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Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Held-to-maturity
Amortized Cost			$ 3,639,850
Allowance for Credit Losses	$ 0	$ (3,214)	(3,214)	$ (3,214)	$ (3,214)	$ (3,214)
Net Carrying Amount			3,636,636
Gross Unrealized Gains			232
Gross Unrealized (Losses)			(686,917)
Estimated Fair Value			2,949,951
Available-for-sale
Amortized Cost	3,723,974		2,852,774
Allowance for Credit Losses	0		0
Gross Unrealized Gains	5,251		307
Gross Unrealized (Losses)	(409,948)		(323,655)
Estimated Fair Value	3,319,277		2,529,426
U.S. Treasury
Available-for-sale
Amortized Cost			999
Allowance for Credit Losses			0
Gross Unrealized Gains			0
Gross Unrealized (Losses)			(3)
Estimated Fair Value			996
U.S. Government agencies
Held-to-maturity
Amortized Cost			455,869
Allowance for Credit Losses			0
Net Carrying Amount			455,869
Gross Unrealized Gains			0
Gross Unrealized (Losses)			(95,961)
Estimated Fair Value			359,908
Available-for-sale
Amortized Cost	49,052		55,589
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2		5
Gross Unrealized (Losses)	(699)		(1,047)
Estimated Fair Value	48,355		54,547
Mortgage-backed securities
Held-to-maturity
Amortized Cost			1,070,032
Allowance for Credit Losses			0
Net Carrying Amount			1,070,032
Gross Unrealized Gains			212
Gross Unrealized (Losses)			(133,746)
Estimated Fair Value			936,498
Available-for-sale
Amortized Cost	2,446,711		1,545,539
Allowance for Credit Losses	0		0
Gross Unrealized Gains	5,032		4
Gross Unrealized (Losses)	(202,150)		(152,784)
Estimated Fair Value	2,249,593		1,392,759
State and political subdivisions
Held-to-maturity
Amortized Cost			1,857,373
Allowance for Credit Losses	0	(202)	(196)	(227)	(191)	(2,006)
Net Carrying Amount			1,857,177
Gross Unrealized Gains			20
Gross Unrealized (Losses)			(436,061)
Estimated Fair Value			1,421,136
Available-for-sale
Amortized Cost	1,045,691		1,015,619
Allowance for Credit Losses	0		0
Gross Unrealized Gains	47		132
Gross Unrealized (Losses)	(200,367)		(157,569)
Estimated Fair Value	845,371		858,182
Other securities
Held-to-maturity
Amortized Cost			256,576
Allowance for Credit Losses	0	$ (3,012)	(3,018)	$ (2,987)	$ (3,023)	$ (1,208)
Net Carrying Amount			253,558
Gross Unrealized Gains			0
Gross Unrealized (Losses)			(21,149)
Estimated Fair Value			232,409
Available-for-sale
Amortized Cost	182,520		235,028
Allowance for Credit Losses	0		0
Gross Unrealized Gains	170		166
Gross Unrealized (Losses)	(6,732)		(12,252)
Estimated Fair Value	$ 175,958		$ 222,942

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