Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 418,253	$ 773,574
Gross Unrealized Gains	5,856	1,499
Gross Unrealized Losses	(952)	(4,369)
Estimated Fair Value	423,157	770,704
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	353,618	699,882
Gross Unrealized Gains	5,371	1,108
Gross Unrealized Losses	(355)	(3,524)
Estimated Fair Value	358,634	697,466
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,169	30,079
Gross Unrealized Gains	12
Gross Unrealized Losses	(26)	(160)
Estimated Fair Value	26,155	29,919
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,646	16,367
Gross Unrealized Gains	138	114
Gross Unrealized Losses	(565)	(573)
Estimated Fair Value	15,219	15,908
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,820	27,246
Gross Unrealized Gains	335	277
Gross Unrealized Losses	(6)	(112)
Estimated Fair Value	$ 23,149	$ 27,411

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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