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, 2019	Dec. 31, 2018
Assets:
Investment securities available for sale, at fair value	$ 659,238	$ 638,618
Equity securities, at fair value	15,232	15,921
Fair Value, Measurements, Recurring
Assets:
Investment securities available for sale, at fair value	659,238	638,618
Equity securities, at fair value	15,232	15,921
Derivative assets	15,178	12,135
Total Assets	689,648	666,674
Liabilities:
Derivative liabilities	14,350	11,036
Total Liabilities	14,350	11,036
Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities available for sale, at fair value	139,581	140,927
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale, at fair value	469,636	447,094
Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities available for sale, at fair value	45,017	45,505
Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities available for sale, at fair value	5,004	5,092
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring
Assets:
Investment securities available for sale, at fair value	16,293	4,920
Equity securities, at fair value	1,864	2,731
Derivative assets	0	0
Total Assets	18,157	7,651
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, at fair value	16,293	4,920
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale, at fair value	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, at fair value	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, at fair value	0	0
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring
Assets:
Investment securities available for sale, at fair value	642,945	633,698
Equity securities, at fair value	13,368	13,190
Derivative assets	15,178	12,135
Total Assets	671,491	659,023
Liabilities:
Derivative liabilities	14,350	11,036
Total Liabilities	14,350	11,036
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities available for sale, at fair value	123,288	136,007
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale, at fair value	469,636	447,094
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities available for sale, at fair value	45,017	45,505
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities available for sale, at fair value	5,004	5,092
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring
Assets:
Investment securities available for sale, at fair value	0	0
Equity securities, at fair value	0	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, at fair value	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale, at fair value	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities available for sale, at fair value	0	0
Significant Unobservable Inputs (Level 3) \| Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities available for sale, at fair value	$ 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 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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