Fair Value Measurement and Fair Value of Financial Instruments - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Investment securities, available for sale	$ 646,135	$ 606,704
U.S. Treasury and U.S. government agencies
Assets:
Investment securities, available for sale	147,255	117,359
Obligations of states and political subdivisions
Assets:
Investment securities, available for sale	51,320	48,326
Equity securities
Assets:
Investment securities, available for sale	18,089	21,882
Fair Value, Measurements, Recurring
Assets:
Investment securities, available for sale	646,135	606,704
Total Assets	652,690	610,082
Liabilities:
Total Liabilities	5,465	2,345
Fair Value, Measurements, Recurring \| Interest Rate Derivatives
Assets:
Other Assets	6,555	3,378
Fair Value, Measurements, Recurring \| Non Hedging \| Interest Rate Derivatives
Liabilities:
Other Liabilities	5,465	2,345
Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities, available for sale	147,255	117,359
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities, available for sale	424,331	413,725
Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities, available for sale	51,320	48,326
Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Investment securities, available for sale	5,140	5,412
Fair Value, Measurements, Recurring \| Equity securities
Assets:
Investment securities, available for sale	18,089	21,882
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring
Assets:
Investment securities, available for sale	10,562	13,679
Total Assets	10,562	13,679
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	5,415	5,931
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring \| Equity securities
Assets:
Investment securities, available for sale	5,147	7,748
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring
Assets:
Investment securities, available for sale	635,573	593,025
Total Assets	642,128	596,403
Liabilities:
Total Liabilities	5,465	2,345
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Interest Rate Derivatives
Assets:
Other Assets	6,555	3,378
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Non Hedging \| Interest Rate Derivatives
Liabilities:
Other Liabilities	5,465	2,345
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities, available for sale	141,840	111,428
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities, available for sale	424,331	413,725
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities, available for sale	51,320	48,326
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Corporate debt securities
Assets:
Investment securities, available for sale	5,140	5,412
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Equity securities
Assets:
Investment securities, available for sale	$ 12,942	$ 14,134

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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-Name Accounting Standards Codification
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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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