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Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Held-to-maturity
Amortized Cost	$ 3,594,745		$ 3,639,850
Allowance for Credit Losses	(3,214)	$ (3,214)	(3,214)	$ (3,214)	$ (3,214)	$ (3,214)
Net Carrying Amount	3,591,531		3,636,636
Gross Unrealized Gains	396		232
Gross Unrealized (Losses)	(699,953)		(686,917)
Estimated Fair Value	2,891,974		2,949,951
Available-for-sale
Amortized Cost	2,764,636		2,852,774
Allowance for Credit Losses	0		0
Gross Unrealized Gains	334		307
Gross Unrealized (Losses)	(359,650)		(323,655)
Estimated Fair Value	2,405,320		2,529,426
U.S. Treasury
Available-for-sale
Amortized Cost	400		999
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	0		(3)
Estimated Fair Value	400		996
U.S. Government agencies
Held-to-maturity
Amortized Cost	457,228		455,869
Allowance for Credit Losses	0		0
Net Carrying Amount	457,228		455,869
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(81,027)		(95,961)
Estimated Fair Value	376,201		359,908
Available-for-sale
Amortized Cost	50,204		55,589
Allowance for Credit Losses	0		0
Gross Unrealized Gains	7		5
Gross Unrealized (Losses)	(713)		(1,047)
Estimated Fair Value	49,498		54,547
Mortgage-backed securities
Held-to-maturity
Amortized Cost	1,024,313		1,070,032
Allowance for Credit Losses	0		0
Net Carrying Amount	1,024,313		1,070,032
Gross Unrealized Gains	337		212
Gross Unrealized (Losses)	(103,501)		(133,746)
Estimated Fair Value	921,149		936,498
Available-for-sale
Amortized Cost	1,464,212		1,545,539
Allowance for Credit Losses	0		0
Gross Unrealized Gains	195		4
Gross Unrealized (Losses)	(114,416)		(152,784)
Estimated Fair Value	1,349,991		1,392,759
State and political subdivisions
Held-to-maturity
Amortized Cost	1,855,816		1,857,373
Allowance for Credit Losses	(202)	(171)	(196)	(191)	(2,252)	(2,006)
Net Carrying Amount	1,855,614		1,857,177
Gross Unrealized Gains	59		20
Gross Unrealized (Losses)	(501,614)		(436,061)
Estimated Fair Value	1,354,059		1,421,136
Available-for-sale
Amortized Cost	1,042,284		1,015,619
Allowance for Credit Losses	0		0
Gross Unrealized Gains	31		132
Gross Unrealized (Losses)	(234,473)		(157,569)
Estimated Fair Value	807,842		858,182
Other securities
Held-to-maturity
Amortized Cost	257,388		256,576
Allowance for Credit Losses	(3,012)	$ (3,043)	(3,018)	$ (3,023)	$ (962)	$ (1,208)
Net Carrying Amount	254,376		253,558
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(13,811)		(21,149)
Estimated Fair Value	240,565		232,409
Available-for-sale
Amortized Cost	207,536		235,028
Allowance for Credit Losses	0		0
Gross Unrealized Gains	101		166
Gross Unrealized (Losses)	(10,048)		(12,252)
Estimated Fair Value	$ 197,589		$ 222,942