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Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 987,495	$ 946,751
Gross Unrealized Gains	25,318	10,721
Gross Unrealized Losses	(16,533)	(7,334)
Allowance for Credit Losses	0
Estimated Fair Value	996,280	950,138
Amortized Cost	337,165	375,606
Gross Unrealized Gains	17,014	6,399
Gross Unrealized Losses	0	(480)
Estimated Fair Value	354,179	381,525
Allowance for Credit Losses	0
Obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	414,494	466,139
Gross Unrealized Gains	20,320	7,261
Gross Unrealized Losses	0	(420)
Allowance for Credit Losses	0
Estimated Fair Value	434,814	472,980
Amortized Cost	324,976	361,785
Gross Unrealized Gains	16,596	6,072
Gross Unrealized Losses	0	(480)
Estimated Fair Value	341,572	367,377
Allowance for Credit Losses	0
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	104,811	106,373
Gross Unrealized Gains	4,835	3,229
Gross Unrealized Losses	0	(1)
Allowance for Credit Losses	0
Estimated Fair Value	109,646	109,601
Amortized Cost	12,189	13,821
Gross Unrealized Gains	418	327
Gross Unrealized Losses	0	0
Estimated Fair Value	12,607	14,148
Allowance for Credit Losses	0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,444	2,430
Gross Unrealized Gains	126	102
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0
Estimated Fair Value	2,570	2,532
Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	465,746	371,809
Gross Unrealized Gains	37	129
Gross Unrealized Losses	(16,533)	(6,913)
Allowance for Credit Losses	0
Estimated Fair Value	$ 449,250	$ 365,025

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