Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Held-to-maturity
Amortized Cost	$ 3,767,371	$ 3,761,094
Allowance for credit losses on HTM	(1,888)	(1,388)	$ (1,377)	$ (1,279)
Net Carrying Amount	3,765,483	3,759,706
Gross Unrealized Gains	216	283
Gross Unrealized (Losses)	(616,723)	(696,756)
Estimated Fair Value	3,148,976	3,063,233
Available-for-sale
Amortized Cost	4,186,431	4,331,413
Allowance for Credit Losses	(5,800)	0
Gross Unrealized Gains	382	373
Gross Unrealized (Losses)	(425,057)	(478,932)
Estimated Fair Value	3,755,956	3,852,854
U.S. Government agencies
Held-to-maturity
Amortized Cost	451,052	448,012
Allowance for credit losses on HTM	0	0
Net Carrying Amount	451,052	448,012
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(92,267)	(102,558)
Estimated Fair Value	358,785	345,454
Available-for-sale
Amortized Cost	188,141	191,498
Allowance for Credit Losses	0	0
Gross Unrealized Gains	73	103
Gross Unrealized (Losses)	(6,371)	(7,322)
Estimated Fair Value	181,843	184,279
U.S. Treasury
Available-for-sale
Amortized Cost	2,264	2,257
Allowance for Credit Losses	0	0
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(44)	(60)
Estimated Fair Value	2,220	2,197
Mortgage-backed securities
Held-to-maturity
Amortized Cost	1,201,418	1,190,781
Allowance for credit losses on HTM	0	0
Net Carrying Amount	1,201,418	1,190,781
Gross Unrealized Gains	27	227
Gross Unrealized (Losses)	(103,863)	(118,960)
Estimated Fair Value	1,097,582	1,072,048
Available-for-sale
Amortized Cost	2,664,735	2,809,319
Allowance for Credit Losses	0	0
Gross Unrealized Gains	37	20
Gross Unrealized (Losses)	(231,242)	(266,437)
Estimated Fair Value	2,433,530	2,542,902
State and political subdivisions
Held-to-maturity
Amortized Cost	1,860,332	1,861,102
Allowance for credit losses on HTM	(362)	(110)	(1,285)	(1,197)
Net Carrying Amount	1,859,970	1,860,992
Gross Unrealized Gains	189	56
Gross Unrealized (Losses)	(393,539)	(446,198)
Estimated Fair Value	1,466,620	1,414,850
Available-for-sale
Amortized Cost	1,060,198	1,056,124
Allowance for Credit Losses	0	0
Gross Unrealized Gains	272	250
Gross Unrealized (Losses)	(164,574)	(185,300)
Estimated Fair Value	895,896	871,074
Other securities
Held-to-maturity
Amortized Cost	254,569	261,199
Allowance for credit losses on HTM	(1,526)	(1,278)	$ (92)	$ (82)
Net Carrying Amount	253,043	259,921
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(27,054)	(29,040)
Estimated Fair Value	225,989	230,881
Available-for-sale
Amortized Cost	271,093	272,215
Allowance for Credit Losses	(5,800)	0
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(22,826)	(19,813)
Estimated Fair Value	$ 242,467	$ 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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