Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($)	Mar. 31, 2022	Dec. 31, 2021
Debt Securities Available for Sale
Amortized Cost	$ 2,474,601,000	$ 2,208,330,000
Gross Unrealized Gains	3,672,000	16,413,000
Gross Unrealized Losses	(115,366,000)	(16,805,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	2,362,907,000	2,207,938,000
Debt Securities Held to Maturity
Amortized Cost	186,748,000	199,759,000
Gross Unrealized Gains	292,000	8,381,000
Gross Unrealized Losses	(3,070,000)	0
Allowance for Credit Losses	0	0
Estimated Fair Value	183,970,000	208,140,000
Obligations of U.S. government agencies
Debt Securities Available for Sale
Amortized Cost	1,789,218,000	1,260,226,000
Gross Unrealized Gains	578,000	8,193,000
Gross Unrealized Losses	(99,052,000)	(11,030,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	1,690,744,000	1,257,389,000
Debt Securities Held to Maturity
Amortized Cost	180,289,000	192,068,000
Gross Unrealized Gains	222,000	8,131,000
Gross Unrealized Losses	(3,070,000)	0
Allowance for Credit Losses	0	0
Estimated Fair Value	177,441,000	200,199,000
Obligations of states and political subdivisions
Debt Securities Available for Sale
Amortized Cost	276,042,000	187,197,000
Gross Unrealized Gains	2,557,000	5,832,000
Gross Unrealized Losses	(11,404,000)	(785,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	267,195,000	192,244,000
Debt Securities Held to Maturity
Amortized Cost	6,459,000	7,691,000
Gross Unrealized Gains	70,000	250,000
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0	0
Estimated Fair Value	6,529,000	7,941,000
Corporate bonds
Debt Securities Available for Sale
Amortized Cost	7,652,000	6,722,000
Gross Unrealized Gains	29,000	34,000
Gross Unrealized Losses	(65,000)	0
Allowance for Credit Losses	0	0
Estimated Fair Value	7,616,000	6,756,000
Asset backed securities
Debt Securities Available for Sale
Amortized Cost	401,689,000	754,185,000
Gross Unrealized Gains	508,000	2,354,000
Gross Unrealized Losses	(4,845,000)	(4,990,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	$ 397,352,000	$ 751,549,000

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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