Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Held-to-maturity
Amortized Cost	$ 3,729,502	$ 3,761,094
Allowance for Credit Losses	(3,214)	(1,388)	$ (1,279)
Net Carrying Amount	3,726,288	3,759,706
Gross Unrealized Gains	638	283
Gross Unrealized (Losses)	(591,556)	(696,756)
Estimated Fair Value	3,135,370	3,063,233
Available-for-sale
Amortized Cost	3,509,709	4,331,413
Allowance for Credit Losses	0	0
Gross Unrealized Gains	230	373
Gross Unrealized (Losses)	(357,786)	(478,932)
Estimated Fair Value	3,152,153	3,852,854
U.S. Treasury
Available-for-sale
Amortized Cost	2,285	2,257
Allowance for Credit Losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(31)	(60)
Estimated Fair Value	2,254	2,197
U.S. Government agencies
Held-to-maturity
Amortized Cost	453,121	448,012
Allowance for Credit Losses	0	0
Net Carrying Amount	453,121	448,012
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(89,203)	(102,558)
Estimated Fair Value	363,918	345,454
Available-for-sale
Amortized Cost	74,460	191,498
Allowance for Credit Losses	0	0
Gross Unrealized Gains	35	103
Gross Unrealized (Losses)	(1,993)	(7,322)
Estimated Fair Value	72,502	184,279
Mortgage-backed securities
Held-to-maturity
Amortized Cost	1,161,694	1,190,781
Allowance for Credit Losses	0	0
Net Carrying Amount	1,161,694	1,190,781
Gross Unrealized Gains	354	227
Gross Unrealized (Losses)	(107,834)	(118,960)
Estimated Fair Value	1,054,214	1,072,048
Available-for-sale
Amortized Cost	2,138,652	2,809,319
Allowance for Credit Losses	0	0
Gross Unrealized Gains	8	20
Gross Unrealized (Losses)	(198,353)	(266,437)
Estimated Fair Value	1,940,307	2,542,902
State and political subdivisions
Held-to-maturity
Amortized Cost	1,858,680	1,861,102
Allowance for Credit Losses	(2,006)	(110)	(1,197)
Net Carrying Amount	1,856,674	1,860,992
Gross Unrealized Gains	284	56
Gross Unrealized (Losses)	(369,509)	(446,198)
Estimated Fair Value	1,487,449	1,414,850
Available-for-sale
Amortized Cost	1,035,147	1,056,124
Allowance for Credit Losses	0	0
Gross Unrealized Gains	187	250
Gross Unrealized (Losses)	(132,541)	(185,300)
Estimated Fair Value	902,793	871,074
Other securities
Held-to-maturity
Amortized Cost	256,007	261,199
Allowance for Credit Losses	(1,208)	(1,278)	$ (82)
Net Carrying Amount	254,799	259,921
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(25,010)	(29,040)
Estimated Fair Value	229,789	230,881
Available-for-sale
Amortized Cost	259,165	272,215
Allowance for Credit Losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(24,868)	(19,813)
Estimated Fair Value	$ 234,297	$ 252,402

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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