Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($)	Mar. 31, 2024	Dec. 31, 2023
Debt Securities Available for Sale
Amortized Cost	$ 2,321,608,000	$ 2,384,325,000
Gross Unrealized Gains	756,000	525,000
Gross Unrealized Losses	(248,476,000)	(232,346,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	2,073,888,000	2,152,504,000
Debt Securities Held to Maturity
Totals	127,811,000	133,494,000
Gross Unrealized Gains	1,000	6,000
Gross Unrealized Losses	(9,284,000)	(8,374,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	118,528,000	125,126,000
Obligations of U.S. government agencies
Debt Securities Available for Sale
Amortized Cost	1,349,679,000	1,386,772,000
Gross Unrealized Gains	1,000	2,000
Gross Unrealized Losses	(179,488,000)	(165,037,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	1,170,192,000	1,221,737,000
Debt Securities Held to Maturity
Totals	125,131,000	130,823,000
Gross Unrealized Gains	1,000	0
Gross Unrealized Losses	(9,239,000)	(8,331,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	115,893,000	122,492,000
Obligations of states and political subdivisions
Debt Securities Available for Sale
Amortized Cost	262,777,000	262,879,000
Gross Unrealized Gains	132,000	268,000
Gross Unrealized Losses	(28,732,000)	(26,772,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	234,177,000	236,375,000
Debt Securities Held to Maturity
Totals	2,680,000	2,671,000
Gross Unrealized Gains	0	6,000
Gross Unrealized Losses	(45,000)	(43,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	2,635,000	2,634,000
Corporate bonds
Debt Securities Available for Sale
Amortized Cost	6,175,000	6,173,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(429,000)	(571,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	5,746,000	5,602,000
Asset backed securities
Debt Securities Available for Sale
Amortized Cost	362,877,000	359,214,000
Gross Unrealized Gains	623,000	255,000
Gross Unrealized Losses	(3,827,000)	(4,188,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	359,673,000	355,281,000
Non-agency collateralized mortgage obligations
Debt Securities Available for Sale
Amortized Cost	340,100,000	369,287,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(36,000,000)	(35,778,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	$ 304,100,000	$ 333,509,000