Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 520,104	$ 542,289
Gross Unrealized Gains	6,833	10,626
Gross Unrealized Losses	(435)	(683)
Debt securities available for sale	526,502	552,232
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	444,695	465,416
Gross Unrealized Gains	5,582	9,820
Gross Unrealized Losses	(422)
Debt securities available for sale	449,855	475,236
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,193	19,697
Gross Unrealized Gains	354	1
Gross Unrealized Losses	(2)	(60)
Debt securities available for sale	23,545	19,638
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	33,025	15,268
Gross Unrealized Gains	546	428
Gross Unrealized Losses	(10)
Debt securities available for sale	33,561	15,696
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,841	28,571
Gross Unrealized Gains	219	377
Gross Unrealized Losses	(1)
Debt securities available for sale	6,059	28,948
Asset backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,350	13,337
Gross Unrealized Gains	132
Gross Unrealized Losses		(623)
Debt securities available for sale	$ 13,482	$ 12,714

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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Amount, before tax, of unrealized loss 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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Amount 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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