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Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
AFS investment securities:
Amortized Cost	$ 2,211,254		$ 2,904,745
Gross Unrealized Gain	57		0
Gross Unrealized Loss	(296,712)		(303,732)
Estimated Fair Value	1,914,599		2,601,013
HTM investment securities:
Amortized Cost	1,737,994		1,388,146
Allowance for Credit Losses	(128)		(43)
Net Carrying Amount	1,737,866		1,388,103
Gross Unrecognized Gain	1,064		44
Gross Unrecognized Loss	(384,496)		(291,051)
Estimated Fair Value	1,354,434		1,097,096
U.S. Treasury
AFS investment securities:
Amortized Cost	14,947		49,156
Gross Unrealized Gain	0		0
Gross Unrealized Loss	(2,033)		(2,139)
Estimated Fair Value	12,914		47,017
Agency
AFS investment securities:
Amortized Cost	457,694		485,331
Gross Unrealized Gain	0		0
Gross Unrealized Loss	(52,180)		(53,893)
Estimated Fair Value	405,514		431,438
Corporate
AFS investment securities:
Amortized Cost	560,873		586,652
Gross Unrealized Gain	57		0
Gross Unrealized Loss	(56,122)		(44,104)
Estimated Fair Value	504,808		542,548
Collateralized mortgage obligations
AFS investment securities:
Amortized Cost	337,195		829,928
Gross Unrealized Gain	0		0
Gross Unrealized Loss	(32,865)		(65,699)
Estimated Fair Value	304,330		764,229
HTM investment securities:
Amortized Cost	341,238
Allowance for Credit Losses	0
Net Carrying Amount	341,238
Gross Unrecognized Gain	1,064
Gross Unrecognized Loss	(17,902)
Estimated Fair Value	324,400
Mortgage-backed securities
AFS investment securities:
Amortized Cost	840,545		953,678
Gross Unrealized Gain	0		0
Gross Unrealized Loss	(153,512)		(137,897)
Estimated Fair Value	687,033		815,781
HTM investment securities:
Amortized Cost	233,881		231,692
Allowance for Credit Losses	0		0
Net Carrying Amount	233,881		231,692
Gross Unrecognized Gain	0		0
Gross Unrecognized Loss	(44,999)		(33,621)
Estimated Fair Value	188,882		198,071
Municipal bonds
HTM investment securities:
Amortized Cost	1,146,583		1,148,055
Allowance for Credit Losses	(128)	$ (113)	(43)	$ (91)	$ (109)	$ (22)
Net Carrying Amount	1,146,455		1,148,012
Gross Unrecognized Gain	0		44
Gross Unrecognized Loss	(321,595)		(257,430)
Estimated Fair Value	824,860		890,626
Other
HTM investment securities:
Amortized Cost	16,292		8,399
Allowance for Credit Losses	0		0
Net Carrying Amount	16,292		8,399
Gross Unrecognized Gain	0		0
Gross Unrecognized Loss	0		0
Estimated Fair Value	$ 16,292		$ 8,399