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	Jun. 30, 2019	Dec. 31, 2018
Assets:
Investment securities available for sale, at fair value	$ 664,122	$ 638,618
Equity securities, at fair value	15,382	15,921
U.S. Treasury and U.S. government agencies
Assets:
Investment securities available for sale, at fair value	139,870	140,927
Obligations of states and political subdivisions
Assets:
Investment securities available for sale, at fair value	45,343	45,505
Fair Value, Measurements, Recurring
Assets:
Investment securities available for sale, at fair value	664,122	638,618
Equity securities, at fair value	15,382	15,921
Derivative assets	23,372	12,135
Total Assets	702,876	666,674
Liabilities:
Derivative liabilities	22,969	11,036
Total Liabilities	22,969	11,036
Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities available for sale, at fair value	139,870	140,927
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale, at fair value	473,786	447,094
Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities available for sale, at fair value	45,343	45,505
Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities available for sale, at fair value	5,123	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	16,525	4,920
Equity securities, at fair value	1,806	2,731
Derivative assets	0	0
Total Assets	18,331	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	16,525	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	647,597	633,698
Equity securities, at fair value	13,576	13,190
Derivative assets	23,372	12,135
Total Assets	684,545	659,023
Liabilities:
Derivative liabilities	22,969	11,036
Total Liabilities	22,969	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	123,345	136,007
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale, at fair value	473,786	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	45,343	45,505
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities available for sale, at fair value	5,123	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

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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