Securities - Summary of Amortized Cost and Fair Value of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 648,230	$ 1,051,436
Gross Unrealized Gains	5,037	971
Gross Unrealized Losses	(1,135)	(21,739)
Fair Value	652,132	1,030,668
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		253,015
Gross Unrealized Gains		59
Gross Unrealized Losses		(60)
Fair Value		253,014
U.S. government sponsored entities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		21,999
Gross Unrealized Gains		1
Gross Unrealized Losses		(112)
Fair Value		21,888
Mortgage-backed securities: residential
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	374,923	596,766
Gross Unrealized Gains	1,876	27
Gross Unrealized Losses	(856)	(16,094)
Fair Value	375,943	580,699
Mortgage-backed securities: commercial
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,858
Gross Unrealized Gains	56
Gross Unrealized Losses	(134)
Fair Value	17,780
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		25,744
Gross Unrealized Gains		0
Gross Unrealized Losses		(900)
Fair Value		24,844
Corporate Notes
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,825	12,480
Gross Unrealized Gains	539	21
Gross Unrealized Losses	(3)	(77)
Fair Value	33,361	12,424
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	222,624	141,432
Gross Unrealized Gains	2,566	863
Gross Unrealized Losses	(142)	(4,496)
Fair Value	$ 225,048	$ 137,799

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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