Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Held-to-maturity
Amortized Cost	$ 3,688,664		$ 3,729,502
Allowance for Credit Losses	(3,214)	$ (3,214)	(3,214)	$ (3,214)	$ (1,888)	$ (1,388)
Net Carrying Amount	3,685,450		3,726,288
Gross Unrealized Gains	218		638
Gross Unrealized (Losses)	(680,144)		(591,556)
Estimated Fair Value	3,005,524		3,135,370
Available-for-sale
Amortized Cost	3,255,738		3,509,709
Allowance for Credit Losses	0		0	(2,396)	(5,800)	0
Gross Unrealized Gains	289		230
Gross Unrealized (Losses)	(370,123)		(357,786)
Estimated Fair Value	2,885,904		3,152,153
U.S. Treasury
Available-for-sale
Amortized Cost	1,294		2,285
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(19)		(31)
Estimated Fair Value	1,275		2,254
U.S. Government agencies
Held-to-maturity
Amortized Cost	454,488		453,121
Allowance for Credit Losses	0		0
Net Carrying Amount	454,488		453,121
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(95,882)		(89,203)
Estimated Fair Value	358,606		363,918
Available-for-sale
Amortized Cost	68,370		74,460
Allowance for Credit Losses	0		0
Gross Unrealized Gains	37		35
Gross Unrealized (Losses)	(1,844)		(1,993)
Estimated Fair Value	66,563		72,502
Mortgage-backed securities
Held-to-maturity
Amortized Cost	1,119,741		1,161,694
Allowance for Credit Losses	0		0
Net Carrying Amount	1,119,741		1,161,694
Gross Unrealized Gains	171		354
Gross Unrealized (Losses)	(128,910)		(107,834)
Estimated Fair Value	991,002		1,054,214
Available-for-sale
Amortized Cost	1,931,923		2,138,652
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		8
Gross Unrealized (Losses)	(201,081)		(198,353)
Estimated Fair Value	1,730,842		1,940,307
State and political subdivisions
Held-to-maturity
Amortized Cost	1,857,600		1,858,680
Allowance for Credit Losses	(191)	(2,252)	(2,006)	(934)	(362)	(110)
Net Carrying Amount	1,857,409		1,856,674
Gross Unrealized Gains	47		284
Gross Unrealized (Losses)	(428,369)		(369,509)
Estimated Fair Value	1,429,087		1,487,449
Available-for-sale
Amortized Cost	1,010,060		1,035,147
Allowance for Credit Losses	0		0	0	0	0
Gross Unrealized Gains	125		187
Gross Unrealized (Losses)	(145,995)		(132,541)
Estimated Fair Value	864,190		902,793
Other securities
Held-to-maturity
Amortized Cost	256,835		256,007
Allowance for Credit Losses	(3,023)	$ (962)	(1,208)	(2,280)	(1,526)	(1,278)
Net Carrying Amount	253,812		254,799
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(26,983)		(25,010)
Estimated Fair Value	226,829		229,789
Available-for-sale
Amortized Cost	244,091		259,165
Allowance for Credit Losses	0		0	$ (2,396)	$ (5,800)	$ 0
Gross Unrealized Gains	127		0
Gross Unrealized (Losses)	(21,184)		(24,868)
Estimated Fair Value	$ 223,034		$ 234,297