Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,028,173	$ 946,751
Gross Unrealized Gains	25,189	10,721
Gross Unrealized Losses	(51,363)	(7,334)
Allowance for Credit Losses	0
Estimated Fair Value	1,001,999	950,138
Amortized Cost	359,770	375,606
Gross Unrealized Gains	17,676	6,399
Gross Unrealized Losses	(4)	(480)
Estimated Fair Value	377,442	381,525
Allowance for Credit Losses	0
Obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	447,736	466,139
Gross Unrealized Gains	21,482	7,261
Gross Unrealized Losses	0	(420)
Allowance for Credit Losses	0
Estimated Fair Value	469,218	472,980
Amortized Cost	345,944	361,785
Gross Unrealized Gains	17,352	6,072
Gross Unrealized Losses	(4)	(480)
Estimated Fair Value	363,292	367,377
Allowance for Credit Losses	0
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	110,564	106,373
Gross Unrealized Gains	3,561	3,229
Gross Unrealized Losses	0	(1)
Allowance for Credit Losses	0
Estimated Fair Value	114,125	109,601
Amortized Cost	13,826	13,821
Gross Unrealized Gains	324	327
Gross Unrealized Losses	0	0
Estimated Fair Value	14,150	14,148
Allowance for Credit Losses	0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,437	2,430
Gross Unrealized Gains	138	102
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0
Estimated Fair Value	2,575	2,532
Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	467,436	371,809
Gross Unrealized Gains	8	129
Gross Unrealized Losses	(51,363)	(6,913)
Allowance for Credit Losses	0
Estimated Fair Value	$ 416,081	$ 365,025

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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