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	Mar. 31, 2018	Dec. 31, 2017
Assets:
Investment securities, available for sale	$ 623,242	$ 628,046
Equity securities, at fair value	18,397	18,089
U.S. Treasury and U.S. government agencies
Assets:
Investment securities, available for sale	144,700	147,255
Obligations of states and political subdivisions
Assets:
Investment securities, available for sale	49,987	51,320
Fair Value, Measurements, Recurring
Assets:
Investment securities, available for sale	623,242	628,046
Equity securities, at fair value	18,397	18,089
Total Assets	650,900	652,690
Liabilities:
Total Liabilities	7,812	5,465
Fair Value, Measurements, Recurring \| Interest Rate Derivatives
Assets:
Derivative assets	9,261	6,555
Liabilities:
Derivative liabilities	7,812	5,465
Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities, available for sale	144,700	147,255
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities, available for sale	423,461	424,331
Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities, available for sale	49,987	51,320
Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities, available for sale	5,094	5,140
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring
Assets:
Investment securities, available for sale	5,138	5,415
Equity securities, at fair value	5,322	5,147
Total Assets	10,460	10,562
Liabilities:
Total Liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring \| Interest Rate Derivatives
Assets:
Derivative assets	0	0
Liabilities:
Derivative 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	5,138	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	618,104	622,631
Equity securities, at fair value	13,075	12,942
Total Assets	640,440	642,128
Liabilities:
Total Liabilities	7,812	5,465
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Interest Rate Derivatives
Assets:
Derivative assets	9,261	6,555
Liabilities:
Derivative liabilities	7,812	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	139,562	141,840
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities, available for sale	423,461	424,331
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities, available for sale	49,987	51,320
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities, available for sale	5,094	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	0
Total Assets	0	0
Liabilities:
Total Liabilities	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Interest Rate Derivatives
Assets:
Derivative assets	0	0
Liabilities:
Derivative 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 investment in debt and equity 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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