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Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
AFS investment securities:
Amortized Cost	$ 1,349,925		$ 1,176,040
Gross Unrealized Gain	386		433
Gross Unrealized Loss	(30,261)		(36,402)
Estimated Fair Value	1,320,050		1,140,071
HTM investment securities:
Amortized Cost	1,710,270		1,729,667
Allowance for Credit Losses	(129)		(126)
Net Carrying Amount	1,710,141		1,729,541
Gross Unrecognized Gain	1,148		2,694
Gross Unrecognized Loss	(283,965)		(246,729)
Estimated Fair Value	1,427,324		1,485,506
U.S. Treasury
AFS investment securities:
Amortized Cost	763,352		538,899
Gross Unrealized Gain	171		381
Gross Unrealized Loss	(412)		(24)
Estimated Fair Value	763,111		539,256
Agency
AFS investment securities:
Amortized Cost	1,583		1,941
Gross Unrealized Gain	0		0
Gross Unrealized Loss	(57)		(73)
Estimated Fair Value	1,526		1,868
Corporate
AFS investment securities:
Amortized Cost	441,365		481,499
Gross Unrealized Gain	207		52
Gross Unrealized Loss	(28,910)		(35,208)
Estimated Fair Value	412,662		446,343
Collateralized mortgage obligations
AFS investment securities:
Amortized Cost	143,625		153,701
Gross Unrealized Gain	8		0
Gross Unrealized Loss	(882)		(1,097)
Estimated Fair Value	142,751		152,604
HTM investment securities:
Amortized Cost	323,532		334,997
Allowance for Credit Losses	0		0
Net Carrying Amount	323,532		334,997
Gross Unrecognized Gain	1,023		1,565
Gross Unrecognized Loss	(11,801)		(9,570)
Estimated Fair Value	312,754		326,992
Municipal bonds
HTM investment securities:
Amortized Cost	1,145,563		1,146,244
Allowance for Credit Losses	(129)	$ (115)	(126)	$ (113)	$ (227)	$ (43)
Net Carrying Amount	1,145,434		1,146,118
Gross Unrecognized Gain	49		819
Gross Unrecognized Loss	(235,842)		(206,361)
Estimated Fair Value	909,641		940,576
Mortgage-backed securities
HTM investment securities:
Amortized Cost	224,953		232,157
Allowance for Credit Losses	0		0
Net Carrying Amount	224,953		232,157
Gross Unrecognized Gain	76		310
Gross Unrecognized Loss	(36,322)		(30,798)
Estimated Fair Value	188,707		201,669
Other
HTM investment securities:
Amortized Cost	16,222		16,269
Allowance for Credit Losses	0		0
Net Carrying Amount	16,222		16,269
Gross Unrecognized Gain	0		0
Gross Unrecognized Loss	0		0
Estimated Fair Value	$ 16,222		$ 16,269