Investment Securities - Securities With Unrealized Losses (Details) $ in Thousands	Mar. 31, 2024 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	$ 43,033
Available-for-sale, less than 12 months, gross unrealized losses	(579)
Available-for-sale, 12 months or more, estimated fair value	2,930,580
Available-for-sale, 12 months or more, gross unrealized losses	(367,066)
Available-for-sale, total, estimated fair value	2,973,613
Available-for-sale, total, gross unrealized losses	(367,645)
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	1,964
Available-for-sale, less than 12 months, gross unrealized losses	(26)
Available-for-sale, 12 months or more, estimated fair value	0
Available-for-sale, 12 months or more, gross unrealized losses	0
Available-for-sale, total, estimated fair value	1,964
Available-for-sale, total, gross unrealized losses	(26)
U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	15,965
Available-for-sale, less than 12 months, gross unrealized losses	(181)
Available-for-sale, 12 months or more, estimated fair value	49,809
Available-for-sale, 12 months or more, gross unrealized losses	(1,814)
Available-for-sale, total, estimated fair value	65,774
Available-for-sale, total, gross unrealized losses	(1,995)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	10,842
Available-for-sale, less than 12 months, gross unrealized losses	(293)
Available-for-sale, 12 months or more, estimated fair value	1,833,560
Available-for-sale, 12 months or more, gross unrealized losses	(203,241)
Available-for-sale, total, estimated fair value	1,844,402
Available-for-sale, total, gross unrealized losses	(203,534)
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	14,262
Available-for-sale, less than 12 months, gross unrealized losses	(79)
Available-for-sale, 12 months or more, estimated fair value	845,801
Available-for-sale, 12 months or more, gross unrealized losses	(138,404)
Available-for-sale, total, estimated fair value	860,063
Available-for-sale, total, gross unrealized losses	(138,483)
Other securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	0
Available-for-sale, less than 12 months, gross unrealized losses	0
Available-for-sale, 12 months or more, estimated fair value	201,410
Available-for-sale, 12 months or more, gross unrealized losses	(23,607)
Available-for-sale, total, estimated fair value	201,410
Available-for-sale, total, gross unrealized losses	$ (23,607)