Estimated Fair Value of Financial Instruments and Fair Value Measurement - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Investment securities, available for sale	$ 613,243	$ 628,046
Equity securities, at fair value	16,038	18,089
Fair Value, Measurements, Recurring
Assets:
Investment securities, available for sale	613,243	628,046
Equity securities, at fair value	16,038
Equity securities, at fair value		18,089
Derivative assets	12,915	6,555
Total Assets	642,196	652,690
Liabilities:
Derivative liabilities	11,391	5,465
Total Liabilities	11,391	5,465
Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities, available for sale	144,474	147,255
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities, available for sale	417,547	424,331
Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities, available for sale	46,181	51,320
Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities, available for sale	5,041	5,140
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring
Assets:
Investment securities, available for sale	4,884	5,415
Equity securities, at fair value	2,996
Equity securities, at fair value		5,147
Derivative assets	0	0
Total Assets	7,880	10,562
Liabilities:
Derivative liabilities	0	0
Total Liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities, available for sale	4,884	5,415
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities, available for sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities, available for sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities, available for sale	0	0
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring
Assets:
Investment securities, available for sale	608,359	622,631
Equity securities, at fair value	13,042
Equity securities, at fair value		12,942
Derivative assets	12,915	6,555
Total Assets	634,316	642,128
Liabilities:
Derivative liabilities	11,391	5,465
Total Liabilities	11,391	5,465
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities, available for sale	139,590	141,840
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities, available for sale	417,547	424,331
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities, available for sale	46,181	51,320
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities, available for sale	5,041	5,140
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring
Assets:
Investment securities, available for sale	0	0
Equity securities, at fair value	0
Equity securities, at fair value		0
Derivative assets	0	0
Total Assets	0	0
Liabilities:
Derivative liabilities	0	0
Total Liabilities	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities, available for sale	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities, available for sale	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities, available for sale	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities, available for sale	$ 0	$ 0

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

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