Investment Securities - Securities With Unrealized Losses (Details) $ in Thousands	Sep. 30, 2023 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	$ 45,424
Available-for-sale, less than 12 months, gross unrealized losses	(4,168)
Available-for-sale, 12 months or more, estimated fair value	3,282,758
Available-for-sale, 12 months or more, gross unrealized losses	(526,085)
Available-for-sale, total, estimated fair value	3,328,182
Available-for-sale, total, gross unrealized losses	(530,253)
U.S. Treasury
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	2,224
Available-for-sale, 12 months or more, gross unrealized losses	(54)
Available-for-sale, total, estimated fair value	2,224
Available-for-sale, total, gross unrealized losses	(54)
U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	9,161
Available-for-sale, less than 12 months, gross unrealized losses	(21)
Available-for-sale, 12 months or more, estimated fair value	158,972
Available-for-sale, 12 months or more, gross unrealized losses	(6,903)
Available-for-sale, total, estimated fair value	168,133
Available-for-sale, total, gross unrealized losses	(6,924)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	9,924
Available-for-sale, less than 12 months, gross unrealized losses	(140)
Available-for-sale, 12 months or more, estimated fair value	2,147,168
Available-for-sale, 12 months or more, gross unrealized losses	(287,571)
Available-for-sale, total, estimated fair value	2,157,092
Available-for-sale, total, gross unrealized losses	(287,711)
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	11,170
Available-for-sale, less than 12 months, gross unrealized losses	(502)
Available-for-sale, 12 months or more, estimated fair value	771,404
Available-for-sale, 12 months or more, gross unrealized losses	(208,638)
Available-for-sale, total, estimated fair value	782,574
Available-for-sale, total, gross unrealized losses	(209,140)
Other securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	15,169
Available-for-sale, less than 12 months, gross unrealized losses	(3,505)
Available-for-sale, 12 months or more, estimated fair value	202,990
Available-for-sale, 12 months or more, gross unrealized losses	(22,919)
Available-for-sale, total, estimated fair value	218,159
Available-for-sale, total, gross unrealized losses	$ (26,424)