Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
AFS investment securities:
Amortized Cost	$ 1,188,053	$ 1,176,040
Gross Unrealized Gain	247	433
Gross Unrealized Loss	(34,279)	(36,402)
Estimated Fair Value	1,154,021	1,140,071
HTM investment securities:
Amortized Cost	1,720,596	1,729,667
Allowance for Credit Losses	(115)	(126)
Net Carrying Amount	1,720,481	1,729,541
Gross Unrecognized Gain	1,698	2,694
Gross Unrecognized Loss	(274,482)	(246,729)
Estimated Fair Value	1,447,697	1,485,506
U.S. Treasury
AFS investment securities:
Amortized Cost	565,128	538,899
Gross Unrealized Gain	4	381
Gross Unrealized Loss	(193)	(24)
Estimated Fair Value	564,939	539,256
Agency
AFS investment securities:
Amortized Cost	1,734	1,941
Gross Unrealized Gain	0	0
Gross Unrealized Loss	(64)	(73)
Estimated Fair Value	1,670	1,868
Corporate
AFS investment securities:
Amortized Cost	471,426	481,499
Gross Unrealized Gain	243	52
Gross Unrealized Loss	(32,787)	(35,208)
Estimated Fair Value	438,882	446,343
Collateralized mortgage obligations
AFS investment securities:
Amortized Cost	149,765	153,701
Gross Unrealized Gain	0	0
Gross Unrealized Loss	(1,235)	(1,097)
Estimated Fair Value	148,530	152,604
HTM investment securities:
Amortized Cost	329,837	334,997
Allowance for Credit Losses	0	0
Net Carrying Amount	329,837	334,997
Gross Unrecognized Gain	1,288	1,565
Gross Unrecognized Loss	(10,867)	(9,570)
Estimated Fair Value	320,258	326,992
Mortgage-backed securities
HTM investment securities:
Amortized Cost	228,611	232,157
Allowance for Credit Losses	0	0
Net Carrying Amount	228,611	232,157
Gross Unrecognized Gain	146	310
Gross Unrecognized Loss	(35,422)	(30,798)
Estimated Fair Value	193,335	201,669
Municipal bonds
HTM investment securities:
Amortized Cost	1,145,902	1,146,244
Allowance for Credit Losses	(115)	(126)	$ (227)	$ (43)
Net Carrying Amount	1,145,787	1,146,118
Gross Unrecognized Gain	264	819
Gross Unrecognized Loss	(228,193)	(206,361)
Estimated Fair Value	917,858	940,576
Other
HTM investment securities:
Amortized Cost	16,246	16,269
Allowance for Credit Losses	0	0
Net Carrying Amount	16,246	16,269
Gross Unrecognized Gain	0	0
Gross Unrecognized Loss	0	0
Estimated Fair Value	$ 16,246	$ 16,269

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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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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