And Thrift [Abstract]	'
Summary of Securities Sold under Agreements to Repurchase	'
	The following is a summary of securities sold under agreements to repurchase as of December 31,
		2013			2012			2011
	(dollars in thousands)
	Amount outstanding at December 31	$	214,440		$	191,390		$	143,320
	Weighted average rate at December 31		0.18	%		0.17	%		0.24	%
	Maximum amount outstanding at any month end	$	214,440		$	213,730		$	152,267
	Daily average balance outstanding during the year	$	203,888		$	174,624		$	129,137
	Weighted average rate during the year		0.18	%		0.21	%		0.29	%

Other_Borrowed_Funds_and_Subor1

Other Borrowed Funds and Subordinated Debentures (Tables)	12 Months Ended
	Dec. 31, 2013
Banking And Thrift [Abstract]	'
Summary of Other Borrowed Funds and Subordinated Debentures	'
	The following is a summary of other borrowed funds and subordinated debentures as of December 31,
		2013			2012			2011
	(dollars in thousands)
	Amount outstanding at December 31	$	291,227		$	231,227		$	280,226
	Weighted average rate at December 31		3.04	%		3.54	%		2.85	%
	Maximum amount outstanding at any month end	$	291,227		$	277,226		$	280,226
	Daily average balance outstanding during the year	$	231,032		$	217,542		$	202,209
	Weighted average rate during the year		3.73	%		3.82	%		3.85	%
Schedule of the Maturity Distribution of FHLBB Advances with the Weighted Average Interest Rates	'
	A schedule of the maturity distribution of FHLBB advances with the weighted average interest rates is as follows:
		2013						2012						2011
	December 31,	Amount			Weighted			Amount			Weighted			Amount		Weighted
					Average						Average					Average
					Rate						Rate					Rate
	(dollars in thousands)
	Within one year	$	53,000			0.4	%	$	46,000			1.86	%	$	81,500		0.42	%
	Over one year to two years		19,500			2.42	%		17,500			3.01	%		23,500		3.34	%
	Over two years to three years		55,000			3.07	%		19,500			2.42	%		17,500		3.01	%
	Over three years to five years		77,000			3.05	%		90,000			3.33	%		74,500		2.9	%
	Over five years		50,500			3.39	%		22,000			4.19	%		47,000		4.38	%
	Total	$	255,000			2.52	%	$	195,000			2.96	%	$	244,000		2.41	%

Reclassifications_Out_of_Accum1

Reclassifications Out of Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Reclassifications Out of Accumulated Other Comprehensive Income	'
			Amount Reclassified from Accumulated
			Other Comprehensive Income
	Details about Accumulated Other		Year ended			Year ended			Affected line item in the Statement
	Comprehensive Income Components		December 31, 2013(a)			December 31, 2012(a)			Where Net Income is Presented
	Unrealized gains and losses on available-for-sale securities		$	3,019		$	1,843		Net gains on sales of investments
				(1,179	)		(728	)	Provision for income taxes
			$	1,840		$	1,115		Net income
	Accretion of unrealized losses transferred		$	3,077		$	—		Securities held-to-maturity
				(1,191	)		—		Provision for income taxes
			$	1,886		$	—		Net Income
	Amortization of defined benefit pension items
	Prior-service costs		$	(10	)	$	(10	)	Salaries and employee benefits(b)
	Actuarial gains (losses)			(1,144	)		(1,071	)	Salaries and employee benefits(b)
	Total before tax			(1,154	)		(1,081	)	Income before taxes
	Tax (expense) or benefit			460			432		Provision for income taxes
	Net of tax		$	(694	)	$	(649	)	Net income
	Total reclassifications for the period		$	3,032		$	466		Net income
	(a)	Amounts in parentheses indicate debits to profit/loss.
	(b)	These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see employee benefits footnote (Note 17) for additional details).

Earnings_Per_Share_EPS_Tables

Earnings Per Share ("EPS") (Tables)	12 Months Ended
	Dec. 31, 2013
Earnings Per Share [Abstract]	'
Reconciliation of Basic EPS and Diluted EPS	'
	The following table is a reconciliation of basic EPS and diluted EPS:
	Year Ended December 31,	2013		2012		2011
	(in thousands except share and per share data)
	BASIC EPS COMPUTATION
	Numerator:
	Net income, Class A	$	15,698	$	14,877	$	13,023
	Net income, Class B		4,348		4,162		3,670
	Denominator:
	Weighted average shares outstanding, Class A		3,575,683		3,557,693		3,543,233
	Weighted average shares outstanding, Class B		1,980,855		1,990,474		1,997,411
	Basic EPS, Class A	$	4.39	$	4.18	$	3.68
	Basic EPS, Class B		2.19		2.09		1.84
	DILUTED EPS COMPUTATION
	Numerator:
	Net income, Class A	$	15,698	$	14,877	$	13,023
	Net income, Class B		4,348		4,162		3,670
	Total net income, for diluted EPS, Class A computation		20,046		19,039		16,693
	Denominator:
	Weighted average shares outstanding, basic, Class A		3,575,683		3,557,693		3,543,233
	Weighted average shares outstanding, Class B		1,980,855		1,990,474		1,997,411
	Dilutive effect of Class A stock options		1,155		1,024		1,150
	Weighted average shares outstanding diluted, Class A		5,557,693		5,549,191		5,541,794
	Weighted average shares outstanding, Class B		1,980,855		1,990,474		1,997,411
	Diluted EPS, Class A	$	3.61	$	3.43	$	3.01
	Diluted EPS, Class B		2.19		2.09		1.84

Stockholders_Equity_Dividends_

Stockholders' Equity Dividends (Tables)	12 Months Ended
	Dec. 31, 2013
Equity [Abstract]	'
Summary of Stock Option Activity Under Stock Option Plan	'
	Stock option activity under the plan is as follows:
		December 31, 2013						December 31, 2012						December 31, 2011
		Amount			Weighted			Amount			Weighted			Amount			Weighted
					Average						Average						Average
					Exercise Price						Exercise Price						Exercise Price
	Shares under option:
	Outstanding at beginning of year		23,350		$	31.17			36,062		$	28.9			38,712		$	28.36
	Forfeited		(1,350	)		26.68			(450	)		22.5			(200	)		15.06
	Exercised		(1,625	)		26.76			(12,262	)		24.82			(2,450	)		21.44
	Outstanding at end of year		20,375		$	31.82			23,350		$	31.17			36,062		$	28.9
	Exercisable at end of year		20,375		$	31.82			23,350		$	31.17			36,062		$	28.9
	Available to be granted at end of year		224,884						223,534						223,084
Summary of the Bank's Actual Capital Amounts and Ratios	'
	The Bank’s actual capital amounts and ratios are presented in the following table:
		Actual			Ratio			For  Capital						To Be Well
		Amount						Adequacy						Capitalized  Under
								Purposes						Prompt Corrective
														Action Provisions
								Amount			Ratio			Amount			Ratio
	As of December 31, 2013
	Total Capital (to Risk-Weighted Assets)	$	230,038			13.91	%	$	132,338			8	%	$	165,422			10	%
	Tier 1 Capital (to Risk-Weighted Assets)		209,360			12.66	%		66,169			4	%		99,253			6	%
	Tier 1 Capital (to 4th Qtr. Average Assets)		209,360			6	%		139,467			4	%		174,334			5	%
	As of December 31, 2012
	Total Capital (to Risk-Weighted Assets)	$	206,464			14.15	%	$	116,726			8	%	$	145,907			10	%
	Tier 1 Capital (to Risk-Weighted Assets)		188,226			12.9	%		58,363			4	%		87,544			6	%
	Tier 1 Capital (to 4th Qtr. Average Assets)		188,226			6.11	%		123,202			4	%		154,002			5	%
Summary of the Company's Actual Capital Amounts and Ratios	'
	The Company’s actual capital amounts and ratios are presented in the following table:
								For  Capital						To Be Well
								Adequacy						Capitalized  Under
								Purposes						Prompt Corrective
														Action Provisions
		Actual			Ratio			Amount			Ratio			Amount			Ratio
		Amount
	As of December 31, 2013
	Total Capital (to Risk-Weighted Assets)	$	247,761			14.92	%	$	132,870			8	%	$	166,088			10	%
	Tier 1 Capital (to Risk-Weighted Assets)		227,000			13.67	%		66,435			4	%		99,653			6	%
	Tier 1 Capital (to 4th Qtr. Average Assets)		227,000			6.5	%		139,769			4	%		174,711			5	%
	As of December 31, 2012
	Total Capital (to Risk-Weighted Assets)	$	227,945			15.59	%	$	116,976			8	%	$	146,220			10	%
	Tier 1 Capital (to Risk-Weighted Assets)		209,668			14.34	%		58,488			4	%		87,732			6	%
	Tier 1 Capital (to 4th Qtr. Average Assets)		209,668			6.8	%		123,377			4	%		154,221			5	%

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'
Summary of Current and Deferred Components of Income Tax Expense	'
	The current and deferred components of income tax expense for the years ended December 31 are as follows:
		2013			2012			2011
	(dollars in thousands)
	Current expense:
	Federal	$	3,520		$	3,181		$	2,198
	State		416			315			309
	Total current expense		3,936			3,496			2,507
	Deferred (benefit) expense:
	Federal		(2,564	)		(1,833	)		(961	)
	State		(365	)		(271	)		8
	Total deferred benefit		(2,929	)		(2,104	)		(953	)
	Provision for income taxes	$	1,007		$	1,392		$	1,554
Income Tax Accounts Included in Other Assets/Liabilities	'
	Income tax accounts included in other assets/liabilities at December 31 are as follows:
		2013			2012
	(dollars in thousands)
	Currently receivable	$	702		$	630
	Deferred income tax asset, net		30,857			14,551
	Total	$	31,559		$	15,181
Summary of Differences between Income Tax Expense at the Statutory Federal Income Tax Rate and Total Income Tax Expense	'
	Differences between income tax expense at the statutory federal income tax rate and total income tax expense are summarized as follows:
		2013			2012			2011
	(dollars in thousands)
	Federal income tax expense at statutory rates	$	7,158		$	6,946		$	6,204
	State income tax, net of federal income tax benefit		34			29			209
	Insurance income		(380	)		(396	)		(396	)
	Effect of tax-exempt interest		(5,348	)		(4,628	)		(3,801	)
	Net tax credit		(572	)		(633	)		(683	)
	Other		115			74			21
	Total	$	1,007		$	1,392		$	1,554
	Effective tax rate		4.8	%		6.8	%		8.5	%
Gross Deferred Income Tax Assets and Gross Deferred Income Tax Liabilities	'
	The following table sets forth the Company’s gross deferred income tax assets and gross deferred income tax liabilities at December 31:
		2013			2012
	(dollars in thousands)
	Deferred income tax assets:
	Allowance for loan losses	$	9,199		$	8,103
	Unrealized losses on securities transferred to held-to-maturity		8,590			—
	Deferred compensation		6,515			5,643
	Pension and SERP liability		4,880			8,621
	AMT credit		3,164			1,908
	Unrealized losses (gains) on securities available-for-sale		652			(7,875	)
	Acquisition premium		438			541
	Nonaccrual interest		136			151
	Depreciation		109			(36	)
	Investments write down		26			26
	Deferred gain		—			11
	Other		198			235
	Gross deferred income tax asset		33,907			17,328
	Deferred income tax liabilities:
	Limited partnerships		(2,769	)		(2,722	)
	Mortgage servicing rights		(281	)		(55	)
	Gross deferred income tax liability		(3,050	)		(2,777	)
	Deferred income tax asset net	$	30,857		$	14,551

Employee_Benefits_Tables

Employee Benefits (Tables)	12 Months Ended
	Dec. 31, 2013
Compensation And Retirement Disclosure [Abstract]	'
Fair Value of Plan Assets and Major Categories	'
	The fair value of plan assets and major categories as of December 31, 2013, is as follows:
	Asset Category	Percent			Total			Level 1			Level 2			Level 3
	(dollars in thousands)
	Collective funds		52.9	%	$	17,092		$	856		$	16,236		$	—
	Equity securities		25.9	%		8,355			8,355			—			—
	Mutual funds		13.1	%		4,250			3,832			418			—
	Hedge funds		7	%		2,256			—			—			2,256
	Short-term investments		1.1	%		369			249			120			—
			100	%	$	32,322		$	13,292		$	16,774		$	2,256
	The fair value of plan assets and major categories as of December 31, 2012, is as follows:
	Asset Category	Percent			Total			Level 1			Level 2			Level 3
	(dollars in thousands)
	Collective funds		52.5	%	$	12,593		$	767		$	11,826		$	—
	Equity securities		24.6	%		5,892			5,892			—			—
	Mutual funds		14.1	%		3,370			3,086			284			—
	Hedge funds		7	%		1,691			—			—			1,691
	Short-term investments		1.8	%		437			62			375			—
			100	%	$	23,983		$	9,807		$	12,485		$	1,691
Changes in Level 3 Securities	'
	The changes in Level 3 securities are shown in the table below:
	Year Ended December 31,	2013			2012
	(dollars in thousands)
	Balance at beginning of year	$	1,691		$	1,522
	Purchases		55			152
	Actual return – assets still being held		510			17
	Balance at end of year	$	2,256		$	1,691
Components of Net Periodic Benefit Cost	'
		Defined Benefit						Supplemental Insurance/
		Pension  Plan						Retirement Plan
		2013			2012			2013			2012
	(dollars in thousands)
	Change projected in benefit obligation
	Benefit obligation at beginning of year	$	31,910		$	28,784		$	25,835		$	21,097
	Service cost		1,196			1,097			1,554			1,425
	Interest cost		1,257			1,295			1,072			923
	Actuarial (gain)/loss		(3,734	)		1,401			(1,916	)		3,482
	Benefits paid		(750	)		(667	)		(1,043	)		(1,092	)
	Projected benefit obligation at end of year	$	29,879		$	31,910		$	25,502		$	25,835
	Change in plan assets
	Fair value of plan assets at beginning of year	$	23,983		$	20,517
	Actual (loss) return on plan assets		4,619			2,333
	Employer contributions		4,470			1,800
	Benefits paid		(750	)		(667	)
	Fair value of plan assets at end of year	$	32,322		$	23,983
	(Unfunded) Funded status	$	2,443		$	(7,927	)	$	(25,502	)	$	(25,835	)
	Accumulated benefit obligation	$	29,747		$	31,773		$	22,278		$	22,181
	Weighted-average assumptions as of December 31
	Discount rate — Liability		5	%		4	%		5	%		4	%
	Discount rate — Expense		4	%		4.5	%		4	%		4.5	%
	Expected return on plan assets		8	%		8	%		NA			NA
	Rate of compensation increase		4	%		4	%		4	%		4	%
	Components of net periodic benefit cost
	Service cost	$	1,196		$	1,097		$	1,554		$	1,425
	Interest cost		1,257			1,295			1,072			923
	Expected return on plan assets		(1,880	)		(1,641	)		—			—
	Recognized prior service cost		(104	)		(104	)		114			114
	Recognized net losses		630			735			514			336
	Net periodic cost	$	1,099		$	1,382		$	3,254		$	2,798
	Other changes in plan assets and benefit obligations recognized in other comprehensive income
	Amortization of prior service cost	$	104		$	104		$	(114	)	$	(114	)
	Net (gain) loss		(6,956	)		(25	)		(2,401	)		3,098
	Total recognized in other comprehensive income		(6,852	)		79			(2,515	)		2,984
	Total recognized in net periodic benefit cost and other comprehensive income	$	(5,753	)	$	1,461		$	739		$	5,782
Summary of Defined Pension Plan and Supplemental Insurance Retirement Plan	'
		December 31, 2013									December 31, 2012
		Plan			Supplemental			Total			Plan			Supplemental			Total
					Plan									Plan
	(dollars in thousands)
	Prior service cost	$	516		$	(991	)	$	(475	)	$	620		$	(1,105	)	$	(485	)
	Net actuarial loss		(3,361	)		(7,901	)		(11,262	)		(10,317	)		(10,302	)		(20,619	)
	Total	$	(2,845	)	$	(8,892	)	$	(11,737	)	$	(9,697	)	$	(11,407	)	$	(21,104	)
Summary of Accumulated Other Comprehensive Loss Expected to be Recognized	'
	The following table summarizes the amounts included in Accumulated Other Comprehensive Loss at December 31, 2013, expected to be recognized as components of net periodic benefit cost in the next year:
		Plan			Supplemental
					Plan
	Amortization of prior service cost to be recognized in 2014	$	(104	)	$	114
	Amortization of loss to be recognized in 2014		12			355

Financial_Instruments_with_Off1

Financial Instruments with Off-Balance-Sheet Risk (Tables)	12 Months Ended
	Dec. 31, 2013
Risks And Uncertainties [Abstract]	'
Summary of Financial Instruments with Off-Balance-Sheet Risk	'
	Financial instruments with off-balance-sheet risk at December 31 are as follows:
	Contract or Notional Amount
		2013		2012
	(dollars in thousands)
	Financial instruments whose contract amount represents credit risk:
	Commitments to originate 1–4 family mortgages	$	3,373	$	13,580
	Standby and commercial letters of credit		7,930		8,411
	Unused lines of credit		249,941		217,246
	Unadvanced portions of construction loans		7,026		17,609
	Unadvanced portions of other loans		17,750		4,872

Other_Operating_Expenses_Table

Other Operating Expenses (Tables)	12 Months Ended
	Dec. 31, 2013
Other Income And Expenses [Abstract]	'
Summary of Other Operating Expenses	'
	Year ended December 31,	2013		2012		2011
	(dollars in thousands)
	Marketing	$	1,749	$	1,853	$	1,575
	Software maintenance/amortization		1,417		1,256		951
	Legal and audit		1,281		1,179		1,140
	Contributions		673		1,074		479
	Processing services		812		921		865
	Consulting		874		890		796
	Postage and delivery		939		877		773
	Supplies		848		849		868
	Telephone		719		750		742
	Directors’ fees		373		330		309
	Insurance		295		279		275
	Core deposit intangible amortization		—		120		388
	Other		1,500		1,230		1,280
	Total	$	11,480	$	11,608	$	10,441

Fair_Values_of_Financial_Instr1

Fair Values of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2013
Text Block [Abstract]	'
Carrying Amounts and Fair Values of Company's Financial Instruments	'
	The following presents (in thousands) the carrying amount, estimated fair value, and placement in the fair value hierarchy of the Company’s financial instruments as of December 31, 2013 and December 31, 2012. This table excludes financial instruments for which the carrying amount approximates fair value. Financial assets for which the fair value approximates carrying value include cash and cash equivalents, short-term investments, FHLBB stock and accrued interest receivable. Financial liabilities for which the fair value approximates carrying value include non-maturity deposits, short-term borrowings and accrued interest payable.
			Carrying Amount		Estimated		Level 1 Inputs		Fair Value		Level 3 Inputs
					Fair Value				Measurements
									Level 2 Inputs
	(dollars in thousands)
	December 31, 2013
	Financial assets:
	Securities held-to-maturity		$	1,487,884	$	1,464,449	$	—	$	1,464,449	$	—
	Loans(1)			1,243,822		1,214,192		—		—		1,214,192
	Financial liabilities:
	Time deposits			382,224		386,742		—		386,742		—
	Other borrowed funds			255,144		254,736		—		254,736		—
	Subordinated debentures			36,083		39,503		—		—		39,503
	December 31, 2012
	Financial assets:
	Securities held-to-maturity		$	275,507	$	281,924	$	—	$	281,924	$	—
	Loans(1)			1,092,591		1,124,716		—		—		1,124,716
	Financial liabilities:
	Time deposits			419,983		424,253		—		424,253		—
	Other borrowed funds			195,144		205,481		—		205,481		—
	Subordinated debentures			36,083		43,423		—		—		43,423
	-1	Comprised of loans (including collateral dependent impaired loans), net of deferred loan costs and the allowance for loan losses.

Quarterly_Results_of_Operation1

Quarterly Results of Operations (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]	'
Quarterly Results of Operations (Unaudited)	'
	2013 Quarters	Fourth		Third		Second		First
	(in thousands, except share data)
	Interest income	$	20,947	$	20,549	$	19,132	$	19,137
	Interest expense		4,842		4,751		4,620		4,592
	Net interest income		16,105		15,798		14,512		14,545
	Provision for loan losses		460		750		750		750
	Net interest income after provision for loan losses		15,645		15,048		13,762		13,795
	Other operating income		4,186		4,774		5,221		4,434
	Operating expenses		14,690		13,995		13,662		13,465
	Income before income taxes		5,141		5,827		5,321		4,764
	Provision for income taxes		116		308		295		288
	Net income	$	5,025	$	5,519	$	5,026	$	4,476
	Share data:
	Average shares outstanding, basic
	Class A		3,580,404		3,578,400		3,574,379		3,569,546
	Class B		1,976,180		1,978,180		1,982,180		1,986,880
	Average shares outstanding, diluted
	Class A		5,557,419		5,558,031		5,557,354		5,557,365
	Class B		1,976,180		1,978,180		1,982,180		1,986,880
	Earnings per share, basic
	Class A	$	1.1	$	1.21	$	1.1	$	0.98
	Class B	$	0.55	$	0.6	$	0.55	$	0.49
	Earnings per share, diluted
	Class A	$	0.9	$	0.99	$	0.9	$	0.81
	Class B	$	0.55	$	0.6	$	0.55	$	0.49
	2012 Quarters	Fourth		Third		Second		First
	(in thousands, except share data)
	Interest income	$	19,738	$	22,079	$	20,312	$	19,365
	Interest expense		4,795		4,859		4,923		4,963
	Net interest income		14,943		17,220		15,389		14,402
	Provision for loan losses		900		1,250		900		1,100
	Net interest income after provision for loan losses		14,043		15,970		14,489		13,302
	Other operating income		4,153		4,105		3,988		3,619
	Operating expenses		13,279		13,708		13,451		12,800
	Income before income taxes		4,917		6,367		5,026		4,121
	Provision for income taxes		139		685		255		313
	Net income	$	4,778	$	5,682	$	4,771	$	3,808
	Share data:
	Average shares outstanding, basic
	Class A		3,564,145		3,559,125		3,556,474		3,550,993
	Class B		1,986,880		1,989,380		1,991,880		1,993,755
	Average shares outstanding, diluted
	Class A		5,552,121		5,549,810		5,548,830		5,545,711
	Class B		1,986,880		1,989,380		1,991,880		1,993,755
	Earnings per share, basic
	Class A	$	1.05	$	1.25	$	1.05	$	0.84
	Class B	$	0.52	$	0.62	$	0.52	$	0.42
	Earnings per share, diluted
	Class A	$	0.86	$	1.02	$	0.86	$	0.69
	Class B	$	0.52	$	0.62	$	0.52	$	0.42

Parent_Company_Financial_State1

Parent Company Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2013
Condensed Financial Information Of Parent Company Only Disclosure [Abstract]	'
Balance Sheets of Parent Company	'
	BALANCE SHEETS
	December 31,	2013			2012
	(dollars in thousands)
	ASSETS:
	Cash	$	12,245		$	19,536
	Investment in subsidiary, at equity		193,783			193,499
	Other assets		6,634			3,145
	Total assets	$	212,662		$	216,180
	LIABILITIES AND STOCKHOLDERS’ EQUITY:
	Liabilities	$	107		$	107
	Subordinated debentures		36,083			36,083
	Stockholders’ equity		176,472			179,990
	Total liabilities and stockholders’ equity	$	212,662		$	216,180
Statements of Income of Parent Company	'
	STATEMENTS OF INCOME
	Year Ended December 31,	2013			2012			2011
	(dollars in thousands)
	Income:
	Interest income from deposits in bank	$	28		$	33		$	100
	Other income		72			72			72
	Total income		100			105			172
	Interest expense		2,400			2,400			2,400
	Operating expenses		208			198			178
	Income before income taxes and equity in undistributed income of subsidiary		(2,508	)		(2,493	)		(2,406	)
	Benefit from income taxes		(853	)		(848	)		(818	)
	Income before equity in undistributed income of subsidiary		(1,655	)		(1,645	)		(1,588	)
	Equity in undistributed income of subsidiary		21,701			20,684			18,281
	Net income	$	20,046		$	19,039		$	16,693
Statements of Cash Flows of Parent Company	'
	STATEMENTS OF CASH FLOWS
	December 31,	2013			2012			2011
	(dollars in thousands)
	CASH FLOWS FROM OPERATING ACTIVITIES:
	Net income	$	20,046		$	19,039		$	16,693
	Adjustments to reconcile net income to net cash provided by operating activities
	Undistributed income of subsidiary		(21,701	)		(20,684	)		(18,281	)
	Depreciation and amortization		12			12			12
	Increase in other assets		(3,500	)		(416	)		(182	)
	Net cash (used in) operating activities		(5,143	)		(2,049	)		(1,758	)
	CASH FLOWS FROM FINANCING ACTIVITIES:
	Net proceeds from the exercise of stock options		43			304			53
	Cash dividends paid		(2,191	)		(2,186	)		(2,180	)
	Net cash used in financing activities		(2,148	)		(1,882	)		(2,127	)
	Net (decrease) in cash		(7,291	)		(3,931	)		(3,885	)
	Cash at beginning of year		19,536			23,467			27,352
	Cash at end of year	$	12,245		$	19,536		$	23,467

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)

0 Months Ended

3 Months Ended

12 Months Ended

Dec. 30, 2005

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Segment

Summary Of Significant Accounting Policies And Other Information [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

Equity ownership interest

'

100.00%

'

'

'

'

'

'

'

100.00%

'

'

Number of reportable segments

'

'

'

'

'

'

'

'

'

1

'

'

Securities held for trading

'

$0

'

'

'

'

'

'

'

$0

'

'

Net income

'

5,025,000

5,519,000

5,026,000

4,476,000

4,778,000

5,682,000

4,771,000

3,808,000

20,046,000

19,039,000

16,693,000

Impairment recognized

'

'

'

'

'

'

'

'

'

0

'

'

Loans discontinued delinquency period

'

'

'

'

'

'

'

'

'

'90 days

'

'

Loan to value ratio

'

'

'

'

'

'

'

'

'

80.00%

'

'

Nonqualified stock options

'

'

'

'

'

'

'

'

'

85.00%

'

'

Options to purchase an aggregate of shares

'

20,375

'

'

'

23,350

'

'

'

20,375

23,350

36,062

Time period over which stock option exercisable

$31.60

'

'

'

'

'

'

'

'

'

'

'

Average price of common stock

$29.28

'

'

'

'

'

'

'

'

'

'

'

Stock option granted

'

'

'

'

'

'

'

'

'

0

0

'

Maximum [Member]

'

'

'

'

'

'

'

'

'

'

'

'

Summary Of Significant Accounting Policies And Other Information [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

Time period over which stock option exercisable

'

'

'

'

'

'

'

'

'

'10 years

'

'

Minimum [Member]

'

'

'

'

'

'

'

'

'

'

'

'

Summary Of Significant Accounting Policies And Other Information [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

Supplemental plan

'

'

'

'

'

'

'

'

'

'1 year

'

'

Federal Home Loan Bank of Boston [Member]

'

'

'

'

'

'

'

'

'

'

'

'

Summary Of Significant Accounting Policies And Other Information [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

Net income

'

'

'

'

'

'

'

'

'

212,300,000

'

'

Declared a dividend equal to an annual yield

'

'

'

'

'

'

'

'

'

1.49%

'

'

Class A Common Stock [Member]

'

'

'

'

'

'

'

'

'

'

'

'

Summary Of Significant Accounting Policies And Other Information [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

Net income

'

'

'

'

'

'

'

'

'

$15,698,000

$14,877,000

$13,023,000

Approved stock option plan for purchase of common stock

'

150,000

'

'

'

'

'

'

'

150,000

'

'

Options to purchase an aggregate of shares

23,950

20,375

'

'

'

'

'

'

'

20,375

'

'

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Revision of EPS Presentation (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Class A Common Stock [Member]	'	'	'	'	'	'	'	'	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Earnings per share, basic	$1.10	$1.21	$1.10	$0.98	$1.05	$1.25	$1.05	$0.84	$4.39	$4.18	$3.68
Earnings per share, diluted	$0.90	$0.99	$0.90	$0.81	$0.86	$1.02	$0.86	$0.69	$3.61	$3.43	$3.01
Class A Common Stock [Member] | As Previously Reported [Member]	'	'	'	'	'	'	'	'	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Earnings per share, basic	'	'	'	'	'	'	'	'	'	'	$3.01
Earnings per share, diluted	'	'	'	'	'	'	'	'	'	'	$3.01
Class B Common Stock [Member]	'	'	'	'	'	'	'	'	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Earnings per share, basic	$0.55	$0.60	$0.55	$0.49	$0.52	$0.62	$0.52	$0.42	$2.19	$2.09	$1.84
Earnings per share, diluted	$0.55	$0.60	$0.55	$0.49	$0.52	$0.62	$0.52	$0.42	$2.19	$2.09	$1.84
Class B Common Stock [Member] | As Previously Reported [Member]	'	'	'	'	'	'	'	'	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Earnings per share, basic	'	'	'	'	'	'	'	'	'	'	$3.01
Earnings per share, diluted	'	'	'	'	'	'	'	'	'	'	$3.01

Cash_and_Due_from_Banks_Additi

Cash and Due from Banks - Additional Information (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Cash And Due From Banks [Abstract]	'	'
Reserve balance of cash and due from banks	$0	$9,608,000

Securities_AvailableforSale_Su

Securities Available-for-Sale - Summary of Securities Available-for-Sale (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized cost	$465,943	$1,414,595
Gross Unrealized Gains	821	21,477
Gross Unrealized Losses	2,519	1,271
Total, Fair Value	464,245	1,434,801
U.S. Treasury [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized cost	1,997	2,000
Gross Unrealized Gains	1	4
Total, Fair Value	1,998	2,004
U.S. Government Sponsored Enterprises [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized cost	9,995	130,048
Gross Unrealized Gains	9	360
Gross Unrealized Losses	'	68
Total, Fair Value	10,004	130,340
SBA Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized cost	7,270	8,043
Gross Unrealized Gains	32	113
Total, Fair Value	7,302	8,156
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized cost	404,103	1,212,953
Gross Unrealized Gains	588	20,816
Gross Unrealized Losses	1,501	412
Total, Fair Value	403,190	1,233,357
Privately Issued Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized cost	2,294	2,938
Gross Unrealized Gains	6	31
Gross Unrealized Losses	23	22
Total, Fair Value	2,277	2,947
Obligations Issued by States and Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized cost	37,578	55,855
Gross Unrealized Gains	15	41
Gross Unrealized Losses	870	722
Total, Fair Value	36,723	55,174
Other Debt Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized cost	2,300	2,300
Gross Unrealized Losses	125	47
Total, Fair Value	2,175	2,253
Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Amortized cost	406	458
Gross Unrealized Gains	170	112
Total, Fair Value	$576	$570

Securities_AvailableforSale_Ad

Securities Available-for-Sale - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]	'	'	'
Realized gross gain	$3,019,000	$1,843,000	$1,940,000
Proceeds from sales of securities available-for-sale	224,045,000	294,881,000	75,615,000
Weighted average remaining life of investment securities available-for-sale	'5 years 1 month 6 days	'	'
Securities at floating rate or adjustable rate	415,692,000	'	'
Securities held-to-maturity at their fair value	987,037,000	'	'
Available-for-sale securities transferred to held-to-maturity securities unrealized loss	25,333,000	'	'
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'
Securities at fair value pledged to secure public deposits and repurchase agreements	368,137,000	665,028,000	'
Federal Home Loan Bank of Boston [Member]	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'
Securities available-for-sale are securities pledged for borrowing	$12,214,000	$220,313,000	'
Securities AFS [Member]	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'
Number of securities, temporarily impaired for less than 12 months	47	20	'
Number of securities, temporarily impaired for 12 months or longer	7	7	'
Number of securities, temporarily impaired, total	234	458	'

Securities_AvailableforSale_Es

Securities Available-for-Sale -Estimated Maturity Distribution of Securities Available-for-Sale (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Investments Debt And Equity Securities [Abstract]	'	'
Within one year, Amortized Cost	$36,942	'
After one but within five years, Amortized Cost	180,676	'
After five but within ten years, Amortized Cost	238,822	'
More than 10 years, Amortized Cost	7,597	'
Nonmaturing, Amortized Cost	1,906	'
Total, Amortized Cost	465,943	'
Within one year, Fair Value	36,964	'
After one but within five years, Fair Value	180,540	'
After five but within ten years, Fair Value	238,018	'
More than 10 years, Fair Value	6,771	'
Nonmaturing, Fair Value	1,952	'
Total, Fair Value	$464,245	$1,434,801

Securities_AvailableforSale_Co

Securities Available-for-Sale - Continuous Unrealized Loss Position for 12 Months or Less and 12 Months or Longer (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Less Than 12 Months, Fair Value	$291,195	$127,973
Less Than 12 Months, Unrealized Losses	1,375	451
12 Months or Longer, Fair Value	29,753	17,448
12 Months or Longer, Unrealized Losses	1,144	820
Total, Fair Value	320,948	145,421
Total, Unrealized Losses	2,519	1,271
U.S. Government Sponsored Enterprises [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less Than 12 Months, Fair Value	'	34,967
Less Than 12 Months, Unrealized Losses	'	68
12 Months or Longer, Fair Value	'	'
12 Months or Longer, Unrealized Losses	'	'
Total, Fair Value	'	34,967
Total, Unrealized Losses	'	68
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less Than 12 Months, Fair Value	289,709	93,006
Less Than 12 Months, Unrealized Losses	1,352	383
12 Months or Longer, Fair Value	24,557	10,169
12 Months or Longer, Unrealized Losses	149	29
Total, Fair Value	314,266	103,175
Total, Unrealized Losses	1,501	412
Privately Issued Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less Than 12 Months, Fair Value	1,486	'
Less Than 12 Months, Unrealized Losses	23	'
12 Months or Longer, Fair Value	'	1,863
12 Months or Longer, Unrealized Losses	'	22
Total, Fair Value	1,486	1,863
Total, Unrealized Losses	23	22
Obligations Issued by States and Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 Months or Longer, Fair Value	3,820	3,963
12 Months or Longer, Unrealized Losses	870	722
Total, Fair Value	3,820	3,963
Total, Unrealized Losses	870	722
Other Debt Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 Months or Longer, Fair Value	1,376	1,453
12 Months or Longer, Unrealized Losses	125	47
Total, Fair Value	1,376	1,453
Total, Unrealized Losses	$125	$47

Investment_Securities_HeldtoMa2

Investment Securities Held-to-Maturity - Unrealized Gain (Losses) on Securities (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]	'	'
Amortized Cost	$1,487,884,000	$275,507,000
Gross Unrealized Gains	2,424,000	6,499,000
Gross Unrealized Losses	25,859,000	82,000
Estimated Fair Value	1,464,449,000	281,924,000
U.S. Government Sponsored Enterprises [Member]	'	'
Schedule of Held-to-maturity Securities [Line Items]	'	'
Amortized Cost	291,779,000	17,747,000
Gross Unrealized Gains	185,000	19,000
Gross Unrealized Losses	5,043,000	8,000
Estimated Fair Value	286,921,000	17,758,000
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]	'	'
Schedule of Held-to-maturity Securities [Line Items]	'	'
Amortized Cost	1,196,105,000	257,760,000
Gross Unrealized Gains	2,239,000	6,480,000
Gross Unrealized Losses	20,816,000	74,000
Estimated Fair Value	$1,177,528,000	$264,166,000

Investment_Securities_HeldtoMa3

Investment Securities Held-to-Maturity - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]	'	'
Securities pledged for borrowing at the Federal Home Loan Bank of Boston	$1,464,449,000	$281,924,000
Weighted average remaining life of investment securities held-to-maturity	'5 years 2 months 12 days	'
Securities held-to-maturity at their fair value	987,037,000	'
Available-for-sale securities transferred to held-to-maturity securities unrealized loss	25,333,000	'
U.S. Government Sponsored Enterprises [Member]	'	'
Schedule of Held-to-maturity Securities [Line Items]	'	'
Securities at fair value pledged to secure public deposits and repurchase agreements	732,144,000	149,366,000
Securities pledged for borrowing at the Federal Home Loan Bank of Boston	286,921,000	17,758,000
Weighted average remaining life	224,663,000	'
Federal Home Loan Bank of Boston [Member]	'	'
Schedule of Held-to-maturity Securities [Line Items]	'	'
Securities pledged for borrowing at the Federal Home Loan Bank of Boston	$510,060,000	$103,617,000
Held-to-Maturity Securities [Member]	'	'
Schedule of Held-to-maturity Securities [Line Items]	'	'
Number of securities, temporarily impaired for less than 12 months	191	3
Number of securities, temporarily impaired for 12 months or longer	13	1
Number of securities, temporarily impaired, total	300	96

Investment_Securities_HeldtoMa4

Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Investments Debt And Equity Securities [Abstract]	'	'
Within one year, Amortized Cost	$5,689	'
After one but within five years, Amortized Cost	827,333	'
After five but within ten years, Amortized Cost	653,784	'
More than ten years, Amortized Cost	1,078	'
Total, Amortized Cost	1,487,884	275,507
Within one year, Fair Value	5,672	'
After one but within five years, Fair Value	818,936	'
After five but within ten years, Fair Value	638,750	'
More than ten years, Fair Value	1,091	'
Estimated Fair Value	$1,464,449	$281,924

Investment_Securities_HeldtoMa5

Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Schedule of Held-to-maturity Securities [Line Items]	'	'
Less Than 12 Months, Fair Value	$1,163,715	$18,930
Less Than 12 Months, Unrealized Losses	23,697	58
12 Months or Longer, Fair Value	80,362	5,371
12 Months or Longer, Unrealized Losses	2,162	24
Total, Fair Value	1,244,077	24,301
Total, Unrealized Losses	25,859	82
U.S. Government Sponsored Enterprises [Member]	'	'
Schedule of Held-to-maturity Securities [Line Items]	'	'
Less Than 12 Months, Fair Value	232,535	9,994
Less Than 12 Months, Unrealized Losses	5,043	8
Total, Fair Value	232,535	9,994
Total, Unrealized Losses	5,043	8
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]	'	'
Schedule of Held-to-maturity Securities [Line Items]	'	'
Less Than 12 Months, Fair Value	931,180	8,936
Less Than 12 Months, Unrealized Losses	18,654	50
12 Months or Longer, Fair Value	80,362	5,371
12 Months or Longer, Unrealized Losses	2,162	24
Total, Fair Value	1,011,542	14,307
Total, Unrealized Losses	$20,816	$74

Loans_Summary_Shows_the_Compos

Loans - Summary Shows the Composition of the Loan Portfolio (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Composition of Nonaccrual Loans and Impaired Loans [Line Items]	'	'
Total	$1,264,763	$1,111,788
Construction and Land Development [Member]	'	'
Composition of Nonaccrual Loans and Impaired Loans [Line Items]	'	'
Total	33,058	38,618
Commercial and Industrial [Member]	'	'
Composition of Nonaccrual Loans and Impaired Loans [Line Items]	'	'
Total	92,402	88,475
Commercial Real Estate [Member]	'	'
Composition of Nonaccrual Loans and Impaired Loans [Line Items]	'	'
Total	713,327	576,465
Residential Real Estate [Member]	'	'
Composition of Nonaccrual Loans and Impaired Loans [Line Items]	'	'
Total	286,041	281,857
Consumer [Member]	'	'
Composition of Nonaccrual Loans and Impaired Loans [Line Items]	'	'
Total	8,824	6,823
Home Equity [Member]	'	'
Composition of Nonaccrual Loans and Impaired Loans [Line Items]	'	'
Total	130,277	118,923
Overdrafts [Member]	'	'
Composition of Nonaccrual Loans and Impaired Loans [Line Items]	'	'
Total	$834	$627

Loans_Additional_Information_D

Loans - Additional Information (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Debt Disclosure [Abstract]	'	'	'
Net of discount on loans	$454,000	$498,000	'
Net deferred fee	174,000	369,000	'
Mortgage loans sold to others	109,301,000	26,786,000	'
Loans held for sale	0	9,378,000	'
Total recorded investment in impaired loans	7,788,000	5,925,000	8,102,000
Impaired loans	6,723,000	5,223,000	'
Specific reserve	$1,019,000	$1,732,000	'

Loans_Composition_of_Nonaccrua

Loans - Composition of Nonaccrual Loans and Impaired Loans (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Debt Disclosure [Abstract]	'	'	'
Loans on nonaccrual	$2,549,000	$4,471,000	$5,827,000
Loans 90 days past due and still accruing	'	'	18,000
Impaired loans on nonaccrual included above	1,819,000	2,878,000	3,468,000
Total recorded investment in impaired loans	7,788,000	5,925,000	8,102,000
Average recorded investment of impaired loans	6,776,000	7,043,000	10,284,000
Accruing troubled debt restructures	5,969,000	3,048,000	4,634,000
Interest income not recorded on nonaccrual loans according to their original terms	711,000	753,000	846,000
Interest income on nonaccrual loans actually recorded	'	'	'
Interest income recognized on impaired loans	$161,000	$180,000	$155,000

Loans_The_Aggregate_Amount_of_

Loans - The Aggregate Amount of Loans to Directors and Officers of the Company and Their Associates (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013
Debt Disclosure [Abstract]	'
Beginning balance	$4,663
Additions	310
Repayments and Deletions	269
Ending balance	$4,704

Allowance_for_Loan_Losses_Summ

Allowance for Loan Losses - Summary of Changes in Allowance for Loan Losses (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Receivables [Abstract]	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses, beginning of year	'	'	'	$19,197	'	'	'	$16,574	$19,197	$16,574	$14,053
Loans charged-off	'	'	'	'	'	'	'	'	-1,813	-2,301	-2,824
Recoveries on loans previously charged-off	'	'	'	'	'	'	'	'	847	774	795
Net charge-offs	'	'	'	'	'	'	'	'	-966	-1,527	-2,029
Provision charged to expense	460	750	750	750	900	1,250	900	1,100	2,710	4,150	4,550
Allowance for loan losses, end of year	$20,941	'	'	'	$19,197	'	'	'	$20,941	$19,197	$16,574

Allowance_for_Loan_Losses_Summ1

Allowance for Loan Losses - Summary of Allowance for Loan Losses (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Allowance for loan losses:	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses, beginning of year	'	'	'	$19,197,000	'	'	'	$16,574,000	$19,197,000	$16,574,000	$14,053,000
Charge-offs	'	'	'	'	'	'	'	'	-1,813,000	-2,301,000	-2,824,000
Recoveries	'	'	'	'	'	'	'	'	847,000	774,000	795,000
Provision	460,000	750,000	750,000	750,000	900,000	1,250,000	900,000	1,100,000	2,710,000	4,150,000	4,550,000
Allowance for loan losses, end of year	20,941,000	'	'	'	19,197,000	'	'	'	20,941,000	19,197,000	16,574,000
Amount of allowance for loan losses for loans deemed to be impaired	1,019,000	'	'	'	1,732,000	'	'	'	1,019,000	1,732,000	'
Amount of allowance for loan losses for loans not deemed to be impaired	'	'	'	'	'	'	'	'	19,922,000	17,465,000	'
Loans:	'	'	'	'	'	'	'	'	'	'	'
Total loans, net	1,264,763,000	'	'	'	1,111,788,000	'	'	'	1,264,763,000	1,111,788,000	'
Loans deemed to be impaired	7,788,000	'	'	'	5,925,000	'	'	'	7,788,000	5,925,000	'
Loans not deemed to be impaired	1,256,975,000	'	'	'	1,105,863,000	'	'	'	1,256,975,000	1,105,863,000	'
Construction and Land Development [Member]	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses:	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses, beginning of year	'	'	'	3,041,000	'	'	'	2,893,000	3,041,000	2,893,000	'
Charge-offs	'	'	'	'	'	'	'	'	-1,000,000	'	'
Provision	'	'	'	'	'	'	'	'	133,000	148,000	'
Allowance for loan losses, end of year	2,174,000	'	'	'	3,041,000	'	'	'	2,174,000	3,041,000	'
Amount of allowance for loan losses for loans deemed to be impaired	12,000	'	'	'	1,000,000	'	'	'	12,000	1,000,000	'
Amount of allowance for loan losses for loans not deemed to be impaired	'	'	'	'	'	'	'	'	2,162,000	2,041,000	'
Loans:	'	'	'	'	'	'	'	'	'	'	'
Total loans, net	33,058,000	'	'	'	38,618,000	'	'	'	33,058,000	38,618,000	'
Loans deemed to be impaired	608,000	'	'	'	1,500,000	'	'	'	608,000	1,500,000	'
Loans not deemed to be impaired	32,450,000	'	'	'	37,118,000	'	'	'	32,450,000	37,118,000	'
Commercial and Industrial [Member]	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses:	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses, beginning of year	'	'	'	3,118,000	'	'	'	3,139,000	3,118,000	3,139,000	'
Charge-offs	'	'	'	'	'	'	'	'	-234,000	-1,253,000	'
Recoveries	'	'	'	'	'	'	'	'	389,000	307,000	'
Provision	'	'	'	'	'	'	'	'	-284,000	925,000	'
Allowance for loan losses, end of year	2,989,000	'	'	'	3,118,000	'	'	'	2,989,000	3,118,000	'
Amount of allowance for loan losses for loans deemed to be impaired	367,000	'	'	'	104,000	'	'	'	367,000	104,000	'
Amount of allowance for loan losses for loans not deemed to be impaired	'	'	'	'	'	'	'	'	2,622,000	3,014,000	'
Loans:	'	'	'	'	'	'	'	'	'	'	'
Total loans, net	92,402,000	'	'	'	88,475,000	'	'	'	92,402,000	88,475,000	'
Loans deemed to be impaired	1,367,000	'	'	'	1,282,000	'	'	'	1,367,000	1,282,000	'
Loans not deemed to be impaired	91,035,000	'	'	'	87,193,000	'	'	'	91,035,000	87,193,000	'
Commercial Real Estate [Member]	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses:	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses, beginning of year	'	'	'	9,065,000	'	'	'	6,566,000	9,065,000	6,566,000	'
Recoveries	'	'	'	'	'	'	'	'	19,000	9,000	'
Provision	'	'	'	'	'	'	'	'	2,134,000	2,490,000	'
Allowance for loan losses, end of year	11,218,000	'	'	'	9,065,000	'	'	'	11,218,000	9,065,000	'
Amount of allowance for loan losses for loans deemed to be impaired	417,000	'	'	'	415,000	'	'	'	417,000	415,000	'
Amount of allowance for loan losses for loans not deemed to be impaired	'	'	'	'	'	'	'	'	10,801,000	8,650,000	'
Loans:	'	'	'	'	'	'	'	'	'	'	'
Total loans, net	713,327,000	'	'	'	576,465,000	'	'	'	713,327,000	576,465,000	'
Loans deemed to be impaired	4,520,000	'	'	'	2,281,000	'	'	'	4,520,000	2,281,000	'
Loans not deemed to be impaired	708,807,000	'	'	'	574,184,000	'	'	'	708,807,000	574,184,000	'
Residential Real Estate [Member]	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses:	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses, beginning of year	'	'	'	1,994,000	'	'	'	1,886,000	1,994,000	1,886,000	'
Charge-offs	'	'	'	'	'	'	'	'	'	-192,000	'
Recoveries	'	'	'	'	'	'	'	'	11,000	17,000	'
Provision	'	'	'	'	'	'	'	'	1,000	283,000	'
Allowance for loan losses, end of year	2,006,000	'	'	'	1,994,000	'	'	'	2,006,000	1,994,000	'
Amount of allowance for loan losses for loans deemed to be impaired	129,000	'	'	'	117,000	'	'	'	129,000	117,000	'
Amount of allowance for loan losses for loans not deemed to be impaired	'	'	'	'	'	'	'	'	1,877,000	1,877,000	'
Loans:	'	'	'	'	'	'	'	'	'	'	'
Total loans, net	286,041,000	'	'	'	281,857,000	'	'	'	286,041,000	281,857,000	'
Loans deemed to be impaired	1,199,000	'	'	'	766,000	'	'	'	1,199,000	766,000	'
Loans not deemed to be impaired	284,842,000	'	'	'	281,091,000	'	'	'	284,842,000	281,091,000	'
Consumer [Member]	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses:	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses, beginning of year	'	'	'	333,000	'	'	'	356,000	333,000	356,000	'
Charge-offs	'	'	'	'	'	'	'	'	-579,000	-697,000	'
Recoveries	'	'	'	'	'	'	'	'	427,000	422,000	'
Provision	'	'	'	'	'	'	'	'	251,000	252,000	'
Allowance for loan losses, end of year	432,000	'	'	'	333,000	'	'	'	432,000	333,000	'
Amount of allowance for loan losses for loans deemed to be impaired	'	'	'	'	'	'	'	'	'	'	'
Amount of allowance for loan losses for loans not deemed to be impaired	'	'	'	'	'	'	'	'	432,000	333,000	'
Loans:	'	'	'	'	'	'	'	'	'	'	'
Total loans, net	9,658,000	'	'	'	7,450,000	'	'	'	9,658,000	7,450,000	'
Loans not deemed to be impaired	9,658,000	'	'	'	7,450,000	'	'	'	9,658,000	7,450,000	'
Home Equity [Member]	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses:	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses, beginning of year	'	'	'	886,000	'	'	'	704,000	886,000	704,000	'
Charge-offs	'	'	'	'	'	'	'	'	'	-159,000	'
Recoveries	'	'	'	'	'	'	'	'	1,000	19,000	'
Provision	'	'	'	'	'	'	'	'	72,000	322,000	'
Allowance for loan losses, end of year	959,000	'	'	'	886,000	'	'	'	959,000	886,000	'
Amount of allowance for loan losses for loans deemed to be impaired	94,000	'	'	'	96,000	'	'	'	94,000	96,000	'
Amount of allowance for loan losses for loans not deemed to be impaired	'	'	'	'	'	'	'	'	865,000	790,000	'
Loans:	'	'	'	'	'	'	'	'	'	'	'
Total loans, net	130,277,000	'	'	'	118,923,000	'	'	'	130,277,000	118,923,000	'
Loans deemed to be impaired	94,000	'	'	'	96,000	'	'	'	94,000	96,000	'
Loans not deemed to be impaired	130,183,000	'	'	'	118,827,000	'	'	'	130,183,000	118,827,000	'
Unallocated [Member]	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses:	'	'	'	'	'	'	'	'	'	'	'
Allowance for loan losses, beginning of year	'	'	'	760,000	'	'	'	1,030,000	760,000	1,030,000	'
Provision	'	'	'	'	'	'	'	'	403,000	-270,000	'
Allowance for loan losses, end of year	1,163,000	'	'	'	760,000	'	'	'	1,163,000	760,000	'
Amount of allowance for loan losses for loans not deemed to be impaired	'	'	'	'	'	'	'	'	$1,163,000	$760,000	'

Allowance_for_Loan_Losses_Loan

Allowance for Loan Losses - Loans by Risk Rating (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Loans by risk rating	'	'
Financing Receivable, Net	$1,264,763	$1,111,788
Construction and Land Development [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	33,058	38,618
Construction and Land Development [Member] | 1-3 (Pass) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	25,138	29,719
Construction and Land Development [Member] | 4 (Monitor) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	7,312	7,399
Construction and Land Development [Member] | 5 (Substandard) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	'	'
Construction and Land Development [Member] | 6 (Doubtful) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	'	'
Construction and Land Development [Member] | Impaired [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	608	1,500
Commercial and Industrial [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	92,402	88,475
Commercial and Industrial [Member] | 1-3 (Pass) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	90,563	86,587
Commercial and Industrial [Member] | 4 (Monitor) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	472	606
Commercial and Industrial [Member] | 5 (Substandard) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	'	'
Commercial and Industrial [Member] | 6 (Doubtful) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	'	'
Commercial and Industrial [Member] | Impaired [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	1,367	1,282
Commercial Real Estate [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	713,327	576,465
Commercial Real Estate [Member] | 1-3 (Pass) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	707,461	569,760
Commercial Real Estate [Member] | 4 (Monitor) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	1,346	4,424
Commercial Real Estate [Member] | 5 (Substandard) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	'	'
Commercial Real Estate [Member] | 6 (Doubtful) [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	'	'
Commercial Real Estate [Member] | Impaired [Member]	'	'
Loans by risk rating	'	'
Financing Receivable, Net	$4,520	$2,281

Allowance_for_Loan_Losses_Agin

Allowance for Loan Losses - Aging of Past-Due Loans (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
Accruing 30-89 Days Past Due	$5,561	$6,690	'
Non Accrual	2,549	4,471	5,827
Accrual Greater Than 90 Days	'	'	'
Total Past Due	8,110	11,161	'
Current Loans	1,256,653	1,100,627	'
Total loans, net	1,264,763	1,111,788	'
Construction and Land Development [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
Accruing 30-89 Days Past Due	'	'	'
Non Accrual	500	1,500	'
Accrual Greater Than 90 Days	'	'	'
Total Past Due	500	1,500	'
Current Loans	32,558	37,118	'
Total loans, net	33,058	38,618	'
Commercial and Industrial [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
Accruing 30-89 Days Past Due	112	1,256	'
Non Accrual	706	676	'
Accrual Greater Than 90 Days	'	'	'
Total Past Due	818	1,932	'
Current Loans	91,584	86,543	'
Total loans, net	92,402	88,475	'
Commercial Real Estate [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
Accruing 30-89 Days Past Due	1,496	3,450	'
Non Accrual	306	674	'
Accrual Greater Than 90 Days	'	'	'
Total Past Due	1,802	4,124	'
Current Loans	711,525	572,341	'
Total loans, net	713,327	576,465	'
Residential Real Estate [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
Accruing 30-89 Days Past Due	2,232	864	'
Non Accrual	1,034	1,597	'
Accrual Greater Than 90 Days	'	'	'
Total Past Due	3,266	2,461	'
Current Loans	282,775	279,396	'
Total loans, net	286,041	281,857	'
Consumer [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
Accruing 30-89 Days Past Due	11	32	'
Non Accrual	3	24	'
Accrual Greater Than 90 Days	'	'	'
Total Past Due	14	56	'
Current Loans	9,644	7,394	'
Total loans, net	9,658	7,450	'
Home Equity [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
Accruing 30-89 Days Past Due	1,710	1,088	'
Accrual Greater Than 90 Days	'	'	'
Total Past Due	1,710	1,088	'
Current Loans	128,567	117,835	'
Total loans, net	$130,277	$118,923	'

Allowance_for_Loan_Losses_Info

Allowance for Loan Losses - Information Pertaining to Impaired Loans (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
With no required reserve recorded, Carrying Value	$1,066,000	$702,000	'
With no required reserve recorded, Unpaid Balance Principal	3,901,000	1,224,000	'
With no required reserve recorded, Required Reserve	'	'	'
With no required reserve recorded, Average Carrying Value	662,000	1,071,000	'
With no required reserve recorded, Interest Income Recognized	1,000	1,000	'
With required reserve recorded, Carrying Value	6,723,000	5,223,000	'
With required reserve recorded, Unpaid Balance Principal	7,135,000	7,277,000	'
With required reserve recorded, Required Reserve	1,019,000	1,732,000	'
With required reserve recorded, Average Carrying Value	6,114,000	5,972,000	'
With required reserve recorded, Interest Income Recognized	160,000	179,000	'
Carrying Value	7,788,000	5,925,000	8,102,000
Unpaid Balance Principal	11,036,000	8,501,000	'
With required reserve recorded, Required Reserve	1,019,000	1,732,000	'
Average Carrying Value	6,776,000	7,043,000	10,284,000
Interest Income Recognized	161,000	180,000	'
Construction and Land Development [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
With no required reserve recorded, Carrying Value	500,000	'	'
With no required reserve recorded, Unpaid Balance Principal	3,292,000	'	'
With no required reserve recorded, Required Reserve	'	'	'
With no required reserve recorded, Average Carrying Value	'	346,000	'
With no required reserve recorded, Interest Income Recognized	'	'	'
With required reserve recorded, Carrying Value	108,000	1,500,000	'
With required reserve recorded, Unpaid Balance Principal	108,000	3,292,000	'
With required reserve recorded, Required Reserve	12,000	1,000,000	'
With required reserve recorded, Average Carrying Value	1,371,000	1,154,000	'
With required reserve recorded, Interest Income Recognized	1,000	'	'
Carrying Value	608,000	1,500,000	'
Unpaid Balance Principal	3,400,000	3,292,000	'
With required reserve recorded, Required Reserve	12,000	1,000,000	'
Average Carrying Value	1,371,000	1,500,000	'
Interest Income Recognized	1,000	'	'
Commercial and Industrial [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
With no required reserve recorded, Carrying Value	238,000	503,000	'
With no required reserve recorded, Unpaid Balance Principal	268,000	994,000	'
With no required reserve recorded, Required Reserve	'	'	'
With no required reserve recorded, Average Carrying Value	361,000	425,000	'
With no required reserve recorded, Interest Income Recognized	1,000	'	'
With required reserve recorded, Carrying Value	1,129,000	779,000	'
With required reserve recorded, Unpaid Balance Principal	1,371,000	995,000	'
With required reserve recorded, Required Reserve	367,000	104,000	'
With required reserve recorded, Average Carrying Value	902,000	1,317,000	'
With required reserve recorded, Interest Income Recognized	37,000	40,000	'
Carrying Value	1,367,000	1,282,000	'
Unpaid Balance Principal	1,639,000	1,989,000	'
With required reserve recorded, Required Reserve	367,000	104,000	'
Average Carrying Value	1,263,000	1,742,000	'
Interest Income Recognized	38,000	41,000	'
Commercial Real Estate [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
With no required reserve recorded, Carrying Value	82,000	169,000	'
With no required reserve recorded, Unpaid Balance Principal	82,000	199,000	'
With no required reserve recorded, Required Reserve	'	'	'
With no required reserve recorded, Average Carrying Value	132,000	176,000	'
With no required reserve recorded, Interest Income Recognized	'	'	'
With required reserve recorded, Carrying Value	4,438,000	2,112,000	'
With required reserve recorded, Unpaid Balance Principal	4,527,000	2,158,000	'
With required reserve recorded, Required Reserve	417,000	415,000	'
With required reserve recorded, Average Carrying Value	2,868,000	2,817,000	'
With required reserve recorded, Interest Income Recognized	120,000	138,000	'
Carrying Value	4,520,000	2,281,000	'
Unpaid Balance Principal	4,609,000	2,357,000	'
With required reserve recorded, Required Reserve	417,000	415,000	'
Average Carrying Value	3,000,000	2,993,000	'
Interest Income Recognized	120,000	138,000	'
Residential Real Estate [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
With no required reserve recorded, Carrying Value	246,000	30,000	'
With no required reserve recorded, Unpaid Balance Principal	259,000	31,000	'
With no required reserve recorded, Required Reserve	'	'	'
With no required reserve recorded, Average Carrying Value	169,000	124,000	'
With no required reserve recorded, Interest Income Recognized	'	'	'
With required reserve recorded, Carrying Value	953,000	736,000	'
With required reserve recorded, Unpaid Balance Principal	1,035,000	736,000	'
With required reserve recorded, Required Reserve	129,000	117,000	'
With required reserve recorded, Average Carrying Value	878,000	640,000	'
With required reserve recorded, Interest Income Recognized	2,000	1,000	'
Carrying Value	1,199,000	766,000	'
Unpaid Balance Principal	1,294,000	767,000	'
With required reserve recorded, Required Reserve	129,000	117,000	'
Average Carrying Value	1,047,000	764,000	'
Interest Income Recognized	2,000	1,000	'
Consumer [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
With no required reserve recorded, Carrying Value	'	'	'
With no required reserve recorded, Unpaid Balance Principal	'	'	'
With no required reserve recorded, Required Reserve	'	'	'
With no required reserve recorded, Average Carrying Value	'	'	'
With no required reserve recorded, Interest Income Recognized	'	'	'
With required reserve recorded, Carrying Value	'	'	'
With required reserve recorded, Unpaid Balance Principal	'	'	'
With required reserve recorded, Required Reserve	'	'	'
With required reserve recorded, Average Carrying Value	'	'	'
With required reserve recorded, Interest Income Recognized	'	'	'
Carrying Value	'	'	'
Unpaid Balance Principal	'	'	'
With required reserve recorded, Required Reserve	'	'	'
Average Carrying Value	'	'	'
Interest Income Recognized	'	'	'
Home Equity [Member]	'	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'	'
With no required reserve recorded, Carrying Value	'	'	'
With no required reserve recorded, Unpaid Balance Principal	'	'	'
With no required reserve recorded, Required Reserve	'	'	'
With no required reserve recorded, Average Carrying Value	'	'	'
With no required reserve recorded, Interest Income Recognized	'	'	'
With required reserve recorded, Carrying Value	94,000	96,000	'
With required reserve recorded, Unpaid Balance Principal	94,000	96,000	'
With required reserve recorded, Required Reserve	94,000	96,000	'
With required reserve recorded, Average Carrying Value	95,000	44,000	'
Carrying Value	94,000	96,000	'
Unpaid Balance Principal	94,000	96,000	'
With required reserve recorded, Required Reserve	94,000	96,000	'
Average Carrying Value	$95,000	$44,000	'

Allowance_for_Loan_Losses_Trou

Allowance for Loan Losses - Troubled Debt Restructurings (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
	Contract	Contract
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'
Number of Contracts	7	3
Pre-modification Outstanding Recorded Investment	$2,836	$854
Post-modification Outstanding Recorded Investment	2,690	838
Construction and Land Development [Member]	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'
Number of Contracts	1	1
Pre-modification Outstanding Recorded Investment	108	750
Post-modification Outstanding Recorded Investment	108	736
Commercial and Industrial [Member]	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'
Number of Contracts	2	1
Pre-modification Outstanding Recorded Investment	67	6
Post-modification Outstanding Recorded Investment	64	6
Commercial Real Estate [Member]	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'
Number of Contracts	3	1
Pre-modification Outstanding Recorded Investment	2,376	98
Post-modification Outstanding Recorded Investment	2,356	96
Residential Real Estate [Member]	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'
Number of Contracts	1	'
Pre-modification Outstanding Recorded Investment	285	'
Post-modification Outstanding Recorded Investment	$162	'

Allowance_for_Loan_Losses_Addi

Allowance for Loan Losses - Additional Information (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'
Troubled debt restructurings, subsequently defaulted	$6,000	$0
Commercial and Industrial [Member]	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'
Financing allowance reduction in principal	808	6,000
Financing allowance reduction in interest payments	606	'
Number of troubled debt restructurings	1	'
Commercial Real Estate [Member]	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'
Financing allowance reduction in interest payments	23,227	'
Financing allowance increase in principal	5,246	'
Construction and Land Development [Member]	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'
Financing allowance reduction in principal	1,515	'
Financing allowance reduction in interest payments	1,098	'
Residential Real Estate [Member]	'	'
Financing Receivable, Allowance for Credit Losses [Line Items]	'	'
Financing allowance reduction in principal	4,704	'
Financing allowance reduction in interest payments	$4,104	$8,000

Bank_Premises_and_Equipment_Sc

Bank Premises and Equipment - Schedule of Bank Premises and Equipment (Detail) (USD $)

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

In Thousands, unless otherwise specified

Land [Member]

Land [Member]

Bank Premises [Member]

Bank Premises [Member]

Furniture and Equipment [Member]

Furniture and Equipment [Member]

Leasehold Improvements [Member]

Leasehold Improvements [Member]

Minimum [Member]

Minimum [Member]

Minimum [Member]

Maximum [Member]

Maximum [Member]

Maximum [Member]

Bank Premises [Member]

Furniture and Equipment [Member]

Leasehold Improvements [Member]

Bank Premises [Member]

Furniture and Equipment [Member]

Leasehold Improvements [Member]

Property, Plant and Equipment [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Bank premises and equipment, gross

$64,688

$63,080

$3,478

$3,478

$19,235

$18,353

$31,965

$31,319

$10,010

$9,930

'

'

'

'

'

'

Accumulated depreciation and amortization

-41,288

-39,181

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Total

$23,400

$23,899

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Estimated Useful Life

'

'

'

'

'

'

'

'

'

'

'30 years

'3 years

'30 years

'39 years

'10 years

'39 years

Bank_Premises_and_Equipment_Ad

Bank Premises and Equipment - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Property Plant And Equipment [Abstract]	'	'	'
Lease expense	$2,094,000	$2,055,000	$2,007,000
Lease rental income	$299,000	$329,000	$455,000
Operating lease premises and equipment lease expiration year, maximum	'2026	'	'

Bank_Premises_and_Equipment_Su

Bank Premises and Equipment - Summary of Future Minimum Rental Commitments for Non-Cancelable Operating Leases (Detail) (USD $)	Dec. 31, 2013
In Thousands, unless otherwise specified
Property Plant And Equipment [Abstract]	'
2014	$2,070
2015	1,824
2016	1,675
2017	1,235
2018	1,035
Thereafter	2,816
Future minimum rental commitments for non-cancelable operating leases, total	$10,655

Goodwill_and_Identifiable_Inta2

Goodwill and Identifiable Intangible Assets - Carrying Amount of Goodwill and Intangibles (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Intangible Assets And Goodwill [Line Items]	'	'
Goodwill, Beginning Balance	$2,714	$2,714
Goodwill, Ending Balance	2,714	2,714
Beginning Balance	'	120
Amortization Expense	'	-120
Ending Balance	'	'
Total, Beginning Balance	2,851	2,957
Additions	653	48
Amortization Expense	-87	-154
Total, Ending Balance	3,417	2,851
Mortgage Servicing Rights [Member]	'	'
Intangible Assets And Goodwill [Line Items]	'	'
Beginning Balance	137	123
Additions	653	48
Amortization Expense	-87	-34
Ending Balance	$703	$137

Fair_Value_Measurements_Financ

Fair Value Measurements - Financial Instruments Measured at Fair Value on a Recurring and Non-recurring Basis (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	$464,245	$1,434,801
U.S. Treasury [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	1,998	2,004
U.S. Government Sponsored Enterprises [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	10,004	130,340
SBA Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	7,302	8,156
Privately Issued Residential Mortgage-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	2,277	2,947
Obligations Issued by States and Political Subdivisions [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	36,723	55,174
Other Debt Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	2,175	2,253
Equity Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	576	570
U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	403,190	1,233,357
Carrying Value [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	464,245	1,434,801
Financial Instruments Measured at Fair Value on a Non-recurring Basis	'	'
Impaired Loans	'	3,587
Carrying Value [Member] | U.S. Treasury [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	1,998	2,004
Carrying Value [Member] | U.S. Government Sponsored Enterprises [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	10,004	130,340
Carrying Value [Member] | SBA Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	7,302	8,156
Carrying Value [Member] | US Government Agency and Sponsored Enterprises [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	'	'
Carrying Value [Member] | Mortgage-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	403,189	'
Carrying Value [Member] | Privately Issued Residential Mortgage-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	2,277	2,947
Carrying Value [Member] | Obligations Issued by States and Political Subdivisions [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	36,723	55,174
Carrying Value [Member] | Other Debt Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	2,176	2,253
Carrying Value [Member] | Equity Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	576	570
Carrying Value [Member] | Impaired Loans [Member]	'	'
Financial Instruments Measured at Fair Value on a Non-recurring Basis	'	'
Impaired Loans	1,747	'
Carrying Value [Member] | U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	'	1,233,357
Fair Value Measurements, Level 1 Inputs [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	286	228
Fair Value Measurements, Level 1 Inputs [Member] | US Government Agency and Sponsored Enterprises [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	'	'
Fair Value Measurements, Level 1 Inputs [Member] | Equity Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	286	228
Fair Value Measurements, Level 2 Inputs [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	427,362	1,380,791
Fair Value Measurements, Level 2 Inputs [Member] | U.S. Treasury [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	1,998	2,004
Fair Value Measurements, Level 2 Inputs [Member] | U.S. Government Sponsored Enterprises [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	10,004	130,340
Fair Value Measurements, Level 2 Inputs [Member] | SBA Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	7,302	8,156
Fair Value Measurements, Level 2 Inputs [Member] | US Government Agency and Sponsored Enterprises [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	'	'
Fair Value Measurements, Level 2 Inputs [Member] | Mortgage-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	403,189	'
Fair Value Measurements, Level 2 Inputs [Member] | Privately Issued Residential Mortgage-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	2,277	2,947
Fair Value Measurements, Level 2 Inputs [Member] | Obligations Issued by States and Political Subdivisions [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	416	1,734
Fair Value Measurements, Level 2 Inputs [Member] | Other Debt Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	2,176	2,253
Fair Value Measurements, Level 2 Inputs [Member] | U.S. Government Agency and Sponsored Enterprises Mortgage-Backed Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	'	1,233,357
Fair Value Measurements, Level 3 Inputs [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	36,597	53,782
Financial Instruments Measured at Fair Value on a Non-recurring Basis	'	'
Impaired Loans	'	3,587
Fair Value Measurements, Level 3 Inputs [Member] | US Government Agency and Sponsored Enterprises [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	'	'
Fair Value Measurements, Level 3 Inputs [Member] | Obligations Issued by States and Political Subdivisions [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	36,307	53,440
Fair Value Measurements, Level 3 Inputs [Member] | Equity Securities [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Securities AFS	290	342
Fair Value Measurements, Level 3 Inputs [Member] | Impaired Loans [Member]	'	'
Financial Instruments Measured at Fair Value on a Non-recurring Basis	'	'
Impaired Loans	$1,747	'

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Impaired loans recognized	$48,000	$1,909,000
Transfers between level 1 and 2	0	0
Liabilities measured at fair value on a recurring or nonrecurring basis	0	0
Fair Value Measurements, Level 3 Inputs [Member]	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Amortized cost of Level 3 securities	37,463,000	54,504,000
Unrealized loss	$866,000	$722,000

Fair_Value_Measurements_Assets

Fair Value Measurements - Assets Measured at Fair Value (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Securities AFS [Member]	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'
Fair Value	$36,597	$53,782
Valuation Technique	'Discounted cash flow	'Discounted cash flow
Unobservable Input	'Discount rate	'Discount rate
Impaired Loans [Member]	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'
Fair Value	$1,747	$3,587
Valuation Technique	'Appraisal of collateral	'Appraisal of collateral
Unobservable Input	'Appraisal adjustments	'Appraisal adjustments
Minimum [Member] | Securities AFS [Member]	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'
Unobservable Input Value or Range	0.00%	0.00%
Minimum [Member] | Impaired Loans [Member]	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'
Unobservable Input Value or Range	0.00%	0.00%
Maximum [Member] | Securities AFS [Member]	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'
Unobservable Input Value or Range	1.00%	1.00%
Maximum [Member] | Impaired Loans [Member]	'	'
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]	'	'
Unobservable Input Value or Range	25.00%	25.00%

Fair_Value_Measurements_Change

Fair Value Measurements - Changes in Level 3 Securities (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning Balance	$53,782	$18,914
Purchases	50,012	90,960
Maturities	-67,028	-56,289
Amortization	-26	-41
Change in fair value	-143	238
Ending Balance	36,597	53,782
Auction Rate Securities [Member]	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning Balance	3,963	3,725
Change in fair value	-143	238
Ending Balance	3,820	3,963
Obligations Issued by States and Political Subdivisions [Member]	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning Balance	49,477	14,772
Purchases	50,012	90,960
Maturities	-66,976	-56,214
Amortization	-26	-41
Ending Balance	32,487	49,477
Equity Securities [Member]	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]	'	'
Beginning Balance	342	417
Maturities	-52	-75
Ending Balance	$290	$342

Deposits_Summary_of_Remaining_

Deposits - Summary of Remaining Maturities or Re-pricing of Time Deposits (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Maturities Of Time Deposits [Abstract]	'	'
Within one year	$248,758	$299,456
Over one year to two years	41,533	67,918
Over two years to three years	48,357	19,834
Over three years to five years	43,576	32,775
Time Deposits, Total	$382,224	$419,983
Within one year, Percent	65.00%	71.00%
Over one year to two years, Percent	11.00%	16.00%
Over two years to three years, Percent	13.00%	5.00%
Over three years to five years, Percent	11.00%	8.00%
Total, Percent	100.00%	100.00%

Deposits_Additional_Informatio

Deposits - Additional Information (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Time Deposits [Abstract]	'	'
Time deposits of $100,000 or more	$259,665,000	$287,048,000

Securities_Sold_under_Agreemen2

Securities Sold under Agreements to Repurchase - Summary of Securities Sold under Agreements to Repurchase (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Subordinated Debentures Activity For Year [Abstract]	'	'	'
Amount outstanding at December 31	$214,440	$191,390	$143,320
Weighted average rate at December 31	0.18%	0.17%	0.24%
Maximum amount outstanding at any month end	214,440	213,730	152,267
Daily average balance outstanding during the year	$203,888	$174,624	$129,137
Weighted average rate during the year	0.18%	0.21%	0.29%

Securities_Sold_under_Agreemen3

Securities Sold under Agreements to Repurchase - Additional Information (Detail) (U.S. Government Sponsored Enterprises [Member], USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
U.S. Government Sponsored Enterprises [Member]	'	'	'
Assets Sold under Agreements to Repurchase [Line Items]	'	'	'
U.S. Government Sponsored Enterprise securities with a total amortized cost	$216,747,000	$187,995,000	$140,891,000
Fair value of the collateral repurchase agreement	$213,350,000	$191,704,000	$143,212,000

Other_Borrowed_Funds_and_Subor2

Other Borrowed Funds and Subordinated Debentures - Summary of Other Borrowed Funds and Subordinated Debentures (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Subordinated Debentures Activity For Year [Abstract]	'	'	'
Amount outstanding at December 31	$291,227	$231,227	$280,226
Weighted average rate at December 31	3.04%	3.54%	2.85%
Maximum amount outstanding at any month end	291,227	277,226	280,226
Daily average balance outstanding during the year	$231,032	$217,542	$202,209
Weighted average rate during the year	3.73%	3.82%	3.85%

Other_Borrowed_Funds_and_Subor3

Other Borrowed Funds and Subordinated Debentures - Additional Information (Detail) (USD $)

12 Months Ended

12 Months Ended

1 Months Ended

12 Months Ended

12 Months Ended

Dec. 31, 2013

Dec. 31, 2011

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Jan. 10, 2005

Dec. 31, 2004

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Federal Home Loan Bank Borrowings [Member]

Subordinated Debt [Member]

Subordinated Debt [Member]

Subordinated Debt [Member]

Subordinated Debt [Member]

Subordinated Debt [Member]

Other Borrowed Funds [Member]

Other Borrowed Funds [Member]

Cumulative Preferred Stock Subject to Mandatory Redemption [Member]

Cumulative Preferred Stock Subject to Mandatory Redemption [Member]

Century Bancorp Capital Trust II [Member]

Century Bancorp Capital Trust II [Member]

Subordinated Debt [Member]

Subordinated Debt [Member]

Century Bancorp Capital Trust II [Member]

Short-term Debt [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

Bank's remaining term borrowing capacity at the FHLBB

'

'

'

$423,143,000

'

'

'

'

'

'

'

'

'

Line of credit with the FHLBB

'

'

'

14,500,000

'

'

'

'

'

'

'

'

'

FHLBB advances

35,000,000

35,000,000

35,000,000

'

'

'

'

'

'

'

'

'

'

Restructuring of FHLBB advances

14,500,000

18,000,000

'

'

'

'

'

'

'

'

'

'

'

Weighted average rate of restructured FHLBB advances

3.16%

4.45%

'

'

'

'

'

'

'

'

'

'

'

Weighted average remaining maturity period of FHLB

'12 months

'25 months

'

'

'

'

'

'

'

'

'

'

'

Weighted average rate subsequent to restructured of FHLBB advances

3.24%

3.50%

'

'

'

'

'

'

'

'

'

'

'

Weighted average remaining maturity subsequent to Restructured FHLBB advances

'68 months

'60 months

'

'

'

'

'

'

'

'

'

'

'

Subordinated debt securities issued

36,083,000

'

36,083,000

'

36,083,000

36,083,000

'

36,083,000

'

'

'

'

'

Subordinated debentures issued for new Unconsolidated subsidiary

'

'

'

'

29,639,000

'

'

'

'

'

'

'

'

Subordinated debt securities due year

'

'

'

'

'2029

'

'

'2034

'

'

'

'

'

Shares of cumulative trust preferred securities

'

'

'

'

'

'

'

'

'

'

'

2,875,000

35,000

Liquidation value of shares of cumulative trust preferred securities

'

'

'

'

$10

'

'

'

$1,000

'

'

'

'

Percentage of preferred trust securities

'

'

'

'

'

'

8.30%

'

'

'

'

'

'

Trust preferred securities annual dividend rate

'

'

'

'

'

'

'

'

'

'

'

8.30%

6.65%

Period of dividend

'

'

'

'

'

'

'

'

'10 years

'

'

'

'

LIBOR rate

'

'

'

'

'

'

'

'

'Three-month LIBOR rate plus 1.87% for the remaining 20 years

'

'

'

'

LIBOR rate trust preferred securities

'

'

'

'

'

'

'

'

1.87%

'

'

'

'

Duration of LIBOR rate

'

'

'

'

'

'

'

'

'20 years

'

'

'

'

Federal funds purchased

'

'

'

'

'

'

'

'

'

0

0

'

'

Outstanding loan

'

'

'

'

'

'

'

'

'

$144,000

$144,000

'

'

Other_Borrowed_Funds_and_Subor4

Other Borrowed Funds and Subordinated Debentures - Schedule of the Maturity Distribution of FHLBB Advances with the Weighted Average Interest Rates (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Federal Home Loan Bank Advances Maturities Summary [Abstract]	'	'	'
Within one year	$53,000	$46,000	$81,500
Over one year to two years	19,500	17,500	23,500
Over two years to three years	55,000	19,500	17,500
Over three years to five years	77,000	90,000	74,500
Over five years	50,500	22,000	47,000
Federal Home Loan Bank, Advances, Total	$255,000	$195,000	$244,000
Within one year, Weighted Average Rate	0.40%	1.86%	0.42%
Over one year to two years, Weighted Average Rate	2.42%	3.01%	3.34%
Over two years to three years, Weighted Average Rate	3.07%	2.42%	3.01%
Over three years to five years, Weighted Average Rate	3.05%	3.33%	2.90%
Over five years, Weighted Average Rate	3.39%	4.19%	4.38%
Weighted Average Rate, Total	2.52%	2.96%	2.41%

Reclassifications_Out_of_Accum2

Reclassifications Out of Accumulated Other Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Net gains on sale of investments	'	'	'	'	'	'	'	'	$3,019	$1,843	$1,940
Securities held-to-maturity	'	'	'	'	'	'	'	'	16,615	6,746	5,816
Provision for income taxes	-116	-308	-295	-288	-139	-685	-255	-313	-1,007	-1,392	-1,554
Net income	5,025	5,519	5,026	4,476	4,778	5,682	4,771	3,808	20,046	19,039	16,693
Salaries and employee benefits	'	'	'	'	'	'	'	'	-35,244	-32,943	-29,630
Reclassification Out of Accumulated Other Comprehensive Income [Member]	'	'	'	'	'	'	'	'	'	'	'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Net income	'	'	'	'	'	'	'	'	3,032	466	'
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains and Losses on Available-for-Sale Securities [Member]	'	'	'	'	'	'	'	'	'	'	'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Net gains on sale of investments	'	'	'	'	'	'	'	'	3,019	1,843	'
Provision for income taxes	'	'	'	'	'	'	'	'	-1,179	-728	'
Net income	'	'	'	'	'	'	'	'	1,840	1,115	'
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Accretion of Unrealized Losses Transferred [Member]	'	'	'	'	'	'	'	'	'	'	'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Securities held-to-maturity	'	'	'	'	'	'	'	'	3,077	'	'
Provision for income taxes	'	'	'	'	'	'	'	'	-1,191	'	'
Net income	'	'	'	'	'	'	'	'	1,886	'	'
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Prior-Service Costs [Member]	'	'	'	'	'	'	'	'	'	'	'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Salaries and employee benefits	'	'	'	'	'	'	'	'	-10	-10	'
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Actuarial Gains (Losses) [Member]	'	'	'	'	'	'	'	'	'	'	'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Salaries and employee benefits	'	'	'	'	'	'	'	'	-1,144	-1,071	'
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Amortization of Defined Benefit Pension Items [Member]	'	'	'	'	'	'	'	'	'	'	'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Total before tax	'	'	'	'	'	'	'	'	-1,154	-1,081	'
Provision for income taxes	'	'	'	'	'	'	'	'	460	432	'
Net income	'	'	'	'	'	'	'	'	($694)	($649)	'

Earnings_Per_Share_EPS_Additio

Earnings Per Share ("EPS") - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Minimum [Member]	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'
Percentage of dividends paid	200.00%	'	'
Class A Common Stock [Member]	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'
Dilutive effect of stock options, increment in shares	1,155	1,024	1,150
Class A Common Stock [Member] | Minimum [Member]	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'
Percentage of dividends paid	200.00%	'	'

Earnings_Per_Share_EPS_Reconci

Earnings Per Share ("EPS") - Reconciliation of Basic EPS and Diluted EPS (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Net income	$5,025	$5,519	$5,026	$4,476	$4,778	$5,682	$4,771	$3,808	$20,046	$19,039	$16,693
Class A Common Stock [Member]	'	'	'	'	'	'	'	'	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Net income	'	'	'	'	'	'	'	'	15,698	14,877	13,023
Weighted average shares outstanding, basic	3,580,404	3,578,400	3,574,379	3,569,546	3,564,145	3,559,125	3,556,474	3,550,993	3,575,683	3,557,693	3,543,233
Basic earnings per share	$1.10	$1.21	$1.10	$0.98	$1.05	$1.25	$1.05	$0.84	$4.39	$4.18	$3.68
Dilutive effect of Class A stock options	'	'	'	'	'	'	'	'	1,155	1,024	1,150
Weighted average shares outstanding, diluted	5,557,419	5,558,031	5,557,354	5,557,365	5,552,121	5,549,810	5,548,830	5,545,711	5,557,693	5,549,191	5,541,794
Diluted earnings per share	$0.90	$0.99	$0.90	$0.81	$0.86	$1.02	$0.86	$0.69	$3.61	$3.43	$3.01
Class B Common Stock [Member]	'	'	'	'	'	'	'	'	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Net income	'	'	'	'	'	'	'	'	$4,348	$4,162	$3,670
Weighted average shares outstanding, basic	1,976,180	1,978,180	1,982,180	1,986,880	1,986,880	1,989,380	1,991,880	1,993,755	1,980,855	1,990,474	1,997,411
Basic earnings per share	$0.55	$0.60	$0.55	$0.49	$0.52	$0.62	$0.52	$0.42	$2.19	$2.09	$1.84
Weighted average shares outstanding, diluted	1,976,180	1,978,180	1,982,180	1,986,880	1,986,880	1,989,380	1,991,880	1,993,755	1,980,855	1,990,474	1,997,411
Diluted earnings per share	$0.55	$0.60	$0.55	$0.49	$0.52	$0.62	$0.52	$0.42	$2.19	$2.09	$1.84

Stockholders_Equity_Dividends_1

Stockholders' Equity Dividends - Additional Information (Detail) (USD $)

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2010

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Stock Option Plan [Member]

Stock Option Plan [Member]

Stock Option Plan [Member]

Minimum [Member]

Minimum [Member]

Maximum [Member]

Class A Common Stock [Member]

Class A Common Stock [Member]

Class A Common Stock [Member]

Class A Common Stock [Member]

Class A Common Stock [Member]

Class A Common Stock [Member]

Stock Option Plan [Member]

Stock Option Plan [Member]

Minimum [Member]

Maximum [Member]

Maximum [Member]

Class of Stock [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Dividend per share percent

'

'

'

'

'

'

'

200.00%

'

'

'

'

'

200.00%

'

'

Stock repurchase program, number of additional shares authorized to be repurchased

'

'

'

'

'

'

'

'

'

'

300,000

300,000

'

'

'

'

Stock repurchase program, number of shares authorized to be repurchased, Percent

'

'

'

'

'

'

'

'

'

'

'

'

'

'

9.00%

9.00%

Share based compensation number of share authorized

'

'

'

'

'

'

'

'

'

150,000

150,000

'

'

'

'

'

Share based compensation of option price based on fair value

'

'

'

'

'

'

'

'

85.00%

'

'

'

'

'

'

'

Number of outstanding options

20,375

23,350

36,062

38,712

20,375

'

'

'

'

'

'

'

'

'

'

'

Stock option exercisable period

'

'

'

'

'

'

'

'

'

'10 years

'

'

'

'

'

'

Exercise prices of shares outstanding, minimum

'

'

'

'

'

'

'

'

'

'

$15.06

$15.06

$15.06

'

'

'

Exercise prices of shares outstanding, maximum

'

'

'

'

'

'

'

'

'

'

$31.83

$31.83

$31.83

'

'

'

Weighted average remaining contractual life

'

'

'

'

'

'

'

'

'

'

'1 year

'2 years

'2 years

'

'

'

Weighted average intrinsic value of options exercised

'

'

'

'

$6.49

'

'

'

'

'

'

'

'

'

'

'

Aggregate value of weighted average intrinsic value of options exercised

'

'

'

'

$10,548

'

'

'

'

'

'

'

'

'

'

'

Aggregate value of weighted average intrinsic value of options exercisable

'

'

'

'

$29,136

$41,549

$49,145

'

'

'

'

'

'

'

'

'

Stockholders_Equity_Dividends_2

Stockholders' Equity Dividends - Summary of Stock Option Activity under Stock Option Plan (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Shares under option:	'	'	'
Outstanding at beginning of year, Amount	23,350	36,062	38,712
Forfeited, Amount	-1,350	-450	-200
Exercised, Amount	-1,625	-12,262	-2,450
Outstanding at end of year, Amount	20,375	23,350	36,062
Exercisable at end of year, Amount	20,375	23,350	36,062
Available to be granted at end of year, Amount	224,884	223,534	223,084
Outstanding at beginning of year, Weighted Average Exercise Price	$31.17	$28.90	$28.36
Forfeited, Weighted Average Exercise Price	$26.68	$22.50	$15.06
Exercised, Weighted Average Exercise Price	$26.76	$24.82	$21.44
Outstanding at end of year, Weighted Average Exercise Price	$31.82	$31.17	$28.90
Exercisable at end of year, Weighted Average Exercise Price	$31.82	$31.17	$28.90

Stockholders_Equity_Dividends_3

Stockholders' Equity Dividends - Summary of the Bank's Actual Capital Amounts and Ratios (Detail) (Century Bancorp Capital Trust II [Member], USD $)	Dec. 31, 2013	Dec. 31, 2012
Century Bancorp Capital Trust II [Member]	'	'
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]	'	'
Total Capital (to Risk-Weighted Assets), Actual Amount	$230,038	$206,464
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	209,360	188,226
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	209,360	188,226
Total Capital (to Risk-Weighted Assets), Ratio	13.91%	14.15%
Tier 1 Capital (to Risk-Weighted Assets), Ratio	12.66%	12.90%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	6.00%	6.11%
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	132,338	116,726
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	66,169	58,363
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	139,467	123,202
Total Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	8.00%	8.00%
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	4.00%	4.00%
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Ratio	4.00%	4.00%
Total Capital (to Risk-Weighted Assets), Well Capitalized Amount	165,422	145,907
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	99,253	87,544
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$174,334	$154,002
Total Capital (to Risk-Weighted Assets), Well Capitalized Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	6.00%	6.00%
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Ratio	5.00%	5.00%

Stockholders_Equity_Dividends_4

Stockholders' Equity Dividends - Summary of the Company's Actual Capital Amounts and Ratios (Detail) (Company [Member], USD $)	Dec. 31, 2013	Dec. 31, 2012
Company [Member]	'	'
Compliance with Regulatory Capital Requirements for Mortgage Companies [Line Items]	'	'
Total Capital (to Risk-Weighted Assets), Actual Amount	$247,761	$227,945
Tier 1 Capital (to Risk-Weighted Assets), Actual Amount	227,000	209,668
Tier 1 Capital (to 4th Qtr. Average Assets), Actual Amount	227,000	209,668
Total Capital (to Risk-Weighted Assets), Ratio	14.92%	15.59%
Tier 1 Capital (to Risk-Weighted Assets), Ratio	13.67%	14.34%
Tier 1 Capital (to 4th Qtr. Average Assets), Ratio	6.50%	6.80%
Total Capital (to Risk-Weighted Assets), Capital Adequacy Amount	132,870	116,976
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Amount	66,435	58,488
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Amount	139,769	123,377
Total Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	8.00%	8.00%
Tier 1 Capital (to Risk-Weighted Assets), Capital Adequacy Ratio	4.00%	4.00%
Tier 1 Capital (to 4th Qtr. Average Assets), Capital Adequacy Ratio	4.00%	4.00%
Total Capital (to Risk-Weighted Assets), Well Capitalized Amount	166,088	146,220
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Amount	99,653	87,732
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Amount	$174,711	$154,221
Total Capital (to Risk-Weighted Assets), Well Capitalized Ratio	10.00%	10.00%
Tier 1 Capital (to Risk-Weighted Assets), Well Capitalized Ratio	6.00%	6.00%
Tier 1 Capital (to 4th Qtr. Average Assets), Well Capitalized Ratio	5.00%	5.00%

Income_Taxes_Summary_of_Curren

Income Taxes - Summary of Current and Deferred Components of Income Tax Expense (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Current expense:	'	'	'	'	'	'	'	'	'	'	'
Federal	'	'	'	'	'	'	'	'	$3,520	$3,181	$2,198
State	'	'	'	'	'	'	'	'	416	315	309
Total current expense	'	'	'	'	'	'	'	'	3,936	3,496	2,507
Deferred (benefit) expense:	'	'	'	'	'	'	'	'	'	'	'
Federal	'	'	'	'	'	'	'	'	-2,564	-1,833	-961
State	'	'	'	'	'	'	'	'	-365	-271	8
Total deferred benefit	'	'	'	'	'	'	'	'	-2,929	-2,104	-953
Provision for income taxes	$116	$308	$295	$288	$139	$685	$255	$313	$1,007	$1,392	$1,554

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]	'	'	'
Penalties expense	$0	$0	$0
Internal Revenue Service for interest	'	$2,000	'

Income_Taxes_Income_Tax_Accoun

Income Taxes - Income Tax Accounts Included in Other Assets/Liabilities (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Income Tax Disclosure [Abstract]	'	'
Currently receivable	$702	$630
Deferred income tax asset, net	30,857	14,551
Total	$31,559	$15,181

Income_Taxes_Summary_of_Differ

Income Taxes - Summary of Differences between Income Tax Expense at the Statutory Federal Income Tax Rate and Total Income Tax Expense (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]	'	'	'	'	'	'	'	'	'	'	'
Federal income tax expense at statutory rates	'	'	'	'	'	'	'	'	$7,158	$6,946	$6,204
State income tax, net of federal income tax benefit	'	'	'	'	'	'	'	'	34	29	209
Insurance income	'	'	'	'	'	'	'	'	-380	-396	-396
Effect of tax-exempt interest	'	'	'	'	'	'	'	'	-5,348	-4,628	-3,801
Net tax credit	'	'	'	'	'	'	'	'	-572	-633	-683
Other	'	'	'	'	'	'	'	'	115	74	21
Provision for income taxes	$116	$308	$295	$288	$139	$685	$255	$313	$1,007	$1,392	$1,554
Effective tax rate	'	'	'	'	'	'	'	'	4.80%	6.80%	8.50%

Income_Taxes_Gross_Deferred_In

Income Taxes - Gross Deferred Income Tax Assets and Gross Deferred Income Tax Liabilities (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Deferred income tax assets:	'	'
Allowance for loan losses	$9,199	$8,103
Unrealized losses on securities transferred to held-to-maturity	8,590	'
Deferred compensation	6,515	5,643
Pension and SERP liability	4,880	8,621
AMT credit	3,164	1,908
Unrealized losses (gains) on securities available-for-sale	652	-7,875
Acquisition premium	438	541
Nonaccrual interest	136	151
Depreciation	109	-36
Investments write down	26	26
Deferred gain	'	11
Other	198	235
Gross deferred income tax asset	33,907	17,328
Deferred income tax liabilities:	'	'
Limited partnerships	-2,769	-2,722
Mortgage servicing rights	-281	-55
Gross deferred income tax liability	-3,050	-2,777
Deferred income tax asset net	$30,857	$14,551

Employee_Benefits_Additional_I

Employee Benefits - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Benefits expected to be paid in 2014	$1,092,000	'	'
Benefits expected to be paid in 2015	1,176,000	'	'
Benefits expected to be paid in 2016	1,247,000	'	'
Benefits expected to be paid in 2017	1,322,000	'	'
Benefits expected to be paid in 2018	1,325,000	'	'
Aggregate benefits expected to paid	8,011,000	'	'
Expected to contribute Pension Plan	1,000,000	'	'
Voluntary contribution of employees	33.30%	'	'
Contributions matched by compensation contribution	6.00%	'	'
Voluntary contribution of employees, amount	322,000	308,000	266,000
Discretionary bonus expense	1,313,000	1,289,000	1,100,000
Supplemental Insurance/ Retirement Plan [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Benefits expected to be paid in 2014	1,108,000	'	'
Benefits expected to be paid in 2015	1,088,000	'	'
Benefits expected to be paid in 2016	1,515,000	'	'
Benefits expected to be paid in 2017	1,973,000	'	'
Benefits expected to be paid in 2018	1,981,000	'	'
Aggregate benefits expected to paid	$10,341,000	'	'
Equity Securities [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Allocation mix for the common and collective trust portfolio, Minimum	40.00%	'	'
Allocation mix for the common and collective trust portfolio, Maximum	64.00%	'	'
Short-Term Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Allocation mix for the common and collective trust portfolio, Minimum	15.00%	'	'
Allocation mix for the common and collective trust portfolio, Maximum	25.00%	'	'
Hedge Funds [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Allocation mix for the common and collective trust portfolio, Minimum	20.00%	'	'
Allocation mix for the common and collective trust portfolio, Maximum	36.00%	'	'

Employee_Benefits_Fair_Value_o

Employee Benefits - Fair Value of Plan Assets and Major Categories (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Defined Benefit Plan Disclosure [Line Items]	'	'
Fair value of plan assets, Percentage	100.00%	100.00%
Total, fair value of plan assets	$32,322	$23,983
Fair Value Measurements, Level 1 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	13,292	9,807
Fair Value Measurements, Level 2 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	16,774	12,485
Fair Value Measurements, Level 3 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	2,256	1,691
Collective Funds [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Fair value of plan assets, Percentage	52.90%	52.50%
Total, fair value of plan assets	17,092	12,593
Collective Funds [Member] | Fair Value Measurements, Level 1 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	856	767
Collective Funds [Member] | Fair Value Measurements, Level 2 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	16,236	11,826
Equity Securities [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Fair value of plan assets, Percentage	25.90%	24.60%
Total, fair value of plan assets	8,355	5,892
Equity Securities [Member] | Fair Value Measurements, Level 1 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	8,355	5,892
Mutual Funds [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Fair value of plan assets, Percentage	13.10%	14.10%
Total, fair value of plan assets	4,250	3,370
Mutual Funds [Member] | Fair Value Measurements, Level 1 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	3,832	3,086
Mutual Funds [Member] | Fair Value Measurements, Level 2 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	418	284
Hedge Funds [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Fair value of plan assets, Percentage	7.00%	7.00%
Total, fair value of plan assets	2,256	1,691
Hedge Funds [Member] | Fair Value Measurements, Level 3 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	2,256	1,691
Short-Term Investments [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Fair value of plan assets, Percentage	1.10%	1.80%
Total, fair value of plan assets	369	437
Short-Term Investments [Member] | Fair Value Measurements, Level 1 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	249	62
Short-Term Investments [Member] | Fair Value Measurements, Level 2 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Total, fair value of plan assets	$120	$375

Employee_Benefits_Changes_in_L

Employee Benefits - Changes in Level 3 Securities (Detail) (Fair Value Measurements, Level 3 Inputs [Member], USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value Measurements, Level 3 Inputs [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Balance at beginning of year	$1,691	$1,522
Purchases	55	152
Actual return - assets still being held	510	17
Balance at end of year	$2,256	$1,691

Employee_Benefits_Components_o

Employee Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Pension Plan [Member]	'	'
Change projected in benefit obligation	'	'
Benefit obligation at beginning of year	$31,910	$28,784
Service cost	1,196	1,097
Interest cost	1,257	1,295
Actuarial (gain)/loss	-3,734	1,401
Benefits paid	-750	-667
Projected benefit obligation at end of year	29,879	31,910
Change in plan assets	'	'
Balance at beginning of year	23,983	20,517
Actual (loss) return on plan assets	4,619	2,333
Employer contributions	4,470	1,800
Benefits paid	-750	-667
Balance at end of year	32,322	23,983
(Unfunded) Funded status	2,443	-7,927
Accumulated benefit obligation	29,747	31,773
Weighted-average assumptions as of December 31	'	'
Discount rate - Liability	5.00%	4.00%
Discount rate - Expense	4.00%	4.50%
Expected return on plan assets	8.00%	8.00%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost	'	'
Service cost	1,196	1,097
Interest cost	1,257	1,295
Expected return on plan assets	-1,880	-1,641
Recognized prior service cost	-104	-104
Recognized net losses	630	735
Net periodic cost	1,099	1,382
Other changes in plan assets and benefit obligations recognized in other comprehensive income	'	'
Amortization of prior service cost	104	104
Net (gain) loss	-6,956	-25
Total recognized in other comprehensive income	-6,852	79
Total recognized in net periodic benefit cost and other comprehensive income	-5,753	1,461
Supplemental Insurance/ Retirement Plan [Member]	'	'
Change projected in benefit obligation	'	'
Benefit obligation at beginning of year	25,835	21,097
Service cost	1,554	1,425
Interest cost	1,072	923
Actuarial (gain)/loss	-1,916	3,482
Benefits paid	-1,043	-1,092
Projected benefit obligation at end of year	25,502	25,835
Change in plan assets	'	'
Benefits paid	-1,043	-1,092
(Unfunded) Funded status	-25,502	-25,835
Accumulated benefit obligation	22,278	22,181
Weighted-average assumptions as of December 31	'	'
Discount rate - Liability	5.00%	4.00%
Discount rate - Expense	4.00%	4.50%
Rate of compensation increase	4.00%	4.00%
Components of net periodic benefit cost	'	'
Service cost	1,554	1,425
Interest cost	1,072	923
Recognized prior service cost	114	114
Recognized net losses	514	336
Net periodic cost	3,254	2,798
Other changes in plan assets and benefit obligations recognized in other comprehensive income	'	'
Amortization of prior service cost	-114	-114
Net (gain) loss	-2,401	3,098
Total recognized in other comprehensive income	-2,515	2,984
Total recognized in net periodic benefit cost and other comprehensive income	$739	$5,782

Employee_Benefits_Summary_of_D

Employee Benefits - Summary of Defined Pension Plan and Supplemental Insurance Retirement Plan (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Net Periodic Benefit Cost [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Prior service cost	($475)	($485)	'
Net actuarial loss	-11,262	-20,619	'
Total	-11,737	-21,104	'
Defined Benefit Pension Plan [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Prior service cost	-104	-104	'
Net actuarial loss	-3,734	1,401	'
Total	29,879	31,910	28,784
Defined Benefit Pension Plan [Member] | Net Periodic Benefit Cost [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Prior service cost	516	620	'
Net actuarial loss	-3,361	-10,317	'
Total	-2,845	-9,697	'
Supplemental Insurance/ Retirement Plan [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Prior service cost	114	114	'
Net actuarial loss	-1,916	3,482	'
Total	25,502	25,835	21,097
Supplemental Insurance/ Retirement Plan [Member] | Net Periodic Benefit Cost [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Prior service cost	-991	-1,105	'
Net actuarial loss	-7,901	-10,302	'
Total	($8,892)	($11,407)	'

Employee_Benefits_Summary_of_A

Employee Benefits - Summary of Accumulated Other Comprehensive Loss Expected to be Recognized (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Amortization of loss to be recognized in 2014	($4,932)	$2,488	$4,047
Defined Benefit Pension Plan [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Amortization of prior service cost to be recognized in 2014	-104	'	'
Amortization of loss to be recognized in 2014	12	'	'
Supplemental Insurance/ Retirement Plan [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Amortization of prior service cost to be recognized in 2014	114	'	'
Amortization of loss to be recognized in 2014	$355	'	'

Financial_Instruments_with_Off2

Financial Instruments with Off-Balance-Sheet Risk - Summary of Financial Instruments with Off-Balance-Sheet Risk (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Commitments to Originate 1-4 Family Mortgages [Member]	'	'
Financial instruments whose contract amount represents credit risk:	'	'
Financial instruments with off-balance-sheet risk	$3,373	$13,580
Standby and Commercial Letters of Credit [Member]	'	'
Financial instruments whose contract amount represents credit risk:	'	'
Financial instruments with off-balance-sheet risk	7,930	8,411
Unused Lines of Credit [Member]	'	'
Financial instruments whose contract amount represents credit risk:	'	'
Financial instruments with off-balance-sheet risk	249,941	217,246
Unadvanced Portions of Construction Loans [Member]	'	'
Financial instruments whose contract amount represents credit risk:	'	'
Financial instruments with off-balance-sheet risk	7,026	17,609
Unadvanced Portions of Other Loans [Member]	'	'
Financial instruments whose contract amount represents credit risk:	'	'
Financial instruments with off-balance-sheet risk	$17,750	$4,872

Other_Operating_Expenses_Summa

Other Operating Expenses - Summary of Other Operating Expenses (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Other Income And Expenses [Abstract]	'	'	'
Marketing	$1,749	$1,853	$1,575
Software maintenance/amortization	1,417	1,256	951
Legal and audit	1,281	1,179	1,140
Contributions	673	1,074	479
Processing services	812	921	865
Consulting	874	890	796
Postage and delivery	939	877	773
Supplies	848	849	868
Telephone	719	750	742
Directors' fees	373	330	309
Insurance	295	279	275
Core deposit intangible amortization	'	120	388
Other	1,500	1,230	1,280
Total	$11,480	$11,608	$10,441

Fair_Values_of_Financial_Instr2

Fair Values of Financial Instruments - Carrying Amounts and Fair Values of Company's Financial Instruments (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Financial assets:	'	'
Securities held-to-maturity	$1,487,884,000	$275,507,000
Loans	1,243,822,000	1,092,591,000
Financial liabilities:	'	'
Time deposits	382,224,000	419,983,000
Other borrowed funds	255,144,000	195,144,000
Subordinated debentures	36,083,000	36,083,000
Carrying Value [Member]	'	'
Financial assets:	'	'
Securities held-to-maturity	1,487,884,000	275,507,000
Loans	1,243,822,000	1,092,591,000
Financial liabilities:	'	'
Time deposits	382,224,000	419,983,000
Other borrowed funds	255,144,000	195,144,000
Subordinated debentures	36,083,000	36,083,000
Estimated Fair Value [Member]	'	'
Financial assets:	'	'
Securities held-to-maturity	1,464,449,000	281,924,000
Loans	1,214,192,000	1,124,716,000
Financial liabilities:	'	'
Time deposits	386,742,000	424,253,000
Other borrowed funds	254,736,000	205,481,000
Subordinated debentures	39,503,000	43,423,000
Fair Value Measurements, Level 1 Inputs [Member]	'	'
Financial assets:	'	'
Securities held-to-maturity	'	'
Loans	'	'
Financial liabilities:	'	'
Time deposits	'	'
Other borrowed funds	'	'
Subordinated debentures	'	'
Fair Value Measurements, Level 2 Inputs [Member]	'	'
Financial assets:	'	'
Securities held-to-maturity	1,464,449,000	281,924,000
Financial liabilities:	'	'
Time deposits	386,742,000	424,253,000
Other borrowed funds	254,736,000	205,481,000
Fair Value Measurements, Level 3 Inputs [Member]	'	'
Financial assets:	'	'
Loans	1,214,192,000	1,124,716,000
Financial liabilities:	'	'
Subordinated debentures	$39,503,000	$43,423,000

Quarterly_Results_of_Operation2

Quarterly Results of Operations - Quarterly Results of Operations (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Interest income	$20,947	$20,549	$19,132	$19,137	$19,738	$22,079	$20,312	$19,365	$79,765	$81,494	$78,065
Interest expense	4,842	4,751	4,620	4,592	4,795	4,859	4,923	4,963	18,805	19,540	22,766
Net interest income	16,105	15,798	14,512	14,545	14,943	17,220	15,389	14,402	60,960	61,954	55,299
Provision for loan losses	460	750	750	750	900	1,250	900	1,100	2,710	4,150	4,550
Net interest income after provision for loan losses	15,645	15,048	13,762	13,795	14,043	15,970	14,489	13,302	58,250	57,804	50,749
Other operating income	4,186	4,774	5,221	4,434	4,153	4,105	3,988	3,619	18,615	15,865	16,240
Operating expenses	14,690	13,995	13,662	13,465	13,279	13,708	13,451	12,800	55,812	53,238	48,742
Income before income taxes	5,141	5,827	5,321	4,764	4,917	6,367	5,026	4,121	21,053	20,431	18,247
Provision for income taxes	116	308	295	288	139	685	255	313	1,007	1,392	1,554
Net income	5,025	5,519	5,026	4,476	4,778	5,682	4,771	3,808	20,046	19,039	16,693
Class A Common Stock [Member]	'	'	'	'	'	'	'	'	'	'	'
Quarterly Financial Information [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Net income	'	'	'	'	'	'	'	'	15,698	14,877	13,023
Share data:	'	'	'	'	'	'	'	'	'	'	'
Average shares outstanding, basic	3,580,404	3,578,400	3,574,379	3,569,546	3,564,145	3,559,125	3,556,474	3,550,993	3,575,683	3,557,693	3,543,233
Average shares outstanding, diluted	5,557,419	5,558,031	5,557,354	5,557,365	5,552,121	5,549,810	5,548,830	5,545,711	5,557,693	5,549,191	5,541,794
Earnings per share, basic	$1.10	$1.21	$1.10	$0.98	$1.05	$1.25	$1.05	$0.84	$4.39	$4.18	$3.68
Earnings per share, diluted	$0.90	$0.99	$0.90	$0.81	$0.86	$1.02	$0.86	$0.69	$3.61	$3.43	$3.01
Class B Common Stock [Member]	'	'	'	'	'	'	'	'	'	'	'
Quarterly Financial Information [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Net income	'	'	'	'	'	'	'	'	$4,348	$4,162	$3,670
Share data:	'	'	'	'	'	'	'	'	'	'	'
Average shares outstanding, basic	1,976,180	1,978,180	1,982,180	1,986,880	1,986,880	1,989,380	1,991,880	1,993,755	1,980,855	1,990,474	1,997,411
Average shares outstanding, diluted	1,976,180	1,978,180	1,982,180	1,986,880	1,986,880	1,989,380	1,991,880	1,993,755	1,980,855	1,990,474	1,997,411
Earnings per share, basic	$0.55	$0.60	$0.55	$0.49	$0.52	$0.62	$0.52	$0.42	$2.19	$2.09	$1.84
Earnings per share, diluted	$0.55	$0.60	$0.55	$0.49	$0.52	$0.62	$0.52	$0.42	$2.19	$2.09	$1.84

Parent_Company_Financial_State2

Parent Company Financial Statements - Balance Sheets of Parent Company (Detail) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
ASSETS:	'	'	'	'
Cash	$59,956,000	$53,646,000	'	'
Other assets	87,897,000	66,031,000	'	'
Total assets	3,431,154,000	3,086,209,000	'	'
LIABILITIES AND STOCKHOLDERS' EQUITY:	'	'	'	'
Liabilities	3,254,682,000	2,906,219,000	'	'
Subordinated debentures	36,083,000	36,083,000	'	'
Stockholders' equity	176,472,000	179,990,000	160,649,000	145,025,000
Total liabilities and stockholders' equity	3,431,154,000	3,086,209,000	'	'
Century Bancorp, Inc. [Member]	'	'	'	'
ASSETS:	'	'	'	'
Cash	12,245,000	19,536,000	'	'
Investment in subsidiary, at equity	193,783,000	193,499,000	'	'
Other assets	6,634,000	3,145,000	'	'
Total assets	212,662,000	216,180,000	'	'
LIABILITIES AND STOCKHOLDERS' EQUITY:	'	'	'	'
Liabilities	107,000	107,000	'	'
Subordinated debentures	36,083,000	36,083,000	'	'
Stockholders' equity	176,472,000	179,990,000	'	'
Total liabilities and stockholders' equity	$212,662,000	$216,180,000	'	'

Parent_Company_Financial_State3

Parent Company Financial Statements - Statements of Income of Parent Company (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income:	'	'	'	'	'	'	'	'	'	'	'
Other income	'	'	'	'	'	'	'	'	$2,583	$2,551	$2,544
Total interest income	20,947	20,549	19,132	19,137	19,738	22,079	20,312	19,365	79,765	81,494	78,065
Interest expense	4,842	4,751	4,620	4,592	4,795	4,859	4,923	4,963	18,805	19,540	22,766
Operating expenses	14,690	13,995	13,662	13,465	13,279	13,708	13,451	12,800	55,812	53,238	48,742
Income before income taxes	5,141	5,827	5,321	4,764	4,917	6,367	5,026	4,121	21,053	20,431	18,247
Benefit from income taxes	116	308	295	288	139	685	255	313	1,007	1,392	1,554
Net income	5,025	5,519	5,026	4,476	4,778	5,682	4,771	3,808	20,046	19,039	16,693
Century Bancorp, Inc. [Member]	'	'	'	'	'	'	'	'	'	'	'
Income:	'	'	'	'	'	'	'	'	'	'	'
Interest income from deposits in bank	'	'	'	'	'	'	'	'	28	33	100
Other income	'	'	'	'	'	'	'	'	72	72	72
Total interest income	'	'	'	'	'	'	'	'	100	105	172
Interest expense	'	'	'	'	'	'	'	'	2,400	2,400	2,400
Operating expenses	'	'	'	'	'	'	'	'	208	198	178
Income before income taxes	'	'	'	'	'	'	'	'	-2,508	-2,493	-2,406
Benefit from income taxes	'	'	'	'	'	'	'	'	-853	-848	-818
Income before equity in undistributed income of subsidiary	'	'	'	'	'	'	'	'	-1,655	-1,645	-1,588
Equity in undistributed income of subsidiary	'	'	'	'	'	'	'	'	21,701	20,684	18,281
Net income	'	'	'	'	'	'	'	'	$20,046	$19,039	$16,693

Parent_Company_Financial_State4

Parent Company Financial Statements - Statements of Cash Flows of Parent Company (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
CASH FLOWS FROM OPERATING ACTIVITIES:	'	'	'
Net income	$20,046	$19,039	$16,693
Adjustments to reconcile net income to net cash provided by operating activities:	'	'	'
Depreciation and amortization	5,358	6,445	5,558
Increase in other assets	-5,693	-3,113	-4,456
Net cash provided by operating activities	31,938	19,426	18,826
CASH FLOWS FROM FINANCING ACTIVITIES:	'	'	'
Net proceeds from the exercise of stock options	43	304	52
Cash dividends paid	-2,191	-2,186	-2,180
Net cash provided by financing activities	351,668	317,678	277,228
Net (decrease) increase in cash and cash equivalents	-57,605	-55,483	19,214
Cash and cash equivalents at beginning of year	152,283	207,766	188,552
Cash and cash equivalents at end of year	94,678	152,283	207,766
Century Bancorp, Inc. [Member]	'	'	'
CASH FLOWS FROM OPERATING ACTIVITIES:	'	'	'
Net income	20,046	19,039	16,693
Adjustments to reconcile net income to net cash provided by operating activities:	'	'	'
Undistributed income of subsidiary	-21,701	-20,684	-18,281
Depreciation and amortization	12	12	12
Increase in other assets	-3,500	-416	-182
Net cash provided by operating activities	-5,143	-2,049	-1,758
CASH FLOWS FROM FINANCING ACTIVITIES:	'	'	'
Net proceeds from the exercise of stock options	43	304	53
Cash dividends paid	-2,191	-2,186	-2,180
Net cash provided by financing activities	-2,148	-1,882	-2,127
Net (decrease) increase in cash and cash equivalents	-7,291	-3,931	-3,885
Cash and cash equivalents at beginning of year	19,536	23,467	27,352
Cash and cash equivalents at end of year	$12,245	$19,536	$23,467