Securities - Available for Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Total securities available for sale	$ 477,979	$ 336,316
Gross Unrealized Gains	21,246	27,183
Gross Unrealized Losses	(1,076)	(35)
Fair Value	498,149	363,464
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Total securities available for sale	53,247	21,479
Gross Unrealized Gains	186	220
Gross Unrealized Losses	(156)	(6)
Fair Value	53,277	21,693
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Total securities available for sale	240,179	208,013
Gross Unrealized Gains	17,086	21,000
Gross Unrealized Losses	(550)	(1)
Fair Value	256,715	229,012
Mortgage-backed Securities in Government Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Total securities available for sale	184,553	106,824
Gross Unrealized Gains	3,974	5,963
Gross Unrealized Losses	(370)	(28)
Fair Value	$ 188,157	$ 112,759

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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