Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($)	Sep. 30, 2023	Dec. 31, 2022
Debt Securities Available for Sale
Amortized Cost	$ 2,509,623,000	$ 2,742,987,000
Gross Unrealized Gains	415,000	421,000
Gross Unrealized Losses	(335,701,000)	(290,970,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	2,174,337,000	2,452,438,000
Debt Securities Held to Maturity
Amortized Cost	139,058,000	160,983,000
Gross Unrealized Gains	0	15,000
Gross Unrealized Losses	(15,000,000)	(11,060,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	124,058,000	149,938,000
Obligations of U.S. government agencies
Debt Securities Available for Sale
Amortized Cost	1,443,038,000	1,568,408,000
Gross Unrealized Gains	0	3,000
Gross Unrealized Losses	(235,866,000)	(195,642,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	1,207,172,000	1,372,769,000
Debt Securities Held to Maturity
Amortized Cost	136,398,000	154,830,000
Gross Unrealized Gains	0	2,000
Gross Unrealized Losses	(14,824,000)	(11,013,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	121,574,000	143,819,000
Obligations of states and political subdivisions
Debt Securities Available for Sale
Amortized Cost	307,291,000	332,625,000
Gross Unrealized Gains	24,000	401,000
Gross Unrealized Losses	(48,646,000)	(39,821,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	258,669,000	293,205,000
Debt Securities Held to Maturity
Amortized Cost	2,660,000	6,153,000
Gross Unrealized Gains	0	13,000
Gross Unrealized Losses	(176,000)	(47,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	2,484,000	6,119,000
Corporate bonds
Debt Securities Available for Sale
Amortized Cost	6,170,000	6,164,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(665,000)	(413,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	5,505,000	5,751,000
Asset backed securities
Debt Securities Available for Sale
Amortized Cost	375,068,000	454,943,000
Gross Unrealized Gains	391,000	17,000
Gross Unrealized Losses	(5,730,000)	(15,193,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	369,729,000	439,767,000
Non-agency collateralized mortgage obligations
Debt Securities Available for Sale
Amortized Cost	378,056,000	380,847,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(44,794,000)	(39,901,000)
Allowance for Credit Losses	0	0
Estimated Fair Value	$ 333,262,000	$ 340,946,000