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Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Held-to-maturity
Amortized Cost	$ 3,618,770	$ 3,639,850
Allowance for Credit Losses	(3,214)	(3,214)	$ (3,214)	$ (3,214)
Net Carrying Amount	3,615,556	3,636,636
Gross Unrealized Gains	357	232
Gross Unrealized (Losses)	(686,288)	(686,917)
Estimated Fair Value	2,929,625	2,949,951
Available-for-sale
Amortized Cost	2,828,940	2,852,774
Allowance for Credit Losses	0	0
Gross Unrealized Gains	306	307
Gross Unrealized (Losses)	(337,397)	(323,655)
Estimated Fair Value	2,491,849	2,529,426
U.S. Treasury
Available-for-sale
Amortized Cost	700	999
Allowance for Credit Losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(1)	(3)
Estimated Fair Value	699	996
U.S. Government agencies
Held-to-maturity
Amortized Cost	456,545	455,869
Allowance for Credit Losses	0	0
Net Carrying Amount	456,545	455,869
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(83,796)	(95,961)
Estimated Fair Value	372,749	359,908
Available-for-sale
Amortized Cost	53,173	55,589
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3	5
Gross Unrealized (Losses)	(858)	(1,047)
Estimated Fair Value	52,318	54,547
Mortgage-backed securities
Held-to-maturity
Amortized Cost	1,048,170	1,070,032
Allowance for Credit Losses	0	0
Net Carrying Amount	1,048,170	1,070,032
Gross Unrealized Gains	313	212
Gross Unrealized (Losses)	(111,288)	(133,746)
Estimated Fair Value	937,195	936,498
Available-for-sale
Amortized Cost	1,508,611	1,545,539
Allowance for Credit Losses	0	0
Gross Unrealized Gains	72	4
Gross Unrealized (Losses)	(127,770)	(152,784)
Estimated Fair Value	1,380,913	1,392,759
State and political subdivisions
Held-to-maturity
Amortized Cost	1,857,076	1,857,373
Allowance for Credit Losses	(171)	(196)	(2,252)	(2,006)
Net Carrying Amount	1,856,905	1,857,177
Gross Unrealized Gains	44	20
Gross Unrealized (Losses)	(474,041)	(436,061)
Estimated Fair Value	1,382,908	1,421,136
Available-for-sale
Amortized Cost	1,031,456	1,015,619
Allowance for Credit Losses	0	0
Gross Unrealized Gains	21	132
Gross Unrealized (Losses)	(198,579)	(157,569)
Estimated Fair Value	832,898	858,182
Other securities
Held-to-maturity
Amortized Cost	256,979	256,576
Allowance for Credit Losses	(3,043)	(3,018)	$ (962)	$ (1,208)
Net Carrying Amount	253,936	253,558
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(17,163)	(21,149)
Estimated Fair Value	236,773	232,409
Available-for-sale
Amortized Cost	235,000	235,028
Allowance for Credit Losses	0	0
Gross Unrealized Gains	210	166
Gross Unrealized (Losses)	(10,189)	(12,252)
Estimated Fair Value	$ 225,021	$ 222,942

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