Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Held-to-maturity
Amortized Cost	$ 3,661,914		$ 3,729,502
Allowance for Credit Losses	(3,214)	$ (3,214)	(3,214)	$ (3,214)	$ (3,214)	$ (1,388)
Net Carrying Amount	3,658,700		3,726,288
Gross Unrealized Gains	747		638
Gross Unrealized (Losses)	(549,837)		(591,556)
Estimated Fair Value	3,109,610		3,135,370
Available-for-sale
Amortized Cost	2,976,811		3,509,709
Allowance for Credit Losses	0		0	(1,196)	(2,396)	0
Gross Unrealized Gains	380		230
Gross Unrealized (Losses)	(286,097)		(357,786)
Estimated Fair Value	2,691,094		3,152,153
U.S. Treasury
Available-for-sale
Amortized Cost	1,297		2,285
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(7)		(31)
Estimated Fair Value	1,290		2,254
U.S. Government agencies
Held-to-maturity
Amortized Cost	455,179		453,121
Allowance for Credit Losses	0		0
Net Carrying Amount	455,179		453,121
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(75,331)		(89,203)
Estimated Fair Value	379,848		363,918
Available-for-sale
Amortized Cost	59,247		74,460
Allowance for Credit Losses	0		0
Gross Unrealized Gains	23		35
Gross Unrealized (Losses)	(873)		(1,993)
Estimated Fair Value	58,397		72,502
Mortgage-backed securities
Held-to-maturity
Amortized Cost	1,093,070		1,161,694
Allowance for Credit Losses	0		0
Net Carrying Amount	1,093,070		1,161,694
Gross Unrealized Gains	571		354
Gross Unrealized (Losses)	(88,660)		(107,834)
Estimated Fair Value	1,004,981		1,054,214
Available-for-sale
Amortized Cost	1,634,391		2,138,652
Allowance for Credit Losses	0		0
Gross Unrealized Gains	18		8
Gross Unrealized (Losses)	(124,007)		(198,353)
Estimated Fair Value	1,510,402		1,940,307
State and political subdivisions
Held-to-maturity
Amortized Cost	1,857,510		1,858,680
Allowance for Credit Losses	(227)	(191)	(2,006)	(1,520)	(934)	(110)
Net Carrying Amount	1,857,283		1,856,674
Gross Unrealized Gains	176		284
Gross Unrealized (Losses)	(368,773)		(369,509)
Estimated Fair Value	1,488,686		1,487,449
Available-for-sale
Amortized Cost	1,042,823		1,035,147
Allowance for Credit Losses	0		0	0	0	0
Gross Unrealized Gains	180		187
Gross Unrealized (Losses)	(144,825)		(132,541)
Estimated Fair Value	898,178		902,793
Other securities
Held-to-maturity
Amortized Cost	256,155		256,007
Allowance for Credit Losses	(2,987)	$ (3,023)	(1,208)	(1,694)	(2,280)	(1,278)
Net Carrying Amount	253,168		254,799
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(17,073)		(25,010)
Estimated Fair Value	236,095		229,789
Available-for-sale
Amortized Cost	239,053		259,165
Allowance for Credit Losses	0		0	$ (1,196)	$ (2,396)	$ 0
Gross Unrealized Gains	159		0
Gross Unrealized (Losses)	(16,385)		(24,868)
Estimated Fair Value	$ 222,827		$ 234,297