Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities Available for Sale
Amortized Cost	$ 1,395,081	$ 946,751
Gross Unrealized Gains	25,225	10,721
Gross Unrealized Losses	(6,042)	(7,334)
Estimated Fair Value	1,414,264	950,138
Debt Securities Held to Maturity
Amortized Cost	284,563	375,606
Gross Unrealized Gains	14,163	6,399
Gross Unrealized Losses	0	(480)
Estimated Fair Value	298,726	381,525
Obligations of U.S. government agencies
Debt Securities Available for Sale
Amortized Cost	795,555	466,139
Gross Unrealized Gains	17,710	7,261
Gross Unrealized Losses	(891)	(420)
Estimated Fair Value	812,374	472,980
Debt Securities Held to Maturity
Amortized Cost	273,667	361,785
Gross Unrealized Gains	13,774	6,072
Gross Unrealized Losses	0	(480)
Estimated Fair Value	287,441	367,377
Obligations of states and political subdivisions
Debt Securities Available for Sale
Amortized Cost	123,347	106,373
Gross Unrealized Gains	5,748	3,229
Gross Unrealized Losses	0	(1)
Estimated Fair Value	129,095	109,601
Debt Securities Held to Maturity
Amortized Cost	10,896	13,821
Gross Unrealized Gains	389	327
Gross Unrealized Losses	0	0
Estimated Fair Value	11,285	14,148
Corporate bonds
Debt Securities Available for Sale
Amortized Cost	2,459	2,430
Gross Unrealized Gains	85	102
Gross Unrealized Losses	0	0
Estimated Fair Value	2,544	2,532
Asset backed securities
Debt Securities Available for Sale
Amortized Cost	473,720	371,809
Gross Unrealized Gains	1,682	129
Gross Unrealized Losses	(5,151)	(6,913)
Estimated Fair Value	$ 470,251	$ 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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