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, 2019	Dec. 31, 2018
Assets:
Investment securities available for sale, at fair value	$ 720,405	$ 638,618
Equity securities, at fair value	15,552	15,921
U.S. Treasury and U.S. government agencies
Assets:
Investment securities available for sale, at fair value	146,330	140,927
Obligations of states and political subdivisions
Assets:
Investment securities available for sale, at fair value	44,044	45,505
Fair Value, Measurements, Recurring
Assets:
Investment securities available for sale, at fair value	720,405	638,618
Equity securities, at fair value	15,552	15,921
Derivative assets	34,722	12,135
Total Assets	770,679	666,674
Liabilities:
Derivative liabilities	34,430	11,036
Total Liabilities	34,430	11,036
Fair Value, Measurements, Recurring \| U.S. Treasury and U.S. government agencies
Assets:
Investment securities available for sale, at fair value	146,330	140,927
Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale, at fair value	524,898	447,094
Fair Value, Measurements, Recurring \| Obligations of states and political subdivisions
Assets:
Investment securities available for sale, at fair value	44,044	45,505
Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities available for sale, at fair value	5,133	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	12,618	4,920
Equity securities, at fair value	1,843	2,731
Derivative assets	0	0
Total Assets	14,461	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	12,618	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	707,787	633,698
Equity securities, at fair value	13,709	13,190
Derivative assets	34,722	12,135
Total Assets	756,218	659,023
Liabilities:
Derivative liabilities	34,430	11,036
Total Liabilities	34,430	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	133,712	136,007
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Mortgage-backed securities
Assets:
Investment securities available for sale, at fair value	524,898	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	44,044	45,505
Significant Other Observable Inputs (Level 2) \| Fair Value, Measurements, Recurring \| Other debt securities
Assets:
Investment securities available for sale, at fair value	5,133	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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