Century Bancorp (CNBKA) 10-Q 7 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 30, 2015
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	CNBKA
Entity Registrant Name	CENTURY BANCORP INC
Entity Central Index Key	812348
Current Fiscal Year End Date	-19
Entity Filer Category	Accelerated Filer
Class A Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		3,600,729
Class B Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		1,967,180

Consolidated_Balance_Sheets_un

Consolidated Balance Sheets (unaudited) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Assets
Cash and due from banks	$59,680	$43,367
Federal funds sold and interest-bearing deposits in other banks	5,945	261,990
Total cash and cash equivalents	65,625	305,357
Short-term investments	2,139	2,131
Securities available-for-sale, amortized cost $455,800 and $448,210, respectively	455,931	448,390
Securities held-to-maturity, fair value $1,796,688 and $1,413,603, respectively	1,773,607	1,406,792
Federal Home Loan Bank of Boston stock, at cost	24,916	24,916
Loans, net
Commercial and industrial	157,637	149,732
Municipal	41,406	41,850
Construction and land development	25,347	22,744
Commercial real estate	691,811	696,272
Residential real estate	258,558	257,305
Home equity	156,063	151,275
Consumer and other	10,508	12,188
Total loans, net	1,341,330	1,331,366
Less: allowance for loan losses	22,529	22,318
Net loans	1,318,801	1,309,048
Bank premises and equipment	24,179	24,182
Accrued interest receivable	6,913	6,241
Goodwill	2,714	2,714
Other assets	98,498	94,265
Total assets	3,773,323	3,624,036
Deposits:
Demand deposits	508,820	484,928
Savings and NOW deposits	1,063,551	978,619
Money Market Accounts	960,668	890,899
Time deposits	382,456	383,145
Total deposits	2,915,495	2,737,591
Securities sold under agreements to repurchase	260,390	212,360
Other borrowed funds	307,642	395,500
Subordinated debentures	36,083	36,083
Due to broker	7,142
Other liabilities	48,743	50,002
Total liabilities	3,575,495	3,431,536
Stockholders' Equity
Preferred stock - $1.00 par value; 100,000 shares authorized; no shares issued and outstanding
Additional paid-in capital	12,292	12,292
Retained earnings	204,821	200,411
Stockholders' equity before adjustment of accumulated other comprehensive income (loss)	222,681	218,271
Unrealized gains on securities available-for-sale, net of taxes	47	77
Unrealized losses on securities transferred to held-to-maturity, net of taxes	-9,744	-10,479
Pension liability, net of taxes	-15,156	-15,369
Total accumulated other comprehensive loss, net of taxes	-24,853	-25,771
Total stockholders' equity	197,828	192,500
Total liabilities and stockholders' equity	3,773,323	3,624,036
Class A Common Stock [Member]
Stockholders' Equity
Common stock value	3,601	3,601
Total stockholders' equity	3,601	3,601
Class B Common Stock [Member]
Stockholders' Equity
Common stock value	1,967	1,967
Total stockholders' equity	$1,967	$1,967

Consolidated_Balance_Sheets_un1

Consolidated Balance Sheets (unaudited) (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, except Share data, unless otherwise specified
Amortized cost	$455,800	$448,210
Held-to-maturity securities, fair value	$1,796,688	$1,413,603
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	3,600,729	3,600,729
Class B Common Stock [Member]
Common stock, par value	$1	$1
Common stock, shares authorized	5,000,000	5,000,000
Common stock, shares issued	1,967,180	1,976,180

Consolidated_Statements_of_Inc

Consolidated Statements of Income (unaudited) (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Interest income
Loans	$12,076	$12,449
Securities held-to-maturity	8,168	7,780
Securities available-for-sale	732	820
Federal funds sold and interest-bearing deposits in other banks	196	82
Total interest income	21,172	21,131
Interest expense
Savings and NOW deposits	628	609
Money market accounts	782	639
Time deposits	1,156	1,085
Securities sold under agreements to repurchase	114	101
Other borrowed funds and subordinated debentures	2,085	2,183
Total interest expense	4,765	4,617
Net interest income	16,407	16,514
Provision for loan losses	200	600
Net interest income after provision for loan losses	16,207	15,914
Other operating income
Service charges on deposit accounts	1,913	2,034
Lockbox fees	788	777
Gains on sales of mortgage loans	99	7
Other income	705	652
Total other operating income	3,505	3,470
Operating expenses
Salaries and employee benefits	9,134	8,875
Occupancy	1,605	1,442
Equipment	593	572
FDIC assessments	503	480
Other	2,703	2,790
Total operating expenses	14,538	14,159
Income before income taxes	5,174	5,225
Provision for income taxes	215	293
Net income	4,959	4,932
Class A Common Stock [Member]
Operating expenses
Net income	3,895	3,867
Share data:
Weighted average number of shares outstanding, basic	3,600,729	3,582,421
Weighted average number of shares outstanding, diluted	5,567,909	5,558,177
Basic earnings per share	$1.08	$1.08
Diluted earnings per share	$0.89	$0.89
Class B Common Stock [Member]
Operating expenses
Net income	$1,064	$1,065
Share data:
Weighted average number of shares outstanding, basic	1,967,180	1,974,180
Weighted average number of shares outstanding, diluted	1,967,180	1,974,180
Basic earnings per share	$0.54	$0.54
Diluted earnings per share	$0.54	$0.54

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (unaudited) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$4,959	$4,932
Unrealized gains (losses) on securities:
Unrealized (losses) gains arising and transferred during period	-30	243
Less: reclassification adjustment for gains included in net income	0	0
Total unrealized (losses) gains on securities	-30	243
Accretion of net unrealized losses transferred	735	924
Defined benefit pension plans:
Amortization of prior service cost and loss included in net periodic benefit cost	213	57
Other comprehensive income (loss)	918	1,224
Comprehensive income (loss)	$5,877	$6,156

Consolidated_Statements_of_Cha

Consolidated Statements of Changes in Shareholders' Equity (unaudited) (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 Income (Loss) [Member]
In Thousands, unless otherwise specified						Class A Common Stock [Member]	Class B Common Stock [Member]
Beginning balance at Dec. 31, 2013	$176,472	$3,580	$1,976	$11,932	$180,747			($21,763)
Net income	4,932	3,867	1,065		4,932
Other comprehensive income, net of tax:
Unrealized holding (losses) gains arising during period, net taxes	243							243
Unrealized holding (losses) gains arising during period, net taxes	243
Accretion of unrealized losses on securities transferred to held-to-maturity net of taxes	924							924
Pension liability adjustment, net of taxes	57							57
Conversion of class B common stock to class A common stock		2
Conversion of class B common stock to class A common stock			-2
Stock options exercised	3	1		2
Cash dividends		-430	-119			-430	-119
Ending balance at Mar. 31, 2014	182,082	3,583	1,974	11,934	185,130			-20,539
Beginning balance at Dec. 31, 2014	192,500	3,601	1,967	12,292	200,411			-25,771
Net income	4,959	3,895	1,064		4,959
Other comprehensive income, net of tax:
Unrealized holding (losses) gains arising during period, net taxes	-30
Unrealized holding (losses) gains arising during period, net taxes	-30							-30
Accretion of unrealized losses on securities transferred to held-to-maturity net of taxes	735							735
Pension liability adjustment, net of taxes	213							213
Cash dividends		-432	-117			-432	-117
Ending balance at Mar. 31, 2015	$197,828	$3,601	$1,967	$12,292	$204,821			($24,853)

Consolidated_Statements_of_Cha1

Consolidated Statements of Changes in Shareholders' Equity (unaudited) (Parenthetical) (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Unrealized holding (losses) gains arising during period, taxes	($19)	$170
Accretion of net unrealized losses transferred during the period, taxes	394	583
Pension liability adjustment, taxes	$142	$38
Stock options exercised, shares		75
Class A Common Stock [Member]
Conversion of class B common stock to class A common stock, shares		2,000
Cash dividends, per share	$0.12	$0.12
Class B Common Stock [Member]
Conversion of class B common stock to class A common stock, shares		-2,000
Cash dividends, per share	$0.06	$0.06

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (unaudited) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$4,959	$4,932
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Gain on sales of mortgage loans	-99	-7
Provision for loan losses	200	600
Deferred income taxes	-175	-744
Net depreciation and amortization	696	867
(Increase) decrease in accrued interest receivable	-672	201
Increase in other assets	-4,621	-998
(Decrease) increase in other liabilities	-904	493
Net cash (used in) provided by operating activities	-616	5,344
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from maturities of short-term investments		1,061
Purchase of short-term investments	-8	-1,069
Proceeds from calls/maturities of securities available-for-sale	34,265	36,157
Purchase of securities available-for-sale	-34,767	-44,658
Proceeds from calls/maturities of securities held-to-maturity	50,467	41,692
Purchase of securities held-to-maturity	-416,137	-30,957
Net increase in loans	-14,903	-127
Proceeds from sales of portfolio loans	5,061	570
Capital expenditures	-621	-636
Net cash (used in) provided by investing activities	-376,643	2,033
CASH FLOWS FROM FINANCING ACTIVITIES:
Net decrease in time deposits	-689	-1,025
Net increase in demand, savings, money market and NOW deposits	178,593	49,199
Net proceeds from exercise of stock options		3
Cash dividends	-549	-549
Net increase (decrease) in securities sold under agreements to repurchase	48,030	-3,790
Net (decrease) increase in other borrowed funds	-87,858	17,500
Net cash provided by financing activities	137,527	61,338
Net (decrease) increase in cash and cash equivalents	-239,732	68,715
Cash and cash equivalents at beginning of period	305,357	94,678
Cash and cash equivalents at end of period	65,625	163,393
Cash paid during the period for:
Interest	4,668	4,583
Income taxes	210	171
Change in unrealized gains (losses) on securities available-for-sale, net of taxes	-30	243
Change in unrealized losses on securities transferred to held-to-maturity, net of taxes	735	924
Pension liability adjustment, net of taxes	213	57
Due to broker	$7,142	$1,000

Basis_of_Financial_Statement_P

Basis of Financial Statement Presentation	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Basis of Financial Statement Presentation	Note 1. 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. The Company’s quarterly report on Form 10-Q should be read in conjunction with the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2014, as filed with the Securities and Exchange Commission.
	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.

Recent_Market_Developments

Recent Market Developments	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Recent Market Developments	Note 2. Recent Market Developments
	The financial services industry continues to face challenges in the aftermath of the recent national and global economic crisis. Since June 2009, the U.S. economy has been recovering from the most severe recession and financial crisis since the Great Depression. There have been some improvements in private sector employment, industrial production and U.S. exports; nevertheless, the pace of economic recovery has been slow. Financial markets have improved since the depths of the crisis but are still unsettled and volatile. There is continued concern about the U.S. economic outlook.
	On July 21, 2010, the Dodd-Frank Wall Street Reform and Consumer Protection Act (the “Act”) became law. The Act was intended to address many issues arising in the recent financial crisis and is exceedingly broad in scope, affecting many aspects of bank and financial market regulation. The Act requires, or permits by implementing regulation, enhanced prudential standards for banks and bank holding companies inclusive of capital, leverage, liquidity, concentration and exposure measures. In addition, traditional bank regulatory principles such as restrictions on transactions with affiliates and insiders were enhanced. The Act also contains reforms of consumer mortgage lending practices and creates a Bureau of Consumer Financial Protection, which is granted broad authority over consumer financial practices of banks and others. It is expected as the specific new or incremental requirements applicable to the Company become effective that the costs and difficulties of remaining compliant with all such requirements will increase. The Act broadens the base for FDIC assessments to average consolidated assets less tangible equity of financial institutions and also permanently raises the current standard maximum FDIC deposit insurance amount to $250,000. The Act extended unlimited deposit insurance on non-interest bearing transaction accounts through December 31, 2012. In addition, the Act added a new Section 13 to the Bank Holding Company Act, the so-called “Volcker Rule,” (the “Rule”) which generally restricts certain banking entities such as the Company and its subsidiaries or affiliates, from engaging in proprietary trading activities and owning equity in or sponsoring any private equity or hedge fund. The Rule became effective July 21, 2012. The final implementing regulations for the Rule were issued by various regulatory agencies in December, 2013 and under an extended conformance regulation compliance must be achieved by July 21, 2015. The conformance period for investments in and relationships with certain “legacy covered funds” has been extended to July 21, 2016. Under the Rule, the Company may be restricted from engaging in proprietary trading, investing in third party hedge or private equity funds or sponsoring new funds unless it qualifies for an exemption from the rule. The Company has little involvement in prohibited proprietary trading or investment activities in covered funds and the Company does not expect that complying with the requirements of the Rule will have any material effect on the Company’s financial condition or results of operation.
	Federal banking regulators have issued risk-based capital guidelines, which assign risk factors to asset categories and off-balance-sheet items. Also, the Basel Committee has issued capital standards entitled “Basel III: A global regulatory framework for more resilient banks and banking systems” (“Basel III”). The Federal Reserve Board has finalized its rule implementing the Basel III regulatory capital framework. The rule that came into effect in January 2015 sets the Basel III minimum regulatory capital requirements for all organizations. It includes a new common equity Tier I ratio of 4.5 percent of risk-weighted assets, raises the minimum Tier I capital ratio from 4 percent to 6 percent of risk-weighted assets and would set a new conservation buffer of 2.5 percent of risk-weighted assets. The implementation of the framework did not have a material impact on the Company’s financial condition or results of operations.

Securities_AvailableforSale

Securities Available-for-Sale	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Securities Available-for-Sale	Note 3. Securities Available-for-Sale
		March 31, 2015								December 31, 2014
		Amortized		Gross		Gross		Fair		Amortized		Gross		Gross		Fair
		Cost		Unrealized		Unrealized		Value		Cost		Unrealized		Unrealized		Value
				Gains		Losses						Gains		Losses
		( In thousands)
	U.S. Treasury	$	2,000	$	—	$	—	$	2,000	$	1,999	$	1	$	—	$	2,000
	Small Business Administration		6,538		32		—		6,570		6,684		33		—		6,717
	U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities		321,978		1,273		374		322,877		336,158		1,387		452		337,093
	Privately Issued Residential Mortgage Backed Securities		1,818		3		18		1,803		1,894		5		25		1,874
	Obligations Issued by States and Political Subdivisions		119,648		—		875		118,773		97,657		—		873		96,784
	Other Debt Securities		3,600		7		96		3,511		3,600		24		100		3,524
	Equity Securities		218		179		—		397		218		180		—		398
	Total	$	455,800	$	1,494	$	1,363	$	455,931	$	448,210	$	1,630	$	1,450	$	448,390
	During the third quarter of 2013, securities available-for-sale with an amortized cost of $1,012,370,000 were transferred to securities held-to-maturity at their fair value of $987,037,000 in response to rising interest rates. Rising interest rates have the potential to increase unrealized losses on the available-for-sale portfolio. The transfer was implemented to lessen the effects of rising interest rates.
	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 $292,887,000 and $301,038,000 at March 31, 2015 and December 31, 2014, respectively. Also included in securities available-for-sale are securities pledged for borrowing at the Federal Home Loan Bank of Boston amounting to $23,339,000 and $24,810,000 at March 31, 2015 and December 31, 2014, respectively. There were no realized gains on sales of investments for the three months ended March 31, 2015 and March 31, 2014, respectively.
	Debt securities of 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 available-for-sale at March 31, 2015.
		Amortized		Fair
		Cost		Value
		( In thousands)
	Within one year	$	115,350	$	115,336
	After one but within five years		198,396		199,214
	After five but within ten years		132,313		132,401
	More than 10 years		8,023		7,179
	Non-maturing		1,718		1,801
	Total	$	455,800	$	455,931
	The weighted average remaining life of investment securities available-for-sale at March 31, 2015 was 3.8 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.
	As of March 31, 2015 and December 31, 2014, 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.
	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 March 31, 2015. 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 4 and 14 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 256 holdings at March 31, 2015.
		March 31, 2015
		Less than 12 months				12 months or longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair Value		Unrealized		Fair Value		Unrealized
				Losses				Losses				Losses
		(In thousands)
	U.S. Government Sponsored Enterprises	$	—	$	—	$	—	$	—	$	—	$	—
	U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities		21,932		37		79,708		337		101,640		374
	Privately Issued Residential Mortgage Backed Securities		—		—		658		18		658		18
	Obligations Issued by States and Political Subdivisions		—		—		3,820		875		3,820		875
	Other Debt Securities		—		—		1,404		96		1,404		96
	Total temporarily impaired securities	$	21,932	$	37	$	85,590	$	1,326	$	107,522	$	1,363
	The following table shows the temporarily impaired securities of the Company’s available-for-sale portfolio at December 31, 2014. 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 14 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 262 holdings at December 31, 2014.
		December 31, 2014
		Less than 12 months				12 months or longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair Value		Unrealized		Fair Value		Unrealized
				Losses				Losses				Losses
		(In thousands)
	U.S. Government Sponsored Enterprises	$	—	$	—	$	—	$	—	$	—	$	—
	U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities		24,457		85		77,585		367		102,042		452
	Privately Issued Residential Mortgage Backed Securities		—		—		678		25		678		25
	Obligations Issued by States and Political Subdivisions		—		—		3,820		873		3,820		873
	Other Debt Securities		—		—		1,400		100		1,400		100
	Total temporarily impaired securities	$	24,457	$	85	$	83,483	$	1,365	$	107,940	$	1,450

Investment_Securities_HeldtoMa

Investment Securities Held-to-Maturity	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Investment Securities Held-to-Maturity	Note 4. Investment Securities Held-to-Maturity
		March 31, 2015								December 31, 2014
		Amortized		Gross		Gross		Estimated		Amortized		Gross		Gross		Estimated
		Cost		Unrealized		Unrealized		Fair		Cost		Unrealized		Unrealized		Fair
				Gains		Losses		Value				Gains		Losses		Value
		(In thousands)
	U.S. Government Sponsored Enterprises	$	362,706	$	5,315	$	65	$	367,956	$	251,617	$	2,707	$	249	$	254,075
	U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities		1,410,901		20,182		2,351		1,428,732		1,155,175		11,185		6,832		1,159,528
	Total	$	1,773,607	$	25,497	$	2,416	$	1,796,688	$	1,406,792	$	13,892	$	7,081	$	1,413,603
	Included in U.S. Government and Agency Securities are securities pledged to secure public deposits and repurchase agreements at fair value amounting to $967,608,000 and $868,924,000 at March 31, 2015 and December 31, 2014, respectively. Also included are securities pledged for borrowing at the Federal Home Loan Bank of Boston at fair value amounting to $449,801,000 and $458,782,000 at March 31, 2015 and December 31, 2014, respectively.
	At March 31, 2015 and December 31, 2014, all mortgage-backed securities are obligations of U.S. Government Agencies and 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 March 31, 2015.
		Amortized		Fair
		Cost		Value
		( In thousands)
	Within one year	$	3,842	$	3,908
	After one but within five years		1,419,300		1,434,605
	After five but within ten years		346,515		354,165
	More than ten years		3,950		4,010
	Total	$	1,773,607	$	1,796,688
	The weighted average remaining life of investment securities held-to-maturity at March 31, 2015 was 4.3 years. Included in the weighted average remaining life calculation at March 31, 2015 were $280,101,000 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.
	As of March 31, 2015 and December 31, 2014, 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 likely 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 likely 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 March 31, 2015 and December 31, 2014.
	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 March 31, 2015. 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 35 and 19 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 357 holdings at March 31, 2015.
		March 31, 2015
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair Value		Unrealized		Fair Value		Unrealized
				Losses				Losses				Losses
		(Dollars in thousands)
	U.S. Government Sponsored Enterprises	$	49,452	$	65	$	—	$	—	$	49,452	$	65
	U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities		180,629		986		109,697		1,365		290,326		2,351
	Total temporarily impaired securities	$	230,081	$	1,051	$	109,697	$	1,365	$	339,778	$	2,416
	The following table shows the temporarily impaired securities of the Company’s held-to-maturity portfolio at December 31, 2014. 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 34 and 48 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 303 holdings at December 31, 2014.
		December 31, 2014
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair Value		Unrealized		Fair Value		Unrealized
				Losses				Losses				Losses
		( In thousands)
	U.S. Government Sponsored Enterprises	$	22,414	$	25	$	14,776	$	224	$	37,190	$	249
	U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities		194,119		1,678		308,526		5,154		502,645		6,832
	Total temporarily impaired securities	$	216,533	$	1,703	$	323,302	$	5,378	$	539,835	$	7,081

Allowance_for_Loan_Losses

Allowance for Loan Losses	3 Months Ended
	Mar. 31, 2015
Receivables [Abstract]
Allowance for Loan Losses	Note 5. 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 periods indicated.
		Three months ended
		March 31,
		2015			2014
		(in thousands)
	Allowance for loan losses, beginning of	$	22,318		$	20,941
	period
	Loans charged off		(81	)		(429	)
	Recoveries on loans previously		92			147
	charged-off
	Net recoveries (charge-offs)		11			(282	)
	Provision charged to expense		200			600
	Allowance for loan losses, end of period	$	22,529		$	21,259
	Further information pertaining to the allowance for loan losses for the three months ending March 31, 2015 follows:
		Construction			Commercial			Municipal			Commercial			Residential			Consumer			Home			Unallocated		Total
		and Land			and						Real			Real						Equity
		Development			Industrial						Estate			Estate
		(Dollars in thousands)
	Allowance for loan losses:
	Balance at December 31, 2014	$	1,592		$	4,758		$	1,488		$	11,199		$	775		$	810		$	599		$	1,097	$	22,318
	Charge-offs		—			—			—			—			—			(81	)		—			—		(81	)
	Recoveries		—			15			—			2			2			73			—			—		92
	Provision		292			(128	)		(115	)		(74	)		22			(88	)		15			276		200
	Ending balance at March 31, 2015	$	1,884		$	4,645		$	1,373		$	11,127		$	799		$	714		$	614		$	1,373	$	22,529
	Amount of allowance for loan losses for loans deemed to be impaired	$	20		$	96		$	—		$	631		$	90		$	—		$	92		$	—		929
	Amount of allowance for loan losses for loans not deemed to be impaired	$	1,864		$	4,549		$	1,373		$	10,496		$	709		$	714		$	522		$	1,373	$	21,600
	Loans:
	Ending balance	$	25,347		$	157,637		$	41,406		$	691,811		$	258,558		$	10,508		$	156,063		$	—	$	1,341,330
	Loans deemed to be impaired	$	102		$	871		$	—		$	4,304		$	952		$	—		$	92		$	—	$	6,321
	Loans not deemed to be impaired	$	25,245		$	156,766		$	41,406		$	687,507		$	257,606		$	10,508		$	155,971		$	—	$	1,335,009
	Further information pertaining to the allowance for loan losses for the three months ending March 31, 2014 follows:
		Construction			Commercial			Municipals			Commercial			Residential			Consumer			Home			Unallocated		Total
		and Land			and						Real			Real						Equity
		Development			Industrial						Estate			Estate
		(Dollars in thousands)
	Allowance for loan losses:
	Balance at December 31, 2013	$	2,174		$	2,617		$	655		$	10,935		$	2,006		$	432		$	959		$	1,163	$	20,941
	Charge-offs		(250	)		—			—			—			—			(179	)		—			—		(429	)
	Recoveries		—			24			—			1			5			116			1			—		147
	Provision		(99	)		(48	)		149			631			(138	)		56			(58	)		107		600
	Ending balance at	$	1,825		$	2,593		$	804		$	11,567		$	1,873		$	425		$	902		$	1,270	$	21,259
	March 31, 2014
	Amount of allowance for loan losses for loans deemed to be impaired	$	25		$	358		$	—		$	382		$	122		$	—		$	94		$	—		981
	Amount of allowance for loan losses for loans not deemed to be impaired	$	1,800		$	2,235		$	804		$	11,185		$	1,751		$	425		$	808		$	1,270	$	20,278
	Loans:
	Ending balance	$	25,928		$	80,478		$	32,307		$	691,541		$	290,465		$	9,542		$	133,800		$	—	$	1,264,061
	Loans deemed to be impaired	$	358		$	1,129		$	—		$	4,408		$	998		$	—		$	94		$	—	$	6,987
	Loans not deemed to be impaired	$	25,570		$	79,349		$	32,307		$	687,133		$	289,467		$	9,542		$	133,706		$	—	$	1,257,074
	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 March 31, 2015 and December 31, 2014.
	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 March 31, 2015 and December 31, 2014.
	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 March 31, 2015 and December 31, 2014 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 March 31, 2015.
		Construction			Commercial			Municipal			Commercial
		and land			and						real
development		industrial			estate
	(Dollars in thousands)
Grade:
1-3 (Pass)	$	18,170		$	156,294		$	41,406		$	686,883
4 (Monitor)		7,075			472			—			624
5 (Substandard)		—			—			—			—
6 (Doubtful)		—			—			—			—
Impaired		102			871			—			4,304
Total	$	25,347		$	157,637		$	41,406		$	691,811
The following table presents the Company’s loans by risk rating at December 31, 2014.
	Construction			Commercial			Municipal			Commercial
	and land			and						real
	development			industrial						estate
	(Dollars in thousands)
Grade:
1-3 (Pass)	$	15,515		$	148,407		$	41,850		$	691,322
4 (Monitor)		7,126			472			—			633
5 (Substandard)		—			—			—			—
6 (Doubtful)		—			—			—			—
Impaired		103			853			—			4,317
Total	$	22,744		$	149,732		$	41,850		$	696,272
The Company utilized payment performance as credit quality indicators for residential real estate, consumer and overdrafts, and the home equity portfolio. The indicators are depicted in the table “aging of past due loans,” below.
Further information pertaining to the allowance for loan losses at March 31, 2015 follows:
	Accruing			Non Accrual			Accrual			Total			Current			Total
	30-89 Days						Greater			Past Due			Loans
	Past Due						Than
							90 Days
	(Dollars in thousands)
Construction and land development	$	—		$	102		$	—		$	102		$	25,245		$	25,347
Commercial and industrial		173			209			—			382			157,255			157,637
Municipal		—			—			—			—			41,406			41,406
Commercial real estate		1,154			2,775			—			3,929			687,882			691,811
Residential real estate		698			632			—			1,330			257,228			258,558
Consumer and overdrafts		6			4			—			10			10,498			10,508
Home equity		733			538			34			1,305			154,758			156,063
Total	$	2,764		$	4,260		$	34		$	7,058		$	1,334,272		$	1,341,330
Further information pertaining to the allowance for loan losses at December 31, 2014 follows:
	Accruing			Non Accrual			Accrual			Total			Current			Total
	30-89 Days						Greater			Past Due			Loans
	Past Due						Than
							90 Days
	(Dollars in thousands)
Construction and land development	$	—		$	103		$	—		$	103		$	22,641		$	22,744
Commercial and industrial		905			157			—			1,062			148,670			149,732
Municipal		—			—			—			—			41,850			41,850
Commercial real estate		1,046			2,781			—			3,827			692,445			696,272
Residential real estate		632			846			—			1,478			255,827			257,305
Consumer and overdrafts		6			5			—			11			12,177			12,188
Home equity		576			254			—			830			150,445			151,275
Total	$	3,165		$	4,146		$	—		$	7,311		$	1,324,055		$	1,331,366
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 collectability of the loan’s principal is not probable. The specific factors that management considers in making the determination that the collectability 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 consolidated financial statements contained in the Company’s Annual Report for the fiscal year ended December 31, 2014.
The following is information pertaining to impaired loans for March 31, 2015:
	Carrying Value			Unpaid			Required			Average			Interest
				Principal			Reserve			Carrying Value			Income
				Balance						For 3 Months			Recognized
										Ending 3/31/15			For 3 Months
													Ending 3/31/15
	(Dollars in thousands)
With no required reserve recorded:
Construction and land development	$	—		$	—		$	—		$	—		$	—
Commercial and industrial		63			105			—			39			—
Municipal		—			—			—			—			—
Commercial real estate		392			396			—			393			—
Residential real estate		132			215			—			134			2
Consumer		—			—			—			—			—
Home equity		—			—			—			—			—
Total	$	587		$	716		$	—		$	566		$	2
With required reserve recorded:
Construction and land development	$	102		$	108		$	20		$	103		$	—
Commercial and industrial		808			1,010			96			821			11
Municipal		—			—			—			—			—
Commercial real estate		3,912			4,006			631			3,918			16
Residential real estate		820			820			90			823			6
Consumer		—			—			—			—			—
Home equity		92			92			92			92			—
Total	$	5,734		$	6,036		$	929		$	5,757		$	33
Total:
Construction and land development	$	102		$	108		$	20		$	103		$	—
Commercial and industrial		871			1,115			96			860			11
Municipal		—			—			—			—			—
Commercial real estate		4,304			4,402			631			4,311			16
Residential real estate		952			1,035			90			957			8
Consumer		—			—			—			—			—
Home equity		92			92			92			92			—
Total	$	6,321		$	6,752		$	929		$	6,323		$	35
The following is information pertaining to impaired loans for March 31, 2014:
	Carrying Value			Unpaid			Required			Average			Interest
				Principal			Reserve			Carrying Value			Income
				Balance						For 3 Months			Recognized
										Ending 3/31/14			For 3 Months
													Ending 3/31/14
	(Dollars in thousands)
With no required reserve recorded:
Construction and land development	$	250		$	3,292		$	—		$	437		$	—
Commercial and industrial		11			42			—			116			—
Municipal		—			—			—			—			—
Commercial real estate		55			56			—			41			—
Residential real estate		—			—			—			164			—
Consumer		—			—			—			—			—
Home equity		—			—			—			—			—
Total	$	316		$	3,390		$	—			758		$	—
With required reserve recorded:
Construction and land development	$	108		$	108		$	25			108		$	—
Commercial and industrial		1,118			1,352			358			1,116			8
Municipal		—			—			—			—			—
Commercial real estate		4,353			4,442			382			4,420			32
Residential real estate		998			1,082			122			949			1
Consumer		—			—			—			—			—
Home equity		94			94			94			94			—
Total	$	6,671		$	7,078		$	981		$	6,687		$	41
Total:
Construction and land development	$	358		$	3,400		$	25			545		$	—
Commercial and industrial		1,129			1,394			358			1,232			8
Municipal		—			—			—			—			—
Commercial real estate		4,408			4,498			382			4,461			32
Residential real estate		998			1,082			122			1,113			1
Consumer		—			—			—			—			—
Home equity		94			94			94			94			—
Total	$	6,987		$	10,468		$	981		$	7,445		$	41
There were no troubled debt restructurings occurring during the three month periods ended March 31, 2015 and March 31, 2014.

Reclassifications_Out_of_Accum

Reclassifications Out of Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Reclassifications Out of Accumulated Other Comprehensive Income	Note 6. Reclassifications Out of Accumulated Other Comprehensive Income (a)
	Amount Reclassified from Accumulated Other Comprehensive Income
	Details about Accumulated		Three Months Ended			Three Months Ended			Affected Line Item
			March 31, 2015			March 31, 2014
	Other Comprehensive								in the Statement
	Income Components								Where Net Income
									is Presented
			(in thousands)
	Accretion of unrealized losses transferred
			$	1,129		$	1,507		Securities held-to-maturity
				(394	)		(583	)	Provision for income taxes
			$	735		$	924		Net income
	Amortization of defined benefit pension items
	Prior-service costs		$	(3	)(b)	$	(3	)(b)	Salaries and employee benefits
	Actuarial gains (losses)			(352	)(b)		(92	)(b)	Salaries and employee benefits
	Total before tax			(355	)		(95	)	Income before taxes
	Tax (expense) or benefit			142			38		Provision for income taxes
	Net of tax		$	(213	)	$	(57	)	Net income
	Total reclassifications for the period		$	(522	)	$	867		Net income, net of tax
	(a)	Amount 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 8) for additional details).

Earnings_per_Share_EPS

Earnings per Share ("EPS")	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Earnings per Share ("EPS")	Note 7. 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 2014 was an increase of 1,534 shares. There were no stock options outstanding during the first quarter of 2015.
	The following table is a reconciliation of basic EPS and diluted EPS for the three months ended March 31,
		2015		2014
	Basic EPS Computation:
	(in thousands except share and per share data)
	Numerator:
	Net income, Class A	$	3,895	$	3,867
	Net income, Class B		1,064		1,065
	Denominator:
	Weighted average shares outstanding, Class A		3,600,729		3,582,421
	Weighted average shares outstanding, Class B		1,967,180		1,974,180
	Basic EPS, Class A	$	1.08	$	1.08
	Basic EPS, Class B		0.54		0.54
	Diluted EPS Computation:
	Numerator:
	Net income, Class A	$	3,895	$	3,867
	Net income, Class B		1,064		1,065
	Total net income, for diluted EPS, Class A computation		4,959		4,932
	Denominator:
	Weighted average shares outstanding, basic, Class A		3,600,729		3,582,421
	Weighted average shares outstanding, Class B		1,967,180		1,974,180
	Dilutive effect of Class A stock options		—		1,576
	Weighted average shares outstanding diluted, Class A		5,567,909		5,558,177
	Weighted average shares outstanding, Class B		1,967,180		1,974,180
	Diluted EPS, Class A	$	0.89	$	0.89
	Diluted EPS, Class B		0.54		0.54

Employee_Benefits

Employee Benefits	3 Months Ended
	Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Employee Benefits	Note 8. Employee Benefits
	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 and its subsidiaries who have at least one year of service may participate in the Supplemental Plan. The Supplemental Plan is voluntary and participants are required to contribute to its cost. Life insurance policies, which are owned by the Company, are purchased covering the lives of each participant.
	Components of Net Periodic Benefit Cost (Credit) for the Three Months Ended March 31,
		Pension Benefits						Supplemental Insurance/
								Retirement Plan
		2015			2014			2015		2014
		(In thousands)
	Service cost	$	336		$	258		$	397	$	389
	Interest		394			367			341		331
	Expected return on plan assets		(688	)		(636	)		—		—
	Recognized prior service cost (benefit)		(26	)		(26	)		29		29
	Recognized net actuarial losses		203			3			150		88
	Net periodic benefit (credit) cost	$	219		$	(34	)	$	917	$	837
	Contributions
	The company intends to contribute $2,000,000 to the Pension Plan in 2015. As of March 31, 2015, $500,000 has been contributed.

Fair_Value_Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements	Note 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 – Instruments that 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 March 31, 2015, are as follows:
	Financial Instruments Measured at Fair Value on a Recurring Basis:
			Securities AFS Fair Value Measurements Using
			Carrying		Quoted Prices			Significant		Significant
			Value		In Active			Observable		Other
					Markets for			Inputs		Unobservable
					Identical			(Level 2)		Inputs
					Assets					(Level 3)
					(Level 1)
			(In thousands)
	U.S. Treasury		$	2,000	$	—		$	2,000	$	—
	SBA Backed Securities			6,570		—			6,570		—
	U.S. Government Agency and Sponsored Mortgage Backed Securities			322,877		—			322,877		—
	Privately Issued Residential Mortgage Backed Securities			1,803		—			1,803		—
	Obligations Issued by States and Political Subdivisions			118,773		—			—		118,773
	Other Debt Securities			3,511		—			3,511		—
	Equity Securities			397		295			—		102
	Total		$	455,931	$	295		$	336,761	$	118,875
	Financial Instruments Measured at Fair Value on a Non-recurring Basis:
	Impaired Loans			1,223		—			—		1,223
	Impaired loan balances 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 the three-month period ended March 31, 2015 amounted to $37,000.
	There were no transfers between level 1 and 2 for the three months ended March 31, 2015. There were no liabilities measured at fair value on a recurring or nonrecurring basis during the three month period ended March 31, 2015.
	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). 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(4)		$	118,875	Discounted cash flow		Discount rate	0% -1% (3)
	Impaired Loans			1,223	Appraisal of collateral (1)		Appraisal adjustments (2)	0%-30% discount
	-1	Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various Level 3 inputs which are not identifiable.
	-2	Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated expenses.
	-3	Weighted averages.
	-4	Municipal securities generally have maturities of one year or less and, therefore, the amortized cost equates to the fair value.
	The changes in Level 3 securities for the three-month period ended March 31, 2015 are shown in the table below:
			Auction		Obligations			Equity		Total
			Rate		Issued by States			Securities
			Securities		& Political
					Subdivisions
			(In thousands)
	Balance at December 31, 2014		$	3,820	$	92,964		$	102	$	96,886
	Purchases			—		41,909			—		41,909
	Maturities and calls			—		(19,914	)		—		(19,914	)
	Amortization			—		(6	)		—		(6	)
	Changes in fair value			—		—			—		—
	Balance at March 31, 2015		$	3,820	$	114,953		$	102	$	118,875
	The amortized cost of Level 3 securities was $119,750,000 at March 31, 2015 with an unrealized loss of $875,000. 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 changes in Level 3 securities for the three-month period ended March 31, 2014, are shown in the table below:
			Auction		Obligations			Equity		Total
			Rate		Issued by States			Securities
			Securities		& Political
					Subdivisions
			(In thousands)
	Balance at December 31, 2013		$	3,820	$	32,487		$	290	$	36,597
	Purchases			—		7,205			—		7,205
	Maturities and calls			—		(10,717	)		—		(10,717	)
	Amortization			—		(5	)		—		(5	)
	Changes in fair value			—		—			—		—
	Balance at March 31, 2014		$	3,820	$	28,970		$	290	$	33,080
	The amortized cost of Level 3 securities was $33,951,000 at March 31, 2014 with an unrealized loss of $870,000. 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, 2014, are as follows:
Financial Instruments Measured at Fair Value on a Recurring Basis:
				Securities AFS Fair Value Measurements Using
		Carrying		Quoted Prices			Significant		Significant
		Value		In Active			Observable		Other
				Markets for			Inputs		Unobservable
				Identical			(Level 2)		Inputs
				Assets					(Level 3)
				(Level 1)
		(In thousands)
U.S. Treasury		$	2,000	$	—		$	2,000	$	—
U.S. Government Sponsored Enterprises			—		—			—		—
SBA Backed Securities			6,717		—			6,717		—
U.S. Government Agency and Sponsored Mortgage Backed Securities			337,093		—			337,093		—
Privately Issued Residential Mortgage Backed Securities			1,874		—			1,874		—
Obligations Issued by States and Political Subdivisions			96,784		—			—		96,784
Other Debt Securities			3,524		—			3,524		—
Equity Securities			398		296			—		102
Total		$	448,390	$	296		$	351,208	$	96,886
Financial Instruments Measured at Fair Value on a Non-recurring Basis:
Impaired Loans			3,410		—			—		3,410
Impaired loan balances 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 the period ended 2014 for the estimated credit loss amounted to $947,000.
There were no transfers between level 1 and 2 for the three months ended December 31, 2014. There were no liabilities measured at fair value on a recurring or nonrecurring basis during the year ended December 31, 2014.
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). Management continues to monitor the assumptions used to value the assets listed below.
Asset		Fair		Valuation Technique		Unobservable Input	Unobservable Input
		Value
							Value or Range
Securities AFS(4)		$	96,886	Discounted cash flow		Discount rate	0% -1% (3)
Impaired Loans			3,410	Appraisal of collateral (1)		Appraisal adjustments (2)	0%-30% discount
-1	Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various Level 3 inputs which are not identifiable.
-2	Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated expenses.
-3	Weighted averages
-4	Municipal securities generally have maturities of one year or less and, therefore, the amortized cost equates to the fair value.

Fair_Values_of_Financial_Instr

Fair Values of Financial Instruments	3 Months Ended
	Mar. 31, 2015
Investments, All Other Investments [Abstract]
Fair Values of Financial Instruments	Note 10. 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 nonfinancial 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 March 31, 2015 and December 31, 2014. 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.
							Fair Value Measurements
			Carrying Amount		Estimated Fair		Level 1 Inputs		Level 2 Inputs		Level 3 Inputs
					Value
	March 31, 2015
	(in thousands)
	Financial assets:
	Securities held-to-maturity		$	1,773,607	$	1,796,688	$	—	$	1,796,688	$	—
	Loans (1)			1,318,801		1,299,012		—		—		1,299,012
	Financial liabilities:
	Time deposits			382,456		386,759		—		386,759		—
	Other borrowed funds			307,642		314,756		—		314,756		—
	Subordinated debentures			36,083		36,083		—		—		36,083
	December 31, 2014
	Financial assets:
	Securities held-to-maturity			1,406,792		1,413,603		—		1,413,603		—
	Loans (1)			1,309,048		1,291,550		—		—		1,291,550
	Financial liabilities:
	Time deposits			383,145		387,919		—		387,919		—
	Other borrowed funds			395,500		400,196		—		400,196		—
	Subordinated debentures			36,083		36,083		—		—		36,083
	-1	Comprised of loans (including collateral dependent impaired loans), net of deferred loan costs and the allowance for loan losses.

Recent_Accounting_Developments

Recent Accounting Developments	3 Months Ended
	Mar. 31, 2015
Accounting Changes and Error Corrections [Abstract]
Recent Accounting Developments	Note 11. Recent Accounting Developments
	In January 2014, the FASB issued ASU 2014-04, Receivables-Troubled Debt Restructurings by Creditors (Subtopic 310-40) Reclassification of Residential Real Estate Collateralized Consumer Mortgage Loans upon Foreclosure. The amendments in this update clarify that an in substance repossession or foreclosure occurs, and a creditor is considered to have received physical possession of residential real estate property collateralizing a consumer mortgage loan, upon either (1) the creditor obtaining legal title to the residential real estate property upon completion of a foreclosure or (2) the borrower conveying all interest in the residential real estate property to the creditor to satisfy that loan through completion of a deed in lieu of foreclosure or through a similar legal agreement. The amendments in this update are effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2014. The Company has assessed the impact of ASU 2014-04 and the adoption of this amendment did not have a material impact on the Company’s financial statements.
	In June 2014, the FASB issued ASU 2014-11, Transfers and Servicing (Topic 860) Repurchase-to-Maturity Transactions, Repurchase Financings, and Disclosures. This ASU eliminates sale accounting for repurchase-to-maturity transactions and supersedes the guidance under which a transfer of a financial asset and a contemporaneous repurchase financing could be accounted for on a combined basis as a forward agreement. In addition, the ASU requires a new disclosure for transactions economically similar to repurchase agreements in which the transferor retains substantially all of the exposure to the economic return on the transferred financial assets throughout the term of the transaction. The ASU is effective for annual periods beginning after December 15, 2014 and interim periods beginning after December 15, 2015; early application is not permitted. The Company assessed the impact of ASU 2014-11 and the adoption of this amendment did not have a material impact on the Company’s financial statements.
	In August 2014, the FASB issued ASU 2014-14, Receivables—Troubled Debt Restructurings by Creditors (Subtopic 310-40) Classification of Certain Government-Guaranteed Mortgage Loans upon Foreclosure. This ASU which will require creditors to derecognize certain foreclosed government-guaranteed mortgage loans and to recognize a separate other receivable that is measured at the amount the creditor expects to recover from the guarantor, and to treat the guarantee and the receivable as a single unit of account. ASU 2014-14 is effective for public business entities for annual periods, and interim periods within those annual periods, beginning after December 15, 2014. For entities other than public business entities, the ASU is effective for annual periods ending after December 15, 2015, and interim periods beginning after December 15, 2015. An entity can elect a prospective or a modified retrospective transition method, but must use the same transition method that it elected under FASB ASU No. 2014-04, Reclassification of Residential Real Estate Collateralized Consumer Mortgage Loans upon Foreclosure. Early adoption, including adoption in an interim period, is permitted if the entity already adopted ASU 2014-04. The Company has assessed the impact of ASU 2014-14 and the adoption of this amendment did not have a material impact on the Company’s financial statements.
	In August 2014, the FASB issued ASU 2014-15, (Subtopic 205-40) Disclosures of Uncertainties about an Entity’s Ability to Continue as a Going Concern. This ASU provides guidance on determining when and how reporting entities must disclose going concern uncertainties in their financial statements. The new standard requires management to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date of issuance of the entity’s financial statements. Further, an entity must provide certain disclosures if there is “substantial doubt about the entity’s ability to continue as a going concern.” The ASU is effective for interim and annual periods beginning after December 15, 2016; early application is permitted. The Company has chosen not to early adopt ASU 2014-15. Management does not expect a material impact, if any, as of March 31, 2015.
	In November 2014, the FASB issued ASU 2014-17, “Business Combinations”. (Topic 805): Pushdown Accounting (a consensus of the FASB Emerging Issues Task Force). This ASU was issued to provide an acquired entity with an option to apply pushdown accounting in its separate financial statements upon occurrence of an event in which an acquirer obtains control of the acquired entity. An acquired entity may elect the option to apply pushdown accounting in the reporting period in which the change-in-control event occurs. An acquired entity should determine whether to elect to apply pushdown accounting for each individual change-in-control event in which an acquirer obtains control of the acquired entity. If pushdown accounting is not applied in the reporting period in which the change-in-control event occurs, an acquired entity will have the option to elect to apply pushdown accounting in a subsequent reporting period to the acquired entity’s most recent change-in-control event. An election to apply pushdown accounting in a reporting period in which the change-in-control event occurred should be considered a change in accounting principle in accordance with Topic 250, Accounting Changes and Error Corrections. If pushdown accounting is applied to an individual change-in-control event, that election is irrevocable. The amendments in this Update are effective on November 18, 2014. The adoption of this standard did not have a material impact on the Company’s financial statements.
	In January 2015, the FASB issued ASU 2015-01, “Income Statement-Extraordinary and Unusual” (Subtopic 225-20): Simplifying Income Statement Presentation by Eliminating the Concept of Extraordinary Items. This ASU eliminates from GAAP the concept of extraordinary items. This Update will align more closely GAAP income statement presentation guidance with International Audit Standards (IAS) 1, Presentation of Financial Statements, which prohibits the presentation and disclosure of extraordinary items. The amendments in this update are effective for fiscal years, and interim periods within those fiscal years, beginning after December  15, 2015. The adoption of this standard is not expected to have a material impact on the Company’s financial statements.

Securities_AvailableforSale_Ta

Securities Available-for-Sale (Tables)	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Summary of Securities Available-for-Sale		March 31, 2015									December 31, 2014
			Amortized		Gross		Gross		Fair			Amortized		Gross		Gross		Fair
			Cost		Unrealized		Unrealized		Value			Cost		Unrealized		Unrealized		Value
					Gains		Losses							Gains		Losses
			( In thousands)
	U.S. Treasury		$	2,000	$	—	$	—	$	2,000		$	1,999	$	1	$	—	$	2,000
	Small Business Administration			6,538		32		—		6,570			6,684		33		—		6,717
	U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities			321,978		1,273		374		322,877			336,158		1,387		452		337,093
	Privately Issued Residential Mortgage Backed Securities			1,818		3		18		1,803			1,894		5		25		1,874
	Obligations Issued by States and Political Subdivisions			119,648		—		875		118,773			97,657		—		873		96,784
	Other Debt Securities			3,600		7		96		3,511			3,600		24		100		3,524
	Equity Securities			218		179		—		397			218		180		—		398
	Total		$	455,800	$	1,494	$	1,363	$	455,931		$	448,210	$	1,630	$	1,450	$	448,390
Estimated Maturity Distribution of Securities Available-for-Sale	The following table shows the maturity distribution of the Company’s securities available-for-sale at March 31, 2015.
			Amortized		Fair
			Cost		Value
			( In thousands)
	Within one year		$	115,350	$	115,336
	After one but within five years			198,396		199,214
	After five but within ten years			132,313		132,401
	More than 10 years			8,023		7,179
	Non-maturing			1,718		1,801
	Total		$	455,800	$	455,931
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 March 31, 2015. 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 4 and 14 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 256 holdings at March 31, 2015.
			March 31, 2015
			Less than 12 months				12 months or longer					Total
	Temporarily Impaired Investments		Fair Value		Unrealized		Fair Value		Unrealized			Fair Value		Unrealized
					Losses				Losses					Losses
			(In thousands)
	U.S. Government Sponsored Enterprises		$	—	$	—	$	—	$	—		$	—	$	—
	U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities			21,932		37		79,708		337			101,640		374
	Privately Issued Residential Mortgage Backed Securities			—		—		658		18			658		18
	Obligations Issued by States and Political Subdivisions			—		—		3,820		875			3,820		875
	Other Debt Securities			—		—		1,404		96			1,404		96
	Total temporarily impaired securities		$	21,932	$	37	$	85,590	$	1,326		$	107,522	$	1,363
	The following table shows the temporarily impaired securities of the Company’s available-for-sale portfolio at December 31, 2014. 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 14 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 262 holdings at December 31, 2014.
			December 31, 2014
			Less than 12 months				12 months or longer					Total
	Temporarily Impaired Investments		Fair Value		Unrealized		Fair Value		Unrealized			Fair Value		Unrealized
					Losses				Losses					Losses
			(In thousands)
	U.S. Government Sponsored Enterprises		$	—	$	—	$	—	$	—		$	—	$	—
	U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities			24,457		85		77,585		367			102,042		452
	Privately Issued Residential Mortgage Backed Securities			—		—		678		25			678		25
	Obligations Issued by States and Political Subdivisions			—		—		3,820		873			3,820		873
	Other Debt Securities			—		—		1,400		100			1,400		100
	Total temporarily impaired securities		$	24,457	$	85	$	83,483	$	1,365		$	107,940	$	1,450

Investment_Securities_HeldtoMa1

Investment Securities Held-to-Maturity (Tables)	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Summary of Held-to-Maturity Securities
		March 31, 2015								December 31, 2014
		Amortized		Gross		Gross		Estimated		Amortized		Gross		Gross		Estimated
		Cost		Unrealized		Unrealized		Fair		Cost		Unrealized		Unrealized		Fair
				Gains		Losses		Value				Gains		Losses		Value
		(In thousands)
	U.S. Government Sponsored Enterprises	$	362,706	$	5,315	$	65	$	367,956	$	251,617	$	2,707	$	249	$	254,075
	U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities		1,410,901		20,182		2,351		1,428,732		1,155,175		11,185		6,832		1,159,528
	Total	$	1,773,607	$	25,497	$	2,416	$	1,796,688	$	1,406,792	$	13,892	$	7,081	$	1,413,603
Company's Securities Held-to-Maturity	The following table shows the maturity distribution of the Company’s securities held-to-maturity at March 31, 2015.
		Amortized		Fair
		Cost		Value
		( In thousands)
	Within one year	$	3,842	$	3,908
	After one but within five years		1,419,300		1,434,605
	After five but within ten years		346,515		354,165
	More than ten years		3,950		4,010
	Total	$	1,773,607	$	1,796,688
Unrealized Market Loss of Securities	The following table shows the temporarily impaired securities of the Company’s held-to-maturity portfolio at March 31, 2015. 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 35 and 19 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 357 holdings at March 31, 2015.
		March 31, 2015
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair Value		Unrealized		Fair Value		Unrealized
				Losses				Losses				Losses
		(Dollars in thousands)
	U.S. Government Sponsored Enterprises	$	49,452	$	65	$	—	$	—	$	49,452	$	65
	U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities		180,629		986		109,697		1,365		290,326		2,351
	Total temporarily impaired securities	$	230,081	$	1,051	$	109,697	$	1,365	$	339,778	$	2,416
	The following table shows the temporarily impaired securities of the Company’s held-to-maturity portfolio at December 31, 2014. 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 34 and 48 securities that are temporarily impaired for less than 12 months and for 12 months or longer, respectively, out of a total of 303 holdings at December 31, 2014.
		December 31, 2014
		Less Than 12 Months				12 Months or Longer				Total
	Temporarily Impaired Investments	Fair Value		Unrealized		Fair Value		Unrealized		Fair Value		Unrealized
				Losses				Losses				Losses
		( In thousands)
	U.S. Government Sponsored Enterprises	$	22,414	$	25	$	14,776	$	224	$	37,190	$	249
	U.S. Government Agency and Sponsored Enterprise Mortgage-Backed Securities		194,119		1,678		308,526		5,154		502,645		6,832
	Total temporarily impaired securities	$	216,533	$	1,703	$	323,302	$	5,378	$	539,835	$	7,081

Allowance_for_Loan_Losses_Tabl

Allowance for Loan Losses (Tables)	3 Months Ended
	Mar. 31, 2015
Receivables [Abstract]
Summary of Changes in Allowance for Loan Losses	The following table summarizes the changes in the Company’s allowance for loan losses for the periods indicated.
		Three months ended
		March 31,
		2015			2014
		(in thousands)
	Allowance for loan losses, beginning of	$	22,318		$	20,941
	period
	Loans charged off		(81	)		(429	)
	Recoveries on loans previously		92			147
	charged-off
	Net recoveries (charge-offs)		11			(282	)
	Provision charged to expense		200			600
	Allowance for loan losses, end of period	$	22,529		$	21,259
Summary of Allowance for Loan Losses	Further information pertaining to the allowance for loan losses for the three months ending March 31, 2015 follows:
		Construction			Commercial			Municipal			Commercial			Residential			Consumer			Home			Unallocated		Total
		and Land			and						Real			Real						Equity
		Development			Industrial						Estate			Estate
		(Dollars in thousands)
	Allowance for loan losses:
	Balance at December 31, 2014	$	1,592		$	4,758		$	1,488		$	11,199		$	775		$	810		$	599		$	1,097	$	22,318
	Charge-offs		—			—			—			—			—			(81	)		—			—		(81	)
	Recoveries		—			15			—			2			2			73			—			—		92
	Provision		292			(128	)		(115	)		(74	)		22			(88	)		15			276		200
	Ending balance at March 31, 2015	$	1,884		$	4,645		$	1,373		$	11,127		$	799		$	714		$	614		$	1,373	$	22,529
	Amount of allowance for loan losses for loans deemed to be impaired	$	20		$	96		$	—		$	631		$	90		$	—		$	92		$	—		929
	Amount of allowance for loan losses for loans not deemed to be impaired	$	1,864		$	4,549		$	1,373		$	10,496		$	709		$	714		$	522		$	1,373	$	21,600
	Loans:
	Ending balance	$	25,347		$	157,637		$	41,406		$	691,811		$	258,558		$	10,508		$	156,063		$	—	$	1,341,330
	Loans deemed to be impaired	$	102		$	871		$	—		$	4,304		$	952		$	—		$	92		$	—	$	6,321
	Loans not deemed to be impaired	$	25,245		$	156,766		$	41,406		$	687,507		$	257,606		$	10,508		$	155,971		$	—	$	1,335,009
	Further information pertaining to the allowance for loan losses for the three months ending March 31, 2014 follows:
		Construction			Commercial			Municipals			Commercial			Residential			Consumer			Home			Unallocated		Total
		and Land			and						Real			Real						Equity
		Development			Industrial						Estate			Estate
		(Dollars in thousands)
	Allowance for loan losses:
	Balance at December 31, 2013	$	2,174		$	2,617		$	655		$	10,935		$	2,006		$	432		$	959		$	1,163	$	20,941
	Charge-offs		(250	)		—			—			—			—			(179	)		—			—		(429	)
	Recoveries		—			24			—			1			5			116			1			—		147
	Provision		(99	)		(48	)		149			631			(138	)		56			(58	)		107		600
	Ending balance at	$	1,825		$	2,593		$	804		$	11,567		$	1,873		$	425		$	902		$	1,270	$	21,259
	March 31, 2014
	Amount of allowance for loan losses for loans deemed to be impaired	$	25		$	358		$	—		$	382		$	122		$	—		$	94		$	—		981
	Amount of allowance for loan losses for loans not deemed to be impaired	$	1,800		$	2,235		$	804		$	11,185		$	1,751		$	425		$	808		$	1,270	$	20,278
	Loans:
	Ending balance	$	25,928		$	80,478		$	32,307		$	691,541		$	290,465		$	9,542		$	133,800		$	—	$	1,264,061
	Loans deemed to be impaired	$	358		$	1,129		$	—		$	4,408		$	998		$	—		$	94		$	—	$	6,987
	Loans not deemed to be impaired	$	25,570		$	79,349		$	32,307		$	687,133		$	289,467		$	9,542		$	133,706		$	—	$	1,257,074
Loans by Risk Rating	The following table presents the Company’s loans by risk rating at March 31, 2015.
		Construction			Commercial			Municipal			Commercial
		and land			and						real
		development			industrial						estate
		(Dollars in thousands)
	Grade:
	1-3 (Pass)	$	18,170		$	156,294		$	41,406		$	686,883
	4 (Monitor)		7,075			472			—			624
	5 (Substandard)		—			—			—			—
	6 (Doubtful)		—			—			—			—
	Impaired		102			871			—			4,304
	Total	$	25,347		$	157,637		$	41,406		$	691,811
	The following table presents the Company’s loans by risk rating at December 31, 2014.
		Construction			Commercial			Municipal			Commercial
		and land			and						real
		development			industrial						estate
		(Dollars in thousands)
	Grade:
	1-3 (Pass)	$	15,515		$	148,407		$	41,850		$	691,322
	4 (Monitor)		7,126			472			—			633
	5 (Substandard)		—			—			—			—
	6 (Doubtful)		—			—			—			—
	Impaired		103			853			—			4,317
	Total	$	22,744		$	149,732		$	41,850		$	696,272
Aging of Past Due Loan Losses	Further information pertaining to the allowance for loan losses at March 31, 2015 follows:
		Accruing			Non Accrual			Accrual			Total			Current			Total
		30-89 Days						Greater			Past Due			Loans
		Past Due						Than
								90 Days
		(Dollars in thousands)
	Construction and land development	$	—		$	102		$	—		$	102		$	25,245		$	25,347
	Commercial and industrial		173			209			—			382			157,255			157,637
	Municipal		—			—			—			—			41,406			41,406
	Commercial real estate		1,154			2,775			—			3,929			687,882			691,811
	Residential real estate		698			632			—			1,330			257,228			258,558
	Consumer and overdrafts		6			4			—			10			10,498			10,508
	Home equity		733			538			34			1,305			154,758			156,063
	Total	$	2,764		$	4,260		$	34		$	7,058		$	1,334,272		$	1,341,330
	Further information pertaining to the allowance for loan losses at December 31, 2014 follows:
		Accruing			Non Accrual			Accrual			Total			Current			Total
		30-89 Days						Greater			Past Due			Loans
		Past Due						Than
								90 Days
		(Dollars in thousands)
	Construction and land development	$	—		$	103		$	—		$	103		$	22,641		$	22,744
	Commercial and industrial		905			157			—			1,062			148,670			149,732
	Municipal		—			—			—			—			41,850			41,850
	Commercial real estate		1,046			2,781			—			3,827			692,445			696,272
	Residential real estate		632			846			—			1,478			255,827			257,305
	Consumer and overdrafts		6			5			—			11			12,177			12,188
	Home equity		576			254			—			830			150,445			151,275
	Total	$	3,165		$	4,146		$	—		$	7,311		$	1,324,055		$	1,331,366
Information Pertaining to Impaired Loans	The following is information pertaining to impaired loans for March 31, 2015:
		Carrying Value			Unpaid			Required			Average			Interest
					Principal			Reserve			Carrying Value			Income
					Balance						For 3 Months			Recognized
											Ending 3/31/15			For 3 Months
														Ending 3/31/15
		(Dollars in thousands)
	With no required reserve recorded:
	Construction and land development	$	—		$	—		$	—		$	—		$	—
	Commercial and industrial		63			105			—			39			—
	Municipal		—			—			—			—			—
	Commercial real estate		392			396			—			393			—
	Residential real estate		132			215			—			134			2
	Consumer		—			—			—			—			—
	Home equity		—			—			—			—			—
	Total	$	587		$	716		$	—		$	566		$	2
	With required reserve recorded:
	Construction and land development	$	102		$	108		$	20		$	103		$	—
	Commercial and industrial		808			1,010			96			821			11
	Municipal		—			—			—			—			—
	Commercial real estate		3,912			4,006			631			3,918			16
	Residential real estate		820			820			90			823			6
	Consumer		—			—			—			—			—
	Home equity		92			92			92			92			—
	Total	$	5,734		$	6,036		$	929		$	5,757		$	33
	Total:
	Construction and land development	$	102		$	108		$	20		$	103		$	—
	Commercial and industrial		871			1,115			96			860			11
	Municipal		—			—			—			—			—
	Commercial real estate		4,304			4,402			631			4,311			16
	Residential real estate		952			1,035			90			957			8
	Consumer		—			—			—			—			—
	Home equity		92			92			92			92			—
	Total	$	6,321		$	6,752		$	929		$	6,323		$	35
	The following is information pertaining to impaired loans for March 31, 2014:
		Carrying Value			Unpaid			Required			Average			Interest
					Principal			Reserve			Carrying Value			Income
					Balance						For 3 Months			Recognized
											Ending 3/31/14			For 3 Months
														Ending 3/31/14
		(Dollars in thousands)
	With no required reserve recorded:
	Construction and land development	$	250		$	3,292		$	—		$	437		$	—
	Commercial and industrial		11			42			—			116			—
	Municipal		—			—			—			—			—
	Commercial real estate		55			56			—			41			—
	Residential real estate		—			—			—			164			—
	Consumer		—			—			—			—			—
	Home equity		—			—			—			—			—
	Total	$	316		$	3,390		$	—			758		$	—
	With required reserve recorded:
	Construction and land development	$	108		$	108		$	25			108		$	—
	Commercial and industrial		1,118			1,352			358			1,116			8
	Municipal		—			—			—			—			—
	Commercial real estate		4,353			4,442			382			4,420			32
	Residential real estate		998			1,082			122			949			1
	Consumer		—			—			—			—			—
	Home equity		94			94			94			94			—
	Total	$	6,671		$	7,078		$	981		$	6,687		$	41
	Total:
	Construction and land development	$	358		$	3,400		$	25			545		$	—
	Commercial and industrial		1,129			1,394			358			1,232			8
	Municipal		—			—			—			—			—
	Commercial real estate		4,408			4,498			382			4,461			32
Residential real estate		998			1,082			122			1,113			1
Consumer		—			—			—			—			—
Home equity		94			94			94			94			—
Total	$	6,987		$	10,468		$	981		$	7,445		$	41

Reclassifications_Out_of_Accum1

Reclassifications Out of Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2015
Text Block [Abstract]
Reclassifications Out of Accumulated Other Comprehensive Income	Amount Reclassified from Accumulated Other Comprehensive Income
	Details about Accumulated		Three Months Ended			Three Months Ended			Affected Line Item
			March 31, 2015			March 31, 2014
	Other Comprehensive								in the Statement
	Income Components								Where Net Income
									is Presented
			(in thousands)
	Accretion of unrealized losses transferred
			$	1,129		$	1,507		Securities held-to-maturity
				(394	)		(583	)	Provision for income taxes
			$	735		$	924		Net income
	Amortization of defined benefit pension items
	Prior-service costs		$	(3	)(b)	$	(3	)(b)	Salaries and employee benefits
	Actuarial gains (losses)			(352	)(b)		(92	)(b)	Salaries and employee benefits
	Total before tax			(355	)		(95	)	Income before taxes
	Tax (expense) or benefit			142			38		Provision for income taxes
	Net of tax		$	(213	)	$	(57	)	Net income
	Total reclassifications for the period		$	(522	)	$	867		Net income, net of tax
	(a)	Amount 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 8) for additional details).

Earnings_per_Share_EPS_Tables

Earnings per Share ("EPS") (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Reconciliation of Basic EPS and Diluted EPS	The following table is a reconciliation of basic EPS and diluted EPS for the three months ended March 31,
		2015		2014
	Basic EPS Computation:
	(in thousands except share and per share data)
	Numerator:
	Net income, Class A	$	3,895	$	3,867
	Net income, Class B		1,064		1,065
	Denominator:
	Weighted average shares outstanding, Class A		3,600,729		3,582,421
	Weighted average shares outstanding, Class B		1,967,180		1,974,180
	Basic EPS, Class A	$	1.08	$	1.08
	Basic EPS, Class B		0.54		0.54
	Diluted EPS Computation:
	Numerator:
	Net income, Class A	$	3,895	$	3,867
	Net income, Class B		1,064		1,065
	Total net income, for diluted EPS, Class A computation		4,959		4,932
	Denominator:
	Weighted average shares outstanding, basic, Class A		3,600,729		3,582,421
	Weighted average shares outstanding, Class B		1,967,180		1,974,180
	Dilutive effect of Class A stock options		—		1,576
	Weighted average shares outstanding diluted, Class A		5,567,909		5,558,177
	Weighted average shares outstanding, Class B		1,967,180		1,974,180
	Diluted EPS, Class A	$	0.89	$	0.89
	Diluted EPS, Class B		0.54		0.54

Employee_Benefits_Tables

Employee Benefits (Tables)	3 Months Ended
	Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Components of Net Periodic Benefit Cost (Credit)	Components of Net Periodic Benefit Cost (Credit) for the Three Months Ended March 31,
		Pension Benefits						Supplemental Insurance/
								Retirement Plan
		2015			2014			2015		2014
		(In thousands)
	Service cost	$	336		$	258		$	397	$	389
	Interest		394			367			341		331
	Expected return on plan assets		(688	)		(636	)		—		—
	Recognized prior service cost (benefit)		(26	)		(26	)		29		29
	Recognized net actuarial losses		203			3			150		88
	Net periodic benefit (credit) cost	$	219		$	(34	)	$	917	$	837

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015, are as follows:
	Financial Instruments Measured at Fair Value on a Recurring Basis:
			Securities AFS Fair Value Measurements Using
			Carrying		Quoted Prices			Significant		Significant
			Value		In Active			Observable		Other
					Markets for			Inputs		Unobservable
					Identical			(Level 2)		Inputs
					Assets					(Level 3)
					(Level 1)
			(In thousands)
	U.S. Treasury		$	2,000	$	—		$	2,000	$	—
	SBA Backed Securities			6,570		—			6,570		—
	U.S. Government Agency and Sponsored Mortgage Backed Securities			322,877		—			322,877		—
	Privately Issued Residential Mortgage Backed Securities			1,803		—			1,803		—
	Obligations Issued by States and Political Subdivisions			118,773		—			—		118,773
	Other Debt Securities			3,511		—			3,511		—
	Equity Securities			397		295			—		102
	Total		$	455,931	$	295		$	336,761	$	118,875
	Financial Instruments Measured at Fair Value on a Non-recurring Basis:
	Impaired Loans			1,223		—			—		1,223
	The results of the fair value hierarchy as of December 31, 2014, are as follows:
	Financial Instruments Measured at Fair Value on a Recurring Basis:
					Securities AFS Fair Value Measurements Using
			Carrying		Quoted Prices			Significant		Significant
			Value		In Active			Observable		Other
					Markets for			Inputs		Unobservable
					Identical			(Level 2)		Inputs
					Assets					(Level 3)
					(Level 1)
			(In thousands)
	U.S. Treasury		$	2,000	$	—		$	2,000	$	—
	U.S. Government Sponsored Enterprises			—		—			—		—
	SBA Backed Securities			6,717		—			6,717		—
	U.S. Government Agency and Sponsored Mortgage Backed Securities			337,093		—			337,093		—
	Privately Issued Residential Mortgage Backed Securities			1,874		—			1,874		—
	Obligations Issued by States and Political Subdivisions			96,784		—			—		96,784
	Other Debt Securities			3,524		—			3,524		—
	Equity Securities			398		296			—		102
	Total		$	448,390	$	296		$	351,208	$	96,886
	Financial Instruments Measured at Fair Value on a Non-recurring Basis:
	Impaired Loans			3,410		—			—		3,410
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). 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(4)		$	118,875	Discounted cash flow		Discount rate	0% -1% (3)
	Impaired Loans			1,223	Appraisal of collateral (1)		Appraisal adjustments (2)	0%-30% discount
	-1	Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various Level 3 inputs which are not identifiable.
	-2	Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated expenses.
	-3	Weighted averages.
	-4	Municipal securities generally have maturities of one year or less and, therefore, the amortized cost equates to the 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). Management continues to monitor the assumptions used to value the assets listed below.
	Asset		Fair		Valuation Technique		Unobservable Input	Unobservable Input
			Value
								Value or Range
	Securities AFS(4)		$	96,886	Discounted cash flow		Discount rate	0% -1% (3)
	Impaired Loans			3,410	Appraisal of collateral (1)		Appraisal adjustments (2)	0%-30% discount
	-1	Fair value is generally determined through independent appraisals of the underlying collateral, which generally include various Level 3 inputs which are not identifiable.
	-2	Appraisals may be adjusted by management for qualitative factors such as economic conditions and estimated expenses.
	-3	Weighted averages
	-4	Municipal securities generally have maturities of one year or less and, therefore, the amortized cost equates to the fair value.
Changes in Level 3 Securities	The changes in Level 3 securities for the three-month period ended March 31, 2015 are shown in the table below:
			Auction		Obligations			Equity		Total
			Rate		Issued by States			Securities
			Securities		& Political
					Subdivisions
			(In thousands)
	Balance at December 31, 2014		$	3,820	$	92,964		$	102	$	96,886
	Purchases			—		41,909			—		41,909
	Maturities and calls			—		(19,914	)		—		(19,914	)
	Amortization			—		(6	)		—		(6	)
	Changes in fair value			—		—			—		—
	Balance at March 31, 2015		$	3,820	$	114,953		$	102	$	118,875
	The changes in Level 3 securities for the three-month period ended March 31, 2014, are shown in the table below:
			Auction		Obligations			Equity		Total
			Rate		Issued by States			Securities
			Securities		& Political
					Subdivisions
			(In thousands)
	Balance at December 31, 2013		$	3,820	$	32,487		$	290	$	36,597
	Purchases			—		7,205			—		7,205
	Maturities and calls			—		(10,717	)		—		(10,717	)
	Amortization			—		(5	)		—		(5	)
	Changes in fair value			—		—			—		—
	Balance at March 31, 2014		$	3,820	$	28,970		$	290	$	33,080

Fair_Values_of_Financial_Instr1

Fair Values of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
Investments, All Other Investments [Abstract]
Carrying Amounts and Fair Values of Company's Financial Instruments	The following presents (in thousands) the carrying amount, estimated fair value, and placement in the fair value hierarchy of the Company’s financial instruments as of March 31, 2015 and December 31, 2014. 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.
							Fair Value Measurements
			Carrying Amount		Estimated Fair		Level 1 Inputs		Level 2 Inputs		Level 3 Inputs
					Value
	March 31, 2015
	(in thousands)
	Financial assets:
	Securities held-to-maturity		$	1,773,607	$	1,796,688	$	—	$	1,796,688	$	—
	Loans (1)			1,318,801		1,299,012		—		—		1,299,012
	Financial liabilities:
	Time deposits			382,456		386,759		—		386,759		—
	Other borrowed funds			307,642		314,756		—		314,756		—
	Subordinated debentures			36,083		36,083		—		—		36,083
	December 31, 2014
	Financial assets:
	Securities held-to-maturity			1,406,792		1,413,603		—		1,413,603		—
	Loans (1)			1,309,048		1,291,550		—		—		1,291,550
	Financial liabilities:
	Time deposits			383,145		387,919		—		387,919		—
	Other borrowed funds			395,500		400,196		—		400,196		—
	Subordinated debentures			36,083		36,083		—		—		36,083
	-1	Comprised of loans (including collateral dependent impaired loans), net of deferred loan costs and the allowance for loan losses.

Basis_of_Financial_Statement_P1

Basis of Financial Statement Presentation - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2015
	Segment
Accounting Policies [Abstract]
Equity ownership interest	100.00%
Number of reportable segments	1

Recent_Market_Developments_Add

Recent Market Developments - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Recent Market Development [Line Items]
Common equity Tier 1 ratio as percentage of risk-weighted assets	4.50%
Minimum Tier 1 capital ratio as percentage of risk-weighted assets	4.00%
Revised minimum Tier 1 capital ratio as percentage of risk-weighted assets	6.00%
Tier 1 capital conservation buffer as percentage of risk-weighted assets	2.50%
Maximum [Member]
Recent Market Development [Line Items]
FDIC deposit insurance amount	250,000

Securities_AvailableforSale_Su

Securities Available-for-Sale - Summary of Securities Available-for-Sale (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$455,800	$448,210
Gross Unrealized Gains	1,494	1,630
Gross Unrealized Losses	1,363	1,450
Total, Fair Value	455,931	448,390
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,000	1,999
Gross Unrealized Gains		1
Total, Fair Value	2,000	2,000
Small Business Administration [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	6,538	6,684
Gross Unrealized Gains	32	33
Total, Fair Value	6,570	6,717
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	321,978	336,158
Gross Unrealized Gains	1,273	1,387
Gross Unrealized Losses	374	452
Total, Fair Value	322,877	337,093
Privately Issued Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,818	1,894
Gross Unrealized Gains	3	5
Gross Unrealized Losses	18	25
Total, Fair Value	1,803	1,874
Obligations Issued by States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	119,648	97,657
Gross Unrealized Losses	875	873
Total, Fair Value	118,773	96,784
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,600	3,600
Gross Unrealized Gains	7	24
Gross Unrealized Losses	96	100
Total, Fair Value	3,511	3,524
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	218	218
Gross Unrealized Gains	179	180
Total, Fair Value	$397	$398

Securities_AvailableforSale_Ad

Securities Available-for-Sale - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Sep. 30, 2013	Mar. 31, 2014	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale with an amortized cost		$1,012,370,000
Securities held-to-maturity at their fair value		987,037,000
Weighted average remaining life of investment securities available-for-sale	3 years 9 months 18 days
Federal Home Loan Bank of Boston [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale are securities pledged for borrowing	23,339,000			24,810,000
Realized gross gain	0		0
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	292,887,000			$301,038,000
Securities AFS [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities, temporarily impaired for less than 12 months	4			3
Number of securities, temporarily impaired for 12 months or longer	14			14
Number of securities, temporarily impaired, total	256			262

Securities_AvailableforSale_Ma

Securities Available-for-Sale - Maturity Distribution of Securities Available-for-Sale (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$115,350
After one but within five years, Amortized Cost	198,396
After five but within ten years, Amortized Cost	132,313
More than 10 years, Amortized Cost	8,023
Non-maturing, Amortized Cost	1,718
Amortized cost	455,800	448,210
Within one year, Fair Value	115,336
After one but within five years, Fair Value	199,214
After five but within ten years, Fair Value	132,401
More than 10 years, Fair Value	7,179
Non-maturing, Fair Value	1,801
Total, Fair Value	$455,931	$448,390

Securities_AvailableforSale_Co

Securities Available-for-Sale - Continuous Unrealized Loss Position for 12 Months or Less and 12 Months or Longer (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$21,932	$24,457
Less than 12 months, Unrealized Losses	37	85
12 months or longer, Fair Value	85,590	83,483
12 months or longer, Unrealized Losses	1,326	1,365
Total, Fair Value	107,522	107,940
Total, Unrealized Losses	1,363	1,450
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	21,932	24,457
Less than 12 months, Unrealized Losses	37	85
12 months or longer, Fair Value	79,708	77,585
12 months or longer, Unrealized Losses	337	367
Total, Fair Value	101,640	102,042
Total, Unrealized Losses	374	452
Privately Issued Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or longer, Fair Value	658	678
12 months or longer, Unrealized Losses	18	25
Total, Fair Value	658	678
Total, Unrealized Losses	18	25
Obligations Issued by States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or longer, Fair Value	3,820	3,820
12 months or longer, Unrealized Losses	875	873
Total, Fair Value	3,820	3,820
Total, Unrealized Losses	875	873
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or longer, Fair Value	1,404	1,400
12 months or longer, Unrealized Losses	96	100
Total, Fair Value	1,404	1,400
Total, Unrealized Losses	$96	$100

Investment_Securities_HeldtoMa2

Investment Securities Held-to-Maturity - Summary of Held-to-Maturity Securities (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$1,773,607	$1,406,792
Gross Unrealized Gains	25,497	13,892
Gross Unrealized Losses	2,416	7,081
Estimated Fair Value	1,796,688	1,413,603
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	362,706	251,617
Gross Unrealized Gains	5,315	2,707
Gross Unrealized Losses	65	249
Estimated Fair Value	367,956	254,075
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,410,901	1,155,175
Gross Unrealized Gains	20,182	11,185
Gross Unrealized Losses	2,351	6,832
Estimated Fair Value	$1,428,732	$1,159,528

Investment_Securities_HeldtoMa3

Investment Securities Held-to-Maturity - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
		Security
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life of investment securities held-to-maturity	4 years 3 months 18 days
Held-to-Maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Number of securities, temporarily impaired for less than 12 months	35	34
Number of securities, temporarily impaired for 12 months or longer	19	48
Number of securities, temporarily impaired, total	357	303
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	449,801,000	458,782,000
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	967,608,000	868,924,000
Weighted average remaining life	280,101,000

Investment_Securities_HeldtoMa4

Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$3,842
After one but within five years, Amortized Cost	1,419,300
After five but within ten years, Amortized Cost	346,515
More than ten years, Amortized Cost	3,950
Amortized Cost	1,773,607	1,406,792
Within one year, Fair Value	3,908
After one but within five years, Fair Value	1,434,605
After five but within ten years, Fair Value	354,165
More than ten years, Fair Value	4,010
Estimated Fair Value	$1,796,688	$1,413,603

Investment_Securities_HeldtoMa5

Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$230,081	$216,533
Less Than 12 Months, Unrealized Losses	1,051	1,703
12 Months or Longer, Fair Value	109,697	323,302
12 Months or Longer, Unrealized Losses	1,365	5,378
Total, Fair Value	339,778	539,835
Total, Unrealized Losses	2,416	7,081
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	49,452	22,414
Less Than 12 Months, Unrealized Losses	65	25
12 Months or Longer, Fair Value		14,776
12 Months or Longer, Unrealized Losses		224
Total, Fair Value	49,452	37,190
Total, Unrealized Losses	65	249
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	180,629	194,119
Less Than 12 Months, Unrealized Losses	986	1,678
12 Months or Longer, Fair Value	109,697	308,526
12 Months or Longer, Unrealized Losses	1,365	5,154
Total, Fair Value	290,326	502,645
Total, Unrealized Losses	$2,351	$6,832

Allowance_for_Loan_Losses_Summ

Allowance for Loan Losses - Summary of Changes in Allowance for Loan Losses (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Receivables [Abstract]
Allowance for loan losses, beginning of period	$22,318	$20,941
Loans charged off	-81	-429
Recoveries on loans previously charged-off	92	147
Net recoveries (charge-offs)	11	-282
Provision charged to expense	200	600
Allowance for loan losses, end of period	$22,529	$21,259

Allowance_for_Loan_Losses_Summ1

Allowance for Loan Losses - Summary of Allowance for Loan Losses (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Allowance for loan losses:
Allowance for loan losses, beginning of period	$22,318	$20,941
Charge-offs	-81	-429
Recoveries	92	147
Provision	200	600
Allowance for loan losses, end of period	22,529	21,259
Amount of allowance for loan losses for loans deemed to be impaired	929	981
Amount of allowance for loan losses for loans not deemed to be impaired	21,600	20,278
Loans:
Total loans, net	1,341,330	1,264,061	1,331,366
Loans deemed to be impaired	6,321	6,987
Loans not deemed to be impaired	1,335,009	1,257,074
Construction and Land Development [Member]
Allowance for loan losses:
Allowance for loan losses, beginning of period	1,592	2,174
Charge-offs		-250
Provision	292	-99
Allowance for loan losses, end of period	1,884	1,825
Amount of allowance for loan losses for loans deemed to be impaired	20	25
Amount of allowance for loan losses for loans not deemed to be impaired	1,864	1,800
Loans:
Total loans, net	25,347	25,928	22,744
Loans deemed to be impaired	102	358
Loans not deemed to be impaired	25,245	25,570
Commercial and Industrial [Member]
Allowance for loan losses:
Allowance for loan losses, beginning of period	4,758	2,617
Recoveries	15	24
Provision	-128	-48
Allowance for loan losses, end of period	4,645	2,593
Amount of allowance for loan losses for loans deemed to be impaired	96	358
Amount of allowance for loan losses for loans not deemed to be impaired	4,549	2,235
Loans:
Total loans, net	157,637	80,478	149,732
Loans deemed to be impaired	871	1,129
Loans not deemed to be impaired	156,766	79,349
Municipal [Member]
Allowance for loan losses:
Allowance for loan losses, beginning of period	1,488	655
Provision	-115	149
Allowance for loan losses, end of period	1,373	804
Amount of allowance for loan losses for loans not deemed to be impaired	1,373	804
Loans:
Total loans, net	41,406	32,307	41,850
Loans not deemed to be impaired	41,406	32,307
Commercial Real Estate [Member]
Allowance for loan losses:
Allowance for loan losses, beginning of period	11,199	10,935
Recoveries	2	1
Provision	-74	631
Allowance for loan losses, end of period	11,127	11,567
Amount of allowance for loan losses for loans deemed to be impaired	631	382
Amount of allowance for loan losses for loans not deemed to be impaired	10,496	11,185
Loans:
Total loans, net	691,811	691,541	696,272
Loans deemed to be impaired	4,304	4,408
Loans not deemed to be impaired	687,507	687,133
Residential Real Estate [Member]
Allowance for loan losses:
Allowance for loan losses, beginning of period	775	2,006
Recoveries	2	5
Provision	22	-138
Allowance for loan losses, end of period	799	1,873
Amount of allowance for loan losses for loans deemed to be impaired	90	122
Amount of allowance for loan losses for loans not deemed to be impaired	709	1,751
Loans:
Total loans, net	258,558	290,465	257,305
Loans deemed to be impaired	952	998
Loans not deemed to be impaired	257,606	289,467
Consumer [Member]
Allowance for loan losses:
Allowance for loan losses, beginning of period	810	432
Charge-offs	-81	-179
Recoveries	73	116
Provision	-88	56
Allowance for loan losses, end of period	714	425
Amount of allowance for loan losses for loans not deemed to be impaired	714	425
Loans:
Total loans, net	10,508	9,542	12,188
Loans not deemed to be impaired	10,508	9,542
Home Equity [Member]
Allowance for loan losses:
Allowance for loan losses, beginning of period	599	959
Recoveries		1
Provision	15	-58
Allowance for loan losses, end of period	614	902
Amount of allowance for loan losses for loans deemed to be impaired	92	94
Amount of allowance for loan losses for loans not deemed to be impaired	522	808
Loans:
Total loans, net	156,063	133,800	151,275
Loans deemed to be impaired	92	94
Loans not deemed to be impaired	155,971	133,706
Unallocated [Member]
Allowance for loan losses:
Allowance for loan losses, beginning of period	1,097	1,163
Provision	276	107
Allowance for loan losses, end of period	1,373	1,270
Amount of allowance for loan losses for loans not deemed to be impaired	$1,373	$1,270

Allowance_for_Loan_Losses_Loan

Allowance for Loan Losses - Loans by Risk Rating (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
In Thousands, unless otherwise specified
Loans by risk rating
Financing Receivable, Net	$1,341,330	$1,331,366	$1,264,061
Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	25,347	22,744	25,928
Construction and Land Development [Member] | 1-3 (Pass) [Member]
Loans by risk rating
Financing Receivable, Net	18,170	15,515
Construction and Land Development [Member] | 4 (Monitor) [Member]
Loans by risk rating
Financing Receivable, Net	7,075	7,126
Construction and Land Development [Member] | Impaired [Member]
Loans by risk rating
Financing Receivable, Net	102	103
Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	157,637	149,732	80,478
Commercial and Industrial [Member] | 1-3 (Pass) [Member]
Loans by risk rating
Financing Receivable, Net	156,294	148,407
Commercial and Industrial [Member] | 4 (Monitor) [Member]
Loans by risk rating
Financing Receivable, Net	472	472
Commercial and Industrial [Member] | Impaired [Member]
Loans by risk rating
Financing Receivable, Net	871	853
Municipal [Member]
Loans by risk rating
Financing Receivable, Net	41,406	41,850	32,307
Municipal [Member] | 1-3 (Pass) [Member]
Loans by risk rating
Financing Receivable, Net	41,406	41,850
Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	691,811	696,272	691,541
Commercial Real Estate [Member] | 1-3 (Pass) [Member]
Loans by risk rating
Financing Receivable, Net	686,883	691,322
Commercial Real Estate [Member] | 4 (Monitor) [Member]
Loans by risk rating
Financing Receivable, Net	624	633
Commercial Real Estate [Member] | Impaired [Member]
Loans by risk rating
Financing Receivable, Net	$4,304	$4,317

Allowance_for_Loan_Losses_Agin

Allowance for Loan Losses - Aging of Past Due Loan Losses (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
In Thousands, unless otherwise specified
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$2,764	$3,165
Non Accrual	4,260	4,146
Accrual Greater Than 90 Days	34
Total Past Due	7,058	7,311
Current Loans	1,334,272	1,324,055
Total loans, net	1,341,330	1,331,366	1,264,061
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Non Accrual	102	103
Total Past Due	102	103
Current Loans	25,245	22,641
Total loans, net	25,347	22,744	25,928
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	173	905
Non Accrual	209	157
Total Past Due	382	1,062
Current Loans	157,255	148,670
Total loans, net	157,637	149,732	80,478
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	41,406	41,850
Total loans, net	41,406	41,850	32,307
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,154	1,046
Non Accrual	2,775	2,781
Total Past Due	3,929	3,827
Current Loans	687,882	692,445
Total loans, net	691,811	696,272	691,541
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	698	632
Non Accrual	632	846
Total Past Due	1,330	1,478
Current Loans	257,228	255,827
Total loans, net	258,558	257,305	290,465
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	6	6
Non Accrual	4	5
Total Past Due	10	11
Current Loans	10,498	12,177
Total loans, net	10,508	12,188	9,542
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	733	576
Non Accrual	538	254
Accrual Greater Than 90 Days	34
Total Past Due	1,305	830
Current Loans	154,758	150,445
Total loans, net	$156,063	$151,275	$133,800

Allowance_for_Loan_Losses_Info

Allowance for Loan Losses - Information Pertaining to Impaired Loans (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	$587	$316
With no required reserve recorded, Unpaid Principal Balance	716	3,390
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value	566	758
With no required reserve recorded, Interest Income Recognized	2
With required reserve recorded, Carrying Value	5,734	6,671
With required reserve recorded, Unpaid Principal Balance	6,036	7,078
With required reserve recorded, Required Reserve	929	981
With required reserve recorded, Average Carrying Value	5,757	6,687
With required reserve recorded, Interest Income Recognized	33	41
Carrying Value	6,321	6,987
Unpaid Principal Balance	6,752	10,468
With required reserve recorded, Required Reserve	929	981
Average Carrying Value	6,323	7,445
Interest Income Recognized	35	41
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value		250
With no required reserve recorded, Unpaid Principal Balance		3,292
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value		437
With required reserve recorded, Carrying Value	102	108
With required reserve recorded, Unpaid Principal Balance	108	108
With required reserve recorded, Required Reserve	20	25
With required reserve recorded, Average Carrying Value	103	108
Carrying Value	102	358
Unpaid Principal Balance	108	3,400
With required reserve recorded, Required Reserve	20	25
Average Carrying Value	103	545
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	63	11
With no required reserve recorded, Unpaid Principal Balance	105	42
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value	39	116
With required reserve recorded, Carrying Value	808	1,118
With required reserve recorded, Unpaid Principal Balance	1,010	1,352
With required reserve recorded, Required Reserve	96	358
With required reserve recorded, Average Carrying Value	821	1,116
With required reserve recorded, Interest Income Recognized	11	8
Carrying Value	871	1,129
Unpaid Principal Balance	1,115	1,394
With required reserve recorded, Required Reserve	96	358
Average Carrying Value	860	1,232
Interest Income Recognized	11	8
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Required Reserve	0	0
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	392	55
With no required reserve recorded, Unpaid Principal Balance	396	56
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value	393	41
With required reserve recorded, Carrying Value	3,912	4,353
With required reserve recorded, Unpaid Principal Balance	4,006	4,442
With required reserve recorded, Required Reserve	631	382
With required reserve recorded, Average Carrying Value	3,918	4,420
With required reserve recorded, Interest Income Recognized	16	32
Carrying Value	4,304	4,408
Unpaid Principal Balance	4,402	4,498
With required reserve recorded, Required Reserve	631	382
Average Carrying Value	4,311	4,461
Interest Income Recognized	16	32
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Carrying Value	132
With no required reserve recorded, Unpaid Principal Balance	215
With no required reserve recorded, Required Reserve	0	0
With no required reserve recorded, Average Carrying Value	134	164
With no required reserve recorded, Interest Income Recognized	2
With required reserve recorded, Carrying Value	820	998
With required reserve recorded, Unpaid Principal Balance	820	1,082
With required reserve recorded, Required Reserve	90	122
With required reserve recorded, Average Carrying Value	823	949
With required reserve recorded, Interest Income Recognized	6	1
Carrying Value	952	998
Unpaid Principal Balance	1,035	1,082
With required reserve recorded, Required Reserve	90	122
Average Carrying Value	957	1,113
Interest Income Recognized	8	1
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Required Reserve	0	0
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
With no required reserve recorded, Required Reserve	0	0
With required reserve recorded, Carrying Value	92	94
With required reserve recorded, Unpaid Principal Balance	92	94
With required reserve recorded, Required Reserve	92	94
With required reserve recorded, Average Carrying Value	92	94
Carrying Value	92	94
Unpaid Principal Balance	92	94
With required reserve recorded, Required Reserve	92	94
Average Carrying Value	$92	$94

Allowance_for_Loan_Losses_Addi

Allowance for Loan Losses - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Receivables [Abstract]
Troubled debt restructurings, subsequently defaulted	$0	$0

Reclassifications_Out_of_Accum2

Reclassifications Out of Accumulated Other Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Securities held-to-maturity	$8,168	$7,780
Provision for income taxes	-215	-293
Net income	4,959	4,932
Salaries and employee benefits	-9,134	-8,875
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	-522	867
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	1,129	1,507
Provision for income taxes	-394	-583
Net income	735	924
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	-3	-3
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	-352	-92
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Amortization of Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before taxes	-355	-95
Provision for income taxes	142	38
Net income	($213)	($57)

Earnings_per_Share_EPS_Additio

Earnings per Share ("EPS") - Additional Information (Detail)	3 Months Ended	12 Months Ended	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2014	Mar. 31, 2015
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Number of Stock options outstanding			0
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	1,576	1,534
Class A Common Stock [Member] | Minimum [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Class A common stock entitled dividend per share percent in comparison to Class B common stock			200.00%

Earnings_Per_Share_EPS_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	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$4,959	$4,932
Class A Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	3,895	3,867
Weighted average shares outstanding, basic	3,600,729	3,582,421
Basic earnings per share	$1.08	$1.08
Dilutive effect of Class A stock options		1,576	1,534
Weighted average shares outstanding, diluted	5,567,909	5,558,177
Diluted earnings per share	$0.89	$0.89
Class B Common Stock [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$1,064	$1,065
Weighted average shares outstanding, basic	1,967,180	1,974,180
Basic earnings per share	$0.54	$0.54
Weighted average shares outstanding, diluted	1,967,180	1,974,180
Diluted earnings per share	$0.54	$0.54

Employee_Benefits_Additional_I

Employee Benefits - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Compensation and Retirement Disclosure [Abstract]
Number of years service to participate in supplemental plan	1 year
Current fiscal year contribution to pension plan	$500,000
Employer expected contribution to Pension Plan in 2015	$2,000,000

Employee_Benefits_Components_o

Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$336	$258
Interest	394	367
Expected return on plan assets	-688	-636
Recognized prior service cost (benefit)	-26	-26
Recognized net actuarial losses	203	3
Net periodic benefit (credit) cost	219	-34
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	397	389
Interest	341	331
Recognized prior service cost (benefit)	29	29
Recognized net actuarial losses	150	88
Net periodic benefit (credit) cost	$917	$837

Fair_Value_Measurements_Financ

Fair Value Measurements - Financial Instruments Measured at Fair Value on a Recurring and Non-recurring Basis (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	$455,931	$448,390
Fair Value Measurements, Level 1 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	295	296
Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	336,761	351,208
Fair Value Measurements, Level 3 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	118,875	96,886
U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	2,000	2,000
U.S. Treasury [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	2,000	2,000
Small Business Administration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	6,570	6,717
Small Business Administration [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	6,570	6,717
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	322,877	337,093
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	322,877	337,093
Privately Issued Residential Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	1,803	1,874
Privately Issued Residential Mortgage Backed Securities [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	1,803	1,874
Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	118,773	96,784
Obligations Issued by States and Political Subdivisions [Member] | Fair Value Measurements, Level 3 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	118,773	96,784
Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	3,511	3,524
Other Debt Securities [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	3,511	3,524
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	397	398
Equity Securities [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	295	296
Equity Securities [Member] | Fair Value Measurements, Level 3 Inputs [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	102	102
Impaired Loans [Member] | Fair Value Measurements, Level 3 Inputs [Member]
Financial Instruments Measured at Fair Value on a Non-recurring Basis
Impaired Loans	1,223	3,410
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	455,931	448,390
Carrying Value [Member] | U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	2,000	2,000
Carrying Value [Member] | Small Business Administration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	6,570	6,717
Carrying Value [Member] | U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	322,877	337,093
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	1,803	1,874
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	118,773	96,784
Carrying Value [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	3,511	3,524
Carrying Value [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities AFS	397	398
Carrying Value [Member] | Impaired Loans [Member]
Financial Instruments Measured at Fair Value on a Non-recurring Basis
Impaired Loans	$1,223	$3,410

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Specific adjustments to impaired loans recognized	$37,000	$947,000
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	119,750,000		33,951,000
Unrealized loss	$875,000		$870,000

Fair_Value_Measurements_Assets

Fair Value Measurements - Assets Measured at Fair Value (Detail) (USD $)	3 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Securities AFS [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Fair Value	$118,875	$96,886
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	$1,223	$3,410
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	30.00%	30.00%

Fair_Value_Measurements_Assets1

Fair Value Measurements - Assets Measured at Fair Value (Parenthetical) (Detail) (Municipal [Member])	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Municipal [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities maturity period	one year or less	one year or less

Fair_Value_Measurements_Change

Fair Value Measurements - Changes in Level 3 Securities (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$96,886	$36,597
Purchases	41,909	7,205
Maturities and calls	-19,914	-10,717
Amortization	-6	-5
Changes in fair value	0	0
Ending Balance	118,875	33,080
Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	3,820	3,820
Changes in fair value	0	0
Ending Balance	3,820	3,820
Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	92,964	32,487
Purchases	41,909	7,205
Maturities and calls	-19,914	-10,717
Amortization	-6	-5
Changes in fair value	0	0
Ending Balance	114,953	28,970
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	102	290
Changes in fair value	0	0
Ending Balance	$102	$290

Fair_Values_of_Financial_Instr2

Fair Values of Financial Instruments - Carrying Amount and Fair Value of Company's Financial Instruments (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Financial assets:
Securities held-to-maturity	$1,773,607	$1,406,792
Loans	1,318,801	1,309,048
Financial liabilities:
Time deposits	382,456	383,145
Other borrowed funds	307,642	395,500
Subordinated debentures	36,083	36,083
Carrying Value [Member]
Financial assets:
Securities held-to-maturity	1,773,607	1,406,792
Loans	1,318,801	1,309,048
Financial liabilities:
Time deposits	382,456	383,145
Other borrowed funds	307,642	395,500
Subordinated debentures	36,083	36,083
Estimated Fair Value [Member]
Financial assets:
Securities held-to-maturity	1,796,688	1,413,603
Loans	1,299,012	1,291,550
Financial liabilities:
Time deposits	386,759	387,919
Other borrowed funds	314,756	400,196
Subordinated debentures	36,083	36,083
Fair Value Measurements, Level 2 Inputs [Member]
Financial assets:
Securities held-to-maturity	1,796,688	1,413,603
Financial liabilities:
Time deposits	386,759	387,919
Other borrowed funds	314,756	400,196
Fair Value Measurements, Level 3 Inputs [Member]
Financial assets:
Loans	1,299,012	1,291,550
Financial liabilities:
Subordinated debentures	$36,083	$36,083