Fair Value Disclosures - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of asset transfers between Level 1 and Level 2	$ 0
Fair value of asset transfers between Level 2 and Level 1	0
Material liabilities measured at fair value	0	$ 0
Fair Value	$ 777,835,000	721,029,000
Minimum discount rate on appraisals of collateral securing loan	10.00%
Maximum discount rate on appraisals of collateral securing loan	50.00%
Collateral for collateral dependent loans	$ 40,458,000	35,086,000
Valuation allowance on impaired loans	3,600,000	2,600,000
Commitments to Extend Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commitments of lending operations	0	0
U.S. Treasury Notes [Member] \| AAA-Rated [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	35,100,000
Federal Agency MBS [Member] \| AAA-Rated [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	742,100,000
Impaired Loans (Collateral Dependent) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Collateral for collateral dependent loans	44,100,000	$ 37,700,000
Visa [Member] \| Common Class B [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated value of shares	$ 13,300,000

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
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Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

+ References

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities.

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Fair value of financial and nonfinancial obligations.

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Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Discount Rate Weighted Average Range Maximum

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